Fast Find >> Mirror,
lap, Polishing cradle, Polishing
strokes, Stroke speed, MOT,
Pits and polishing out, Testing, The laser test, The Jerry test, Ronchi test, More Ronchi, The star test, Rez test, Aluminizing,
Collimation, Sky compass, Foreleg, Focusers, Glow finder, Cleaning, Modernity, Null test, Reflectivity, Coatings,
You're at: http://craigeroochi.neocities.org/dog-2.html
(last update: December 13th, 2017)
>This page attempts to confine itself to just the making, accessorizing and the using --of a Dogson telescope<
contact: craig er oochi a t outlook dotty com
* For a more complete and traditional background on ATMing, try the following link: https://stellafane.org/tm/index.html
For an excellent overview of amateur astronomy, see: https://www.handprint.com/ASTRO/atm.html
Hold "Ctrl" and tap "+" for larger text. My 800x600 authored pages look best if shaped squarishly:
It turned out to be a 5.75" by f/8.74 (as masked) with 75% illumination (secondary limited) to the edge of a .85 degree (51 minute) field of view, and is quite usable for a (one magnitude dimmer at the edge) 1.25 degree field with an affordable (say: $40) 32mm eyepiece.
For wider angle observing, I've got a pair of Russian 20/80 binoculars
(2.75 degrees) and a pair of excellent 10x50 Brunton binoculars (6+ degrees,
purchased at a close-out price).
* An eye full of stars is an eye full of stars, whether seen through a honking "richest field" telescope or a blackened toilet paper tube. The point is to pay attention, know or learn what you're looking at, to log what you can of it ("progress made good"), and to possibly make a discovery --but without turning an enjoyable avocation into a miserable pain-in-the-ass (I'm still working on that part :-) and letting astronomy overload your life. Since observing is mostly done in the dark, damp, cold hours of the night, it can easily get out of proportion and become an experience that you tend to postpone. Have goals, but reasonable ones. "Happiness is in your own back yard."
* Where I live, it's particularly chilly and damp at night, often with but brief clear sky opportunities. Lately I've been wondering if I should take another stab at celestial photography --and study my astrographs instead of standing out there in the cold (then use my good Dogson for a closer look at a thing or two in particular). (In that regard, I finally broke down and tried doing some image stacking, which gave me significantly more magnitude reach with just a few frames.)
* There will always be stuff that's beyond the reach of your eye and your instruments. Instead of straining at your limits, simply pay good and accurate attention to what's at hand. Useful and even innovative work (VSOing, meteor counts, binaries, comets, Lunar events) can be done with a log book/device, method, clock, and minimal observing equipment.
* As to telescopes, Nils Olof Carlin's suggested collimation tolerances imply that the making of a successful telescope can be a fairly casually executed project --or it can become a bit of a struggle to maintain your instrument within tight error limits. (If your telescope is well made (ie: rigid) then even a short focus 'scope might only need a little tweak at the start of each session, after which the alignment will hold as you swing it about.)
Carlin suggests that your scope's optical tube assembly ("OTA") and the alignment of its optics should be within the following tolerances --through to the lateral drift of its prime focus (regardless of the objective's diameter):
f/4 = +/-0.7mm; f/4.5 = +/-1mm; f/5 = +/-1.4mm; f/6 = +/-2.4mm; f/8 = +/-5.5mm; f/10 = +/-11mm
Comparatively, then: the build quality and maintenance of an f/8
or longer Newtonian scope appears to be a "walk in the park" --if its aperture
doesn't exceed about 8 inches or 200mm. (Any larger/longer and you'll be
stumbling around in the dark with a slippery wet step-stool or ladder.)
** Another consideration in the decision to make, buy or repurpose optics for a telescope --is that the grind and polish ATM will be working with an assortment of unusual substances. Aside from silvering chemistry (which few of us any longer engage in), there's glass dust, fine grinding and polishing compounds. You want to avoid scattering, inhaling and ingesting these items, or letting kids play in or have access to the area. You also want to avoid flushing coarser grinding compunds and sludge/slurry down household drains --where it might clog up.
The polishing compound sold as "cerium oxide" is special in that it's made up of variable components, one of which might be radioactive (as is the batch I'm using). Here's the list from an old MSDS sheet:
"Rare Earth Oxide": 30% - 65% (presumably: that's the
nominal cerium oxide --Craig)
Calcium Oxide: 10% - 15%
Strontium Oxide: 1% - 6%
Alumina Silicate: 1% - 40%
Fluorides: 4% - 10%
* The stuff I bought and used was tan colored and it really made my Geiger counter click. Clearly, this is a poorly defined and controlled substance. Though not listed, the radioactive component is sometimes said to be thorium. My sample was a heavy beta emitter. [I don't know if there's any alpha.]
But there's good news on that. "Got Grit" sells "white" cerium oxide.
Jerry Oltion kindly sent me a sample and I got no clicks above normal background
** As you'll see by my polishing and figuring notes, I found my 6 inch mirror damnably difficult to make --at least this first time through the course. Such problems as I experienced are usually written off to the errors of newbie ATMers, but an experienced and very active voice at the Eugene, Oregon club advises beginners to start instead with a shorter focus 8 or 10 inch mirror. He recounts that, despite his many [exemplary, I'll add] telescope project successes since, his only failed and abandoned mirror was a traditional 6 inch. More-over, he's not seen but one really good 6" mirror. (I made a good spherical 8 inch x f/10 mirror 60 years ago with no special problems.)
So at this point, I've made a "good" 6 inch mirror (by my humble standards), but only after 76 hours of polishing(!) --work which should have taken me about 6 hours. Only the making of my second 6 inch mirror will demonstrate if I've finally got the small mirror process bolted down.
Never-the-less, I'm presenting what appear to be the methods, tools and materials which worked.
* Here's the main page of my as-built drawings for Dog-2 (click to enlarge):
* Let's address the Dogson design/concept's deficiencies (that I'm aware of):
~ Chief among them is low altitude (below 30 degrees) sky and terrestrial observational performance. You can hoist the butt end up onto a stool or table, but the balance and vibration gets bad. (Dog-2's low altitude accommodation was somewhat designed around a Cosco brand step stool that we own. With added crutch tip feet, it seemed ideal to use with this scope --sitting, small kids stepping up, and "butt hauling" for the range of 15 to 30 degrees.)
~ While (screw) jacking the Dogson up and down in altitude goes swimmingly, some won't like turning this 'scope in azimuth --with its non-turning wheels dragging in the dirt (and there's usually a bit of backlash).
~ By today's strut and string 'scope standards, at 30 pounds it's heavy for a 6 inch telescope, though maybe not when compared to other long focus, closed tube scopes.
~ The Dogson is not a scope for swinging about and earning your Messier certificate. It's for picking and finding a target/object, studying it, and maybe shooting an afocally coupled snapshot (if the object is bright enough).
~ So far, I can't think of any way to make this stick-in-the-mud telescope track.
* Dogson Advantages:
~ As a one-hand lift, one-piece telescope (not counting a stool and kneeling pad), you can take it and go (the separately stored mirror already having been slipped into place). (I only "go" out into the side yard of our home.)
~ At astronomical altitudes (30 degrees and above) the OTA/tube is itself two parts of a steady tripod to the ground. There are no vibrations, no shaking when you adjust the focus, and no balance problem when you use a large eyepiece or mount a camera. If you're tired, rest an arm on the scope.
~ By using the Cosco step stool or a typical plastic garden chair (which has about an 8 inches lower seat), an adult can comfortably observe from about 45 degrees of altitude to the zenith. For the range of 30 to 45 degrees, an adult who's able to use a kneeling pad or a low stool is accommodated.
~ The mirror is securely held captive, but easily removed with a single thumbscrew --such that it's convenient to store the mirror at outside temperatures (in a cheap, desiccated, air tight container). There's a mark on the mirror's periphery so I can always install it the same way.
~ The primary mirror cell rides on a sled which can be moved to adjust fine focus and/or to accommodate gross prime focus changing accessories --like maybe an erecting Amici prism or a Barlow fore-lens, and to accommodate close subjects (to 100 feet). More such accommodation can easily be designed in.
This gross focus adjustment range can also make up for a mirror that fell short or long of its target focal length without having to bore another focuser hole.
~ The "focuser" also focuses a bit --helically, possibly to estimate distances in the 1000 to 10,000 foot range.
~ Despite not having a rocker box, the Dogson does have setting circles
~ It also has a swing out (and swing around to the other side, if desired) lightweight finder --
~ --plus a swing-up camera mount.
~ Since the tube is square, it's easy to mount accessories and features like those black "Fomecore" top-side insulation panels, internal baffles, the cell sled, sled tension arms, the secondary strut positioning "float block", a plumbing parts focuser, slide-off mirror access panel, a frame attached Dog leg and feet, a swing-out finder and the DDNU navigation platform.
This limited helical focuser is cobbled from plumbing parts (you can see the inside detail here). It can hold the waist banded, short barreled 32mm Celestron eyepiece in the below image. However, except for range estimation (1000 to 10,000 feet), such slow focusing adjustment isn't needed. Given smooth barreled eyepieces and the sled approach to primary focusing, one can easily do fine focusing with less than par focal eyepieces by simply turning and push-pulling them in a simple "eyepiece holder" like the below pictured plumbing item: a 1-1/2" to 1-1/4" ABS, male trap adapter (under $2, and thanks to Richard Berry for the suggestion). The nut adjustment and washer produces any desired degree of buttery smooth grip upon a standard (smooth) eyepiece barrel.
The traditional focuser at left is usually fitted so as to allow an inch of inward travel. As such, and with an eyepiece fully seated, the top of its tube stands an eyepiece 4 inches off of a round tube, and 4-1/4" off a square tube. Using a low profile helical or friction focuser like that ABS plumbing item can bring the eyepiece in to only an inch and a half (leaving a bit of inward travel), which in turn allows a significant reduction in diagonal size/obstruction, diagonal weight and potential vibration problems. (Again: that plumbing item is a no-go with a waisted eyepiece barrel.)
* Yes, I'm aware of "goto" 'scopes and the amazing "Safari" ($20 --!) application for i-Pads --which I've played with for a few minutes, and which affordably turns any telescope into a goto instrument --or buy a long focus optic for the i-Pad, which then itself becomes a navigating astrophotographic telescope --!-- (but you might then want to mount it onto a good tripod).
* Most everyone we know, young and old, has a hand-held computer/cell phone/Internet device --except ourselves. My understanding is that once you've acquired an I-thing (used ones are affordable or free) with GPS and inertial/gyroscopic capabilities --and have downloaded a Safari-like sky program, it can then be operated (sans any updates) without a communications contract. But --the Luddite in me says: "whoa".
* But what do I know? Our great-grandchildren had to tell us about WiFi a few years back. We stood there like amazed bush people as they sucked the Internet out of thin air. We only pack a bottom-feeder CDMA Tracfone (under $10/month) for hello-goodbye voice and have no Internet access at home --which would cost us $55/month --more than water and sewer!-- unless part of a $90/month "bundle". (We remember $10/month for an Internet connection.) So instead, we use a pair of Chromebooks to cadge free public WiFi.
Some of the "passions of our times" (like posting this page to the Internet) we must share --even embrace (and ain't HTML grand?), but I feel alienated from things that are price fixed and far beyond my understanding. (This is really more about my own struggles here with modernity --than any retro advocacy.)
The 6 inch mirror that I made for my "Dogson-2" telescope has only been my 2nd time at grinding and polishing in 60 years. My first mirror was an 8 inch x f/10, which I finished to a spherical figure.
To begin: * If the back of your blank isn't smooth and clear, make it so --by grinding with #120 and then #500 grit against flat glass or ceramic tile. Follow that up with some sleazy polishing technique --like #500 grit against a plastic tool --whatever works to make it transparent.
* Next: put a good 45 degree bevel (or "chamfer") around the peripheries of the back and the face of the mirror. Yes: you'll be initially losing some diameter --as much as 1/4 inch (1/8" bevel), but you get some back as you grind, and you want enough bevel so as not to be doing it twice. I used a typical, traditional, two-sided (rough and fine), carborundum sharpening stone --and the fine side seemed fine enough. (This beveling work takes a while.)
~ When cleaning my finished, polished mirror in preparation to send it in for aluminizing, I was unable to simply wash off a ring of cerium oxide (polishing agent) which was crusted onto the bevel. I had to go around the bevel again with the carborundum stone --holding the stone such that I could not possibly scratch the mirror.
I also gathered what I could from the literature in our library and summarized it:
~ The rough and fine grinding both went fast --maybe too fast for good control. I spent more time gauging my progress (ended up using automotive leaf gauges and a truly straight [hard to find] steel rule for the sagitta) --
~ --and trying to judge when #500 pits had vanquished the #120 pits. (Which was fairly easy, after I made a glass microscope slide with #120 grind on one end and #500 grind on the other --for microscopic comparison.)
~ You can switch to "wet testing" the mirror (plain water is best) by reflection (an AA battery flashlight next to your eye) shortly after starting the #500 grit grind.
~ Be very sure to do the "pencil" or "felt tip marker" test for contact --before calling it quits with your finest fine grind. (Simply mark up the mirror, then see if a few strokes cleans your marks off evenly.)
~ Yes: it is plenty tough to
polish #500 grit pits out of Pyrex --something like 20 hours
to pass traditional microscopic inspection. The alternative is to use a
final, very fine grind with "micron grit" --maybe 12 micron. However, this
is the point at which your mirror is most vulnerable to getting scratched
(one of the reasons why I skipped using micron grits). Next time I'm going
to try following the #500 grit with 12 micron grit on a dedicated pitch
lap --which "will not scratch".
* There were many little questions and decisions:
~ How much grit per "wet" (1/8th teaspoon);
~ How long is a "wet" (go by ear, and you can add drops of water to extend it a bit).
~ How hard to bear down during rough grinding (as hard as you can comfortably sustain without getting sloppy, which was about 20 pounds total pressure and mirror weight for me).
~ How wet is a wet (sluice off the tool and mirror in your plastic bucket, leaving both wet).
~ How wide & long is the "hogging" stroke (what you're comfortable with -maybe "4/5ths". Don't tip the mirror.)
~ How close to the RC (radius of curvature) should I rough grind (1% long, since the "1/3rd W" "maintenance stroke" pretty much stops RC progress when you need to (during fine grind or polish, and assuming the use of a polishing cradle). You can "hog" #500 grit until the focus is about 0.5% longer than your target.
* Again: For a more complete background on ATMing, follow this link: https://stellafane.org/tm/index.html
* I repeatedly experienced the common problem of a blankety-blank "turned down edge" (TDE) plus it had a stubbornly oblate figure --even with an undersized lap (MOT). I've read and heard that a TDE is especially common for beginners and for those who make longer focus 6 inch mirror --for over 100 years, and maybe for 300 years. I think it's time to get a better consensus as to what causes a TDE and what doesn't.
** For sure: since the mirror's figure at the edge of a mirror must "go around the corner", there will always be a bit of TDE. One ATMer routinely flats and frosts it with fine grit, then paints it flat black. Many others, some commercial, routinely mask off a wee bit of TDE --perhaps using a metal band with a rolled edge. I mask the edge with a black paper collar and a cut-to-pop-in-fit O-ring (thickness/diameter as needed).
Fingers are often pointed at soft pitch laps as the TDE culprit and one can imagine the mirror's leading edge plowing into soft pitch. Mel Bartels and others, however, can work with a relatively soft lap and not end up with a serious TDE. Also, the periphery of my mirror was the last to polish out (which is typical), so the TDE didn't develop because I was knocking down the edge --but because I was deepening the slope of the central zones (despite that it somehow remained oblate, overall), and/or the polishing action wasn't getting to the edge (duh: since the edge spends half the time over-hanging the lap).
Never-the-less, I slowly came to the realization that my pitch was indeed too soft, which is at least a nuisance in that the lap needs frequent trimming.
It also dawned on me that we needed a much better way to gauge pitch hardness. Traditionally, one simply bends a small piece or pushes his/her thumbnail into a sample, but touching, holding, or the very work of pushing your thumbnail into it will heat and sharply soften the pitch. (In India, ATMs try chewing a piece -!- but before you write them off as primitives, I also have an account of that being the practice in a pre-WW-2 optical lab here in the states.) Consequently, I took a long detour in order to nail down the hardness of my pitch --against temperature.
While I suspected that I'd not have developed such a horrible turned down edge with harder pitch, I first went (by stages) to a 17% (1 inch) undersized lap (polishing with mirror on top = MOT) to see if that alone (using my rather soft 58 minute [plunge test] pitch lap) --might roll back a turned down edge --without producing other problems. It didn't, and I've gotten the impression that undersized and otherwise "mutilated" laps are a zone problem making bad idea.
My pitch strip mold and grinding tool, marked for a 6" lap with 1" tiles
--and the failed undersized lap effort (click to enlarge)
* I eventually settled on using somewhat soft (166 minute plunge test) pitch --which is about the same viscosity/hardness as (brand name) "Gulgolz-64", and I reduced my TDE with a 4/5th "W stroke".
The negative consequences which ATMs have experienced by polishing "too fast" and the "strokes per second" or minute advice we get on that matter must also be weighed against mirror size and stroke length --since (obviously) a "1/3rd" W stroke per second with a 12 inch mirror requires moving it twice as far per second than with a 6" mirror.
I've decided against blaming inertial tipping/plowing and drag heating (unless polishing squeaky dry), but it seems intuitive that if the "give" of pitch isn't fast enough for cerium oxide particles to yield (when they snag into the pitch), then they might start to tumble --in the fashion of grinding particles.
However: one can simply use the normal temperature and environmental controls of the room the illustrated apparatus is located in, making sure to keep the air moving for uniform temperature. The below graph will help you compare a single plunge time that you've clocked against my measurements (at least for the two brands of pitch I've tested) such that you don't have to worry about what temperature spot on the graph you use --just temperature uniformity (a fan and mid-day) and wait for the pitch under test to arrive at a uniform temperature.
* Optical pitch is a rather viscous substance. My plunge times for
both Willmann-Bell "hard" pitch and Gulgolz-64 were running to 12 and 9
hours --at 68 degrees Fahrenheit with 400 grams of weight force. Consequently,
I switched to lead shot and doubled the plunging force to a weight of 800
grams. That didn't simply halve the initial plunge times, since the work
performed in plunging the BB ball contributes to the local temperature
of the pitch. Results will always be peculiar to the test method used,
so try to replicate my test rig, develop methods and
graphs of your own, or use the test rig published in later editions of
Jean Texereau's book (see next item).
Update (10/05/2014): My 1957 copy of Jean Texereau's "How To Make A Telescope" advocates the old thumbnail pitch hardness test, but thanks to Jerry Oltion, I've been alerted that later editions include simple plans for making a pitch hardness/viscosity test rig:
The penetrating tip is tapered to 14 degrees and its tip is flattened back to a width of 0.04 inch (1mm), the details of which will be found *here* on Jerry's pages, since he built a Texereau tester.
Jerry also checked some new Gulgolz-64 pitch, finding it to fall 0.021 inch at 68 degrees F. Earlier, he'd sent me a sample of the same pitch and it corresponded to 247 minutes on the chart below when using my rig. That happens to be the hardest pitch Texereau recommends for polishing and figuring medium to long focus mirrors (at a work room temperature of 68 degrees Fahrenheit).
Judging from my chart and the one in Texereau's newer editions, this pitch would remain usable at 80 or so degrees --but I couldn't recommend that for long focus mirrors. Texereau's pitch hardness "goal" would be about 78 minutes on my chart, but I ended up working with 166 minute test pitch (corrected to 68 degrees F., or 20 degrees centigrade --which is the normal lab standard.)
** When doing hardness tests, it's very important to know what the temperature of your pitch is. It's very unlikely to be at room temperature if that temperature has changed in the last hour, or if you've handled the pitch. I soaked my test tiles in a temperature monitored bath (a glass of water at room temperature) before testing and transferred the test squares/tiles with tweezers. Again: hardness changes drastically with temperature.
** Since the flow rate of pitch is affected by thickness, tile
size, and clear channels between, your pitch tiles should be made to a
standard --and that might as well be the 1 by 1 by 1/4 inch tile size assumed
here and by Texereau. They'll seem harder as they get thinner. (Never allow
the channels to close.)
Test methods compared:
* The most obvious difference between my test method and Texereau's is that his gauges depth, while mine measures time --both graphed on a logarithmic "Y"/ordinate axis. I think my method is a better way to go, since standard tiles are only 0.25 inches thick, whereas we've (hopefully) got plenty of spare time.
* A major advantage of the Texereau test is that the steel rod can be expected to conduct heat away from the worked point of penetration, thus more truly representing pitch viscosity/hardness at ambient/room temperature. However, if you decide to capitalize on that by (say) making the rod out of copper, your results might be at some small variance from Texereau's.
Aside from convenience, I don't think that the tile being mounted on a glass tool is a factor in carrying away heat since neither the pitch nor the glass is very conductive.
* To somewhat compensate for the confinement of heat (to the B-B
and point of contact) when using my tester, I kept a fan running on it
and used rather long run times (first at 400 grams, then 800 grams of force
--and I suggest that you use no more).
* I suspect that a lot of the advice we read about pitch and laps is based on several times reheated/recycled pitch, which then ends up pretty hard. However, the beginner starts with freshly delivered pitch, some of which is impossible (judging from the Willmann-Bell "hard" pitch I purchased) to heat to a pouring temperature without a lot of froth. Perhaps beginners, having been soundly counseled against "boiling" the pitch, consequently end up with a poorly poured, soft lap. (The Gulgolz-64 product I tested did not foam when heated to pouring temperature --a bigga point in its favor.)
My approach to Willmann-Bell's "hard" pitch was to first slowly heat fresh pitch past the pouring temperature (I reached 230 Fahrenheit, and I've seen 257 degrees suggested), drop back to my pouring temperature (205) and stir down the foam for a fairly bubble free pour. (It's not to worry about little bubbles. They make good micro facets.)
* Judging by the rudeness of traditional hardness tests, I suspect there's a wide range of hardness which will work well, especially for small, long focus mirrors. (Again: deep mirrors are said to need softer pitch.)
* Making a batch of pitch harder seems to require either adding rosin or enough heat for sustained "ebullience": the frothing off of volatile components in the pitch --which begins below the 212 degree boiling point of water with fresh W-B pitch, but such frothing/foaming tends to be less in once heated pitch --until you exceed the last temperature reached. (I've also seen ATM advice specifically warning against any foaming, but -again- that might be based on recycling long since foamed off pitch at reasonable pour temperatures.)
** Please go slow, use low electric heat and be prepared to smother a fire. (Don't use water!) **
* I used linseed oil to soften pitch. It's less volatile than turpentine, and you need only a few milliliters/cc per pound of pitch to make a big difference. The downside: it's then tough to harden it again with heat.
* I used a candy thermometer, which wipes pretty clean when hot. Later (burner off and away from the stove) you can wipe the rest of the pitch off with a little mineral spirits (paint thinner) and a paper towel.
* My W-B pitch was pour-able at about 200 degrees, so I stopped heating it at 230, stirred down the foam/bubbles, and took it off the kitchen range at 205 degrees.
~ However, you shouldn't need to take the temperature of your pitch, especially when using the Gulgolz brand. Just slowly heat it to an easy pouring temperature.
** Again: go slow when adding heat and do not leave the stove unattended.
Hardness increases as temperature drops --and at a steep rate.
The average of these rates is +22% per -degree F.
(click to enlarge)
* After much experimentation, I ended up with a batch of pitch that plunge tested at 166 minutes (corrected to 68 degrees). I can't tell you how I got to that hardness (too many detours and reheats), but you can get there with W-B pitch by adding very little Linseed oil (maybe 1cc per pound) --or: simply buy some Gulgolz-64 (which is said to have conditioners against chipping) --which checks out at about the same hardness.
The Pitch Lap
This is far from a complete overview of the pitch lap and polishing --but it might hold few useful items.
* In view of the above graph, simply raising the temperature in my (small) mirror shop let me do the initial lap pressing ("cold pressing"^) in 3 hours. Another time it took 9 hours at about 70 degrees with the same pitch. I see no reason to do "hot pressing" and risk mushrooming the lap tiles --or thermally imprinting the mirror. (Those who make monolithic, poured and pressed laps must, of course, do it hot --so hot as to almost be liquid.)
* I ended up making 7 laps for this one 6 inch mirror. In the future, I'll only need to make the polishing lap once, plus maybe a dedicated 12 micron grit lap.
* What really works well for adhering the pitch tiles onto your ceramic (floor tile) grinding tool is to first wipe an almost dry sheen of turpentine (the real stuff, not "mineral spirits") onto the ceramic face.
~ Problem: wiping turps will erase your lap lay-out lines, so deftly wipe/blot within the marked one inch squares.)
~ I see no need to warm the tool if the tiles are well candled. (Hold the squares by their opposite corners --thumb and forefinger. Bring the rough side in close and somewhat under the base of the candle flame, lest you be burning your fingers. Melt the pitch just short of it running, then gently wring it down in place.)
~ For the 5th (a full sized) lap, I tried to stick the pitch tiles onto residual pitch from the recently removed 4th lap. That was a no-go. Every one of those tiles easily popped off when tested.
~ It might be that turpentine only works well for the pine tar based pitch that Willmann-Bell sells.
* I ended up pressing (using 10 pounds total weight, including the weight of my 6" mirror --or about 6oz/square inch) with a disk of synthetic window screen against the lap, and a single sheet disc of (Reynolds brand) non-stick baker's parchment paper between the mirror and the screen (to keep pitch from sticking to the glass). As the pitch tiles comes into conformance with the mirror, that screen gave their shiny surfaces a cerium oxide holding micro-faceted finish, so it's easy to tell when pressing is sufficiently complete. (Baker's parchment also covers the floor of my pitch strip mold.)
~ I think it's important to follow screen pressing with 15 minutes (using 166m test pitch) of pressing with just parchment (or tracing) paper, since the upwelling of the pitch flow into the screen is surely irregular.
~ I do not like "direct" cold pressing, using only cerium oxide or rouge slurry between. Such long contact has transferred pitch deposits onto the mirror, which then become polish and test fouling smears.
~ Again: I don't feel that initial "hot pressing" is necessary for a traditional, discrete tiled lap. Extended cold pressing works well for me (with 166m test pitch). One might raise the polishing room temperature 5 degrees.
* Eventually, I ended up using a nominally full size lap, but with a diminishing bevel --as is the normal practice.
* Mel Bartels (no slouch!) holds the opinion that an over-sized lap prevents a TDE. (Truth be known, Mel makes lots of great big mirrors, while I struggled mightily with my one little piss-ass mirror.)
** The common wisdom is to ensure that the central tile/square is well off center, so as to not polish in a pattern of zonal departures, ripples or whatever into the mirror. So far I've been following that advice, even though I suspect it's hogwash. What's more: I once managed to "dog-biscuit" my mirror (meaning: mottling/choppiness) --anyway.
Although our optics goods suppliers, books and Web resources bespeak surface departure errors in terms of a wave length of light --implying that our criteria has to do with the heights of a mirror profile (assuming that the mirror resting on its back), although venerable authorities would have us graph out those profiles using the very sensitive Foucault test, those heights are simply a convenient way to reference mirror quality. It's actually the slopes which produce those profile heights --it's the average slopes that we see in our tests --and which confound our efforts to confine a star's image to a point at prime focus.
These departures, seen at the radius of curvature, are (of course) from what would be a spherical surface. When we do test a spherical mirror, it looks perfect: no shadows in the Foucault test, straight bars in the Ronchi test --which is often called a "null". What we seek when "figuring" a well polished spherical mirror, however, is a paraboloidal surface. Interpreting its departure from sphere in an easy but precision way --is the essence of what follows.
The Ronchi Test
* Some parties in our ATM circles (most notably: Willmann-Bell) count both the black lines of a Ronchi grating and the equal width spaces between --such that a grating with 50 black lines per inch is said to be a "100 line" grating. However, the vast majority of the optical and science industry outside of our ATM community either counts only the black lines --or: refers to the (black and white) "line pair" count of a grating or a resolution target. Seldom do vendors, ATM web pages and other published articles state which definition is being used.
* Most ATM workers who report and image or illustrate Ronchi grating results place the grating just inside the focus. Fortunately, most everyone is keen on stating whether a given Ronchi image or drawing was inside or outside --because the results are opposite each other. However --
Jerry Oltion pointed out to me that a grating placed outside of focus gives a better indication of the shape of the peripheral zone of a mirror, and he's right. Once I tried outside-the-focus with a good Willmann-Bell grating, I converted over from having been an "inside man".
* I can't think of an explanation as to why, but the Ronchi image is also much sharper outside the focus, and that's with the (W-B) grating turned to face either way. This phenomenon is much less apparent when using a laser printed Ronchi grating. Presumably, that's because a laser printed transparency grating is so crummy:
The basic Ronchi images: A - B - C - D
At left is what we see if the mirror has a nice spherical surface, for which a Ronchi setup can be considered a fairly good "null test". The following three Ronchi illustrations (all just outside of focus) describe other surface shapes, which require seasoned, subjective judgment to interpret: B: "oblate", meaning flatter (more shallow) than spherical; C: deeper than spherical (might be paraboloid, might be "hyperboloid", or too deep); D: deeper than spherical and the edge/periphery of the mirror either remains sphere --or worse/flatter: the "turned down edge".
* A subtle variation on the above might be a central area that's sphere, plus an outer/peripheral area that's either paraboloidal, a different sphere, or oblate/TDE. In either case I go carefully (15 minutes per session, minimum pressure, using rouge) with a long and wide (4/5ths) "W stroke". This tends to over-run a TDE, but it also deepens the center^.
Should the center go too deep ("over-corrected"), I resort to a 1/6th to 1/4th W stroke with medium pressure. That raises the center --with less tendency toward a TDE than a 1/3rd W smoothing stroke, but it also tends to make the surface choppy ("dog biscuit") --though not as badly as does the short "I-stroke".
^ If you're making a small, long focus mirror (say: f/9), you might be going for a spherical surface, or "figure", which brings you within 1/8 wave when working a 6 inch mirror. (Presumably, shadowing by the secondary lessens the effective error of a spherical mirror, but not significantly for long focus scopes with minimal secondaries.)
** Being such a lazy barsted, I've found that a (good) Ronchi grating can be used like a Foucault knife edge (as others have noticed). I've also skated out on having to add a micrometer pusher or a manipulator to delicately move the grating left and right. I do lots of nudging, although it's possible to move the Ronchi bars and adjust the focus point without touching the Ronchi apparatus --by introducing and twisting glass slides.
I'm holding a pair of microscope slides (home made, chamfered edge, 1" x 3.5" window glass). The slides don't have to be optically flat. (Any introduced problem would be readily apparent, of course).
By placing slides into the beams, the focus point of the RC (radius of curvature) is lengthened. By checking a stack of slides, I found that a single piece of 0.110 inch thick, "single strength" window glass placed in both beams lengthens the focus by 0.035 inch, and by 0.0175 inch in just the return beam. A real (0.050" thick) microscope slide should lengthen the focus by 0.016", a 0.025" thick cover slip by 0.008". (I've since made a little slide-around wooden caddy for holding my glass slides upright.)
If the mirror is on the paraboloidal^ side (its center having a shorter radius of curvature focus), then use (say) 2 or 3 slides for a long focus mirror and nudge the Ronchi apparatus to evenly darken an outer donut area of the mirror first. Next, find out how many slides to remove in order to darken the center of the donut --and to stop the apparent Ronchi bars there from moving in the opposite direction when you budge the apparatus (always moving in from outside the focus).
* I've repeatedly tried using "Couder"^ type masks and variations on the simpler center-and-periphery masks we see in the ATM-1 book^, but I feel much more aware of what's going on without masks. As ATM-1 counsels, we're simply looking at the over-all difference in radius of curvature, center to edge. That, together with seeing and knowing that the surface smoothly transits from center to edge --is enough information for completion.
(^See section II-31 of Texereau's book: "How To Make A Telescope", pages 94-98 and 220 of ATM-1.)
When such measurements are made by way of using masks and a Foucault knife edge on (say) a 6 inch mirror, the outer 1/2 inch is looked at, and maybe a central diameter of 1.5 inches. The simple formula: difference = r2/RC, gives us a good idea of where we're at, mirror figure-wise. However: that formula applies to the focus difference from the dead center to the very edge, versus the actual mean radius we're looking at somewhat inward from the edge, and somewhat outward from the center --since we need a goodly area for judging the shadows. Consequently, "r" might be better taken as 2.5 inches, rather than 3, especially if the outer edge has a strong bevel, or some TDE is to be abandoned. That would yield 6.25/102" = 0.061".
~ The inner (say) 1.5 inches of mirror (ie: a 3/4" radius) only accounts for .563/102 = 0.0055" of the would-be difference, whereas the jump from r = 2.5 to r = 3 extends the RC from 0.061" to 0.088" --something worth paying attention to.
~ Using my old eyes (and mind), and not using a mask, it takes me a while to get grounded as to what I'm looking and nudging at. I'm watching no less than a central 2 inches of blob-like diameter (there goes 0.010") and maybe as little as 2.25 inches of radius for the average "donut" diameter --for a perceived difference of (maybe) only 0.039". For my (nominally) 6" x f/8.4 mirror (which would probably work okay even if left spherical), I decided that just one glass slide's worth of parabolizing difference was going to be just fine.
^ If your mirror is on the "oblate spheroid" side, you don't need to know by how much. Just fix it.
* In order to tweak a single Ronchi line across your mirror --or just the inner or the outer zone of your mirror, that can be done by twisting the slides in the beam a little. However, I find it easier to just nudge the apparatus or table a little. Sometimes shifting my weight on the floor will do the adjustment.
* I mark a big fat arrow on the back of my mirror so that it's always oriented the same for tests. Again: I always come into the RC focus from outside the focus. The bars are cleaner and I'm better oriented that way.
~ Be sure to turn off any heaters in the room, lower the lights, banish your enthusiastic dog and your clumpity-clomp restless spouse to a distant room, because your home is now made of quivering Jell-O.
~ Famously: you can fairly determine whether your mirror is sphere: the Ronchi lines are straight across.
~ When it's deeper than sphere (possibly a paraboloid): the lines curve to cup away from the center of the mirror (when seen outside of the RC focus).
~ But: when the Ronchi lines curve a little to cup the center of the mirror, it's --
Not only is this mirror "over-corrected",
but it's got bad TDE as well.
(All of my Ronchigrams are outside of focus)
Is the mirror sufficiently polished out?
* Although any pits were long gone under microscopic examination, I kept seeing the same, soft, granular light scatter when giving the surface a laser light test --through 10 hours of polishing. I switched to using rouge for the final hours of polishing and figuring, but that made no difference. Perhaps this is just the nature of Pyrex.
A big tip from Jerry Oltion is to illuminate the surface from just under the mirror's edge --which eliminates lighting up surface dust --and which might more selectively illuminate any actual pits. (I've not tried this yet.)
It looked the same at 76 hours, so maybe Pyrex always shows some sort of surface "grain"?
The brighter spot and that scabby looking stuff is due to the casting impression on the back (far)
side of the mirror. Next time I'll first grind and flash polish the back, since it's nice to have a clear
view of the grinding and polishing action (MOT). (Click to enlarge.)
* At 40 hours, I was still fending off the TDE and trying to coax
a paraboloid out of the recurring sphere. What finally answered was the
classical parabolizing stroke.
The Jerry Test:
Jerry Oltion recently discovered an excellent way to inspect for pits --which I look forward to trying. Having previously smoothed and visually polished the back of your mirror (well enough to observe your progress while grinding and polishing), turn your cleaned mirror over --face down on a black mat surface. Shine a bright light into the edge of the mirror, close to the face. This method makes the remaining pits stand out sharply, but not any dust stuck to the mirror's face.
Just as the single "blinking" line Ronchi test, which is about the
same as a Foucault test (which Jerry Oltion calls the "red blood cell test"
--which it resembles) --seems far and away more sensitive than just looking
at Ronchi grating lines, the "star test" is (reportedly) yet again an order
more sensitive. The basic idea is that you need your telescope in good
alignment/collimation and you need to do this test on a night with good
and steady "seeing", such that you can compare a bright star's highly magnified
image to either side of prime focus. Mel Bartels tells us all about it
--from which page I call your attention to Mel saying: "--you must learn the star test on small long focus mirrors. This is the only way to learn the subtleties of the star test. Small long focus mirrors tend to have fewer confounding defects, are more easily mounted, tend to be less affected by cooling night air, and [by allowing the use of less powerful eyepieces] keeps the eye's afflictions out of the picture. Scopes with 4 to 10 inches aperture and a focal ratio of f/6 to f/10 are best [to learn with]." With that statement, and with the striking similarity of Mel's and Jerry Oltion's star test rigs to a Dogson, I feel comfortable resting my case and advocacies for choosing a Dogson scope project.
* However, I find good seeing rather rare where I live (near the Oregon coast) and the high power star test difficult to interpret. Polaris seems too dim for my small mirror before it's aluminized, so I picked out a bright star and manually tracked it. (I'm trying to remember if I've ever seen a star here that wasn't somehow "boiling".)
* I've several times tried to set up an artificial star --way up on a great sand dune which peaks about two blocks from our front yard. However, my solar powered lights up there (clearly visible, even at dusk) were short lived, thanks to bugs gathering in the dark and kids gathering by day to tear them up. My impression was that for critical testing, one must wait until the early morning hours --when the ground is too cold to be sending up turbulent waves of warmed air. (That might also be the best time to observe.)
* I tried it one more time with a convex mirror reflector (that protectively bagged automotive accessory rear view mirror, glued to the top of my stake, along with a yellow automotive reflector to help find the stake at night):
I had problems:
~ As mentioned at the top of this page, the Dogson telescope design is a poor one for low altitude/terrestrial observation. Although one can hoist the butt end up onto a high stool, the balance and vibration gets bad. (Adding a purpose built stool would help solve that problem, but I'm staying with the high astronomical aim of my Dogson for now.)
~ The thin lens formula has served me well over the years, but this time I didn't have as much back-focus (closer subject/longer focus) as I should have. I did managed to squeak out a 100 foot focus, however.
~ The faint diffraction disk I ended up seeing was too dim to evaluate, so I'll just forget about testing a non-aluminized 6 inch mirror this way.
~ The automotive mirror's surface is simply not good enough. I saw smatterings of more than one point of light.
~ Some while ago I made an artificial star source out of a white LED for modeling small obstructed lenses at 50 feet or so --inside the house. Under a microscope its pinhole is quite round and measures 15 microns (0.006 inch, but to subtend 1/4 arc-sec, I'd have to place it 400 feet distant --and probably for checkered results in the early morning hours. ---Hmmnnn
** So: unless someone can think of a much better way to do it, I vote that star testing, artificial or real, is yet another "hobby killer" which drains the sap, takes blocks of time away from the goals of documented celestial observation, creative interpretation and (at least) personal discovery.
I can see doing a star test at long last with a finished and aluminized mirror --to document/log the mirror's true condition --should one ever happen to have a night of great seeing, a high power eyepiece handy, and a fully collimated 'scope at the same time --but not as yet another hurdle to clear on your way to the observing chair.
* That leaves me with the fabulous "blinking" test --aka: the cheap and easy, brighter alternative to Foucault testing, by using a good Ronchi grating at focus.
* Standard Ronchi testing is, of course, also very useful.
* The resolution target test:
I printed this target one inch square, laminated it with shipping tape and made it self-adhesive with double-stick carpet tape --then stuck it high onto the post of a stop sign --about 330 feet distant from our front porch. This is a traditional photographic standard for testing/proving throughput resolution. If my mirrors can separate those bars at an arc-second or better, what more could I want out of my optics? As it turned out, I was able to distinguish the line pairs at only 0.81 arc-second --which worked out to "32" (line pairs per inch) on the target --the diffraction limit. Although line pairs are easier to see than close binaries or low contrast planetary details, I can have confidence in this repeatable, "case closed" test and I'm happy with the result. (Do this test in the early morning hours, before sidewalks and pavement starts heating up.)
* The "hardware test": There's also the possibility of using power pole hardware, but I found little by way of bolts with threads or other repetitive shapes --save for transformer insulators. Those insulators are too big for "neighborhood" use, but might be great if you have some distant poles and transformers to examine.
* --And then there's the flattering, feel-good, "Ronchi eyepiece" test, which is a paraboloidal null test (you see straight lines). It's not very sensitive, but will reveal gross problems. It consists of a 35mm film can with a hole in the cap's center --covered with (say) a small piece of "150 line" (75 line pair) Ronchi grating. If your mirror is at least a fair paraboloid, you see straight lines when looking at the light of a star --spread out across your mirror. If that star is other than Polaris, turn the Ronchi grating to align with the star's drift to make the lines stand still. Otherwise, let the lines drift across your image of the mirror and watch to see if they bend and wiggle --meaning your telescope mirror is a can of worms :-) since this is not a very sensitive test. (But perhaps it's sensitive enough.)
I'll be trying this test using a piece of positioning film strip (150 LP/inch) from an old HP printer. and so will another member of our e-group: Jerry Oltion (Astronomer's Workbench editor for Sky & Telescope) --who has a much better observing location.
* Update: Jerry did so, carefully checking with 4 telescopes --the mirrors of which he is very familiar with. Using 150 LP/inch is, as expected, more sensitive (than half that line count) --perhaps good enough to prove that a mirror is within 1/4 wave of true, but it's not as prescriptively sensitive as a proper (in and out of focus), traditional "star test". My own tests here, under a sky with rotten seeing, exhibited straight lines across my 6 inch mirror, so two things: The Ronchi eyepiece test seems to work when the seeing won't support a regular star test --and: this test (like the others cited on this page) agree that I have a "good enough" mirror.
In trying to speed up the process of grinding and polishing, I became persuaded that the rapid reversal of the mirror's direction (MOT), when being normally pushed and pulled by gripping the mirror's top edges --and especially when being moved by means of a traditional "handle", results in a torque arm (against the inertia of the mirror's mass), which tends to tip the mirror's leading edge down into the pitch lap --hypothetically resulting in a TDE. Only after building a "cradle" to minimize that effect, I found this problem well described on page #344, and then about the same cradle idea on page #370 of Amateur Telescope Making, Book #1, but manifested as the "steering wheel dingbat". It was invented (circa 1930s?) by one J.V. McAdam.
** Although I no longer think that inertial tipping is much of a factor, a cradle also keeps the heat of my fingers and palms off the mirror, vastly improves my view of the lap at work, of my stroke limits and of the periodic rotation of the marked mirror.
* Eventually, I realized that hand heat is very likely a big factor in pitch hardness for small mirrors, and might well be why so many 6 inch mirror efforts go south. Obviously, a proportionately larger (say a) 10 inch mirror has 4.6 times as much mass to heat up, and 2.8 times the radiation area to shed heat.
Those 3 top Teflon pads applied even downward pressure on the mirror (or the option of no additional pressure) and the handles provide alternative positions for old arthritic hands. Unseen is a pad moderated thumb screw on the far side. Also: the right handle can be turned to adjust the clearance of the right side bumper pad. The other two pads are fixed.
Here's a further development---
Aside from being spoiled by the cradle's advantages, I've come to realize how different are my efforts with a polishing cradle, compared to the hands-on work we read about in our literature and ATM Web sites. When using such a cradle:
* The mirror is significantly and uniformly colder, with little or no temperature gradient, mirror back to face. How much the mirror's temperature gets raised by hands-on polishing is, of course, an individualized and variable factor that makes advice between workers iffy.
* The lap tiles also run at a significantly lower temperature (8 degrees lower for me^) --and pitch viscosity is very sensitive to temperature.
^** I borrowed our son-in-law's optical thermometer, which I first checked out for accuracy and consistency with different materials (including pitch), finding it to be a good instrument.
^* After 30 minutes of hand "W" (1 inch stroke) polishing on a 5-1/8" lap --mediated only by surgical gloves for non-slip friction, I measured (a black tape target on) the mirror back at 82 degrees, (a cerium oxide slick on) the mirror face at 79 degrees, and the tile faces at 78 degrees. Room temperature was 70.2 degrees.
^* Earlier, after polishing a normal W stroke with the cradle on a full size lap for 15 minutes, I measured no more than +/- 1 degree differences across the lap face (I was expecting a significant center to edge difference), and those temps were within 2 degrees of room temperature. Even with a bit of demon polishing, the difference between a tile in good contact and one in poor contact was about one degree. (I had to work quickly to get near instantaneous readings, but I also did second and third readings after waiting 30 and then 90 seconds for any deeper tile heat to reach the facet faces.)
* I've read, and it seems intuitive, that there will always be a bit of TDE --that the simplest cure is to mask it off --maybe with flat black paint, but I'd use an O-ring.
* My cradle polishing did not go well. Mel suggested I go to softer pitch and Jerry suggested going to mini or micro facets --which effectively makes the pitch softer (aside from avoiding possible stroke length resonance issues). I ended up doing both, but first I changed just the best candidate: pitch hardness --and in view of the fact that my polishing lap and mirror temperatures remain near ambient. I made another, standard, one inch square tiled lap with softer (166 minute plunge test) pitch --and got much better results.
At the 59th Hour (9/15/2015):
Ah do declare: neber did see such a hellatious "hill" --!-- over half an inch longer in radius of curvature.
* After seeing that hill, I took notice of a crack across the central most lap tile. It must have settled lower than the others (pushed further down by the "bakers parchment paper" I use) --and I failed to notice the lack of polishing action there. Next I pressed at length to equalize the tiles --plus I raised the shop temperature to 70+ degrees, which made my (then) 418 minute test pitch nearly as soft as Gulgolz-64 247 minute pitch.
* ATM mirror work is not yet a science, it refuses to be so constrained (at least for me), the advice we get is all over the map --and "the map is not the territory" (Alfred Korzybski).
11/03/2015: Having gone to softer pitch (166 minute test), I used short (1/6th to 1/4th) "W" stroking to address the TDE. That "raised" the center, and while it reliably brought a slightly slightly flatter curve outward from middle to edge with harder pitch (to either match or to over-run a TDE), the advance of that corrective curve stalled out with the new 166 minute pitch.
It was back to using the 1/3rd "W" stroke for smoothing. Then I had to use an all-out, classical, 4/5 (or so) "parabolizing W stroke" to get even a hint of producing a parabolic curve. A pleasant and unexpected benefit of this stroke was that it appeared to keep a TDE at bay.
Here's a dark and grainy actual Foucault-gram (made with a 30 to 40 micron vertical slit)--
My mirror seemed to be getting smoother, but I was concerned about over-correcting --by continuing with a 4/5th "W" stroke, 1 minute rotations, minimum pressure, and red rouge. -----Hmnnnnn
Again: my polishing was not going well. Mel suggested I go to softer pitch and Jerry suggested going to mini or micro facets --which effectively makes the pitch softer (aside from avoiding possible stroke length resonance issues). I ended up doing both, adjusting the pitch hardness first. The new, standard, one inch square tiled lap with softer pitch gave much better results. By impressing micro-facets with window screen, I got much better "drag".
* Since the Ronchigrams at left are outside of the RC focus, the mirror, overall, is slightly "oblate".
* My short "W" stroking nearly eliminated the 1/4 inch or so of "turned down edge" I'd been looking at for days --presumably, by adjusting the rest of the mirror to match. Perhaps you can see a vestige of it by clicking on and enlarging the first image.
* While that slight kink near the ends of the Ronchi lines might escape casual notice, the reason for it is clearly shown in the 3rd image, when the Ronchi grating is nudged into position to act like a bright Foucault test.
* I find this 3rd image difficult to visualize. My mind's eye wants to imagine a light source to the left. However: once you "get it" with imagined light from the right, it snaps in and that central area becomes a "hill" --a big smooth hill, sitting in the middle of a spherical looking dish.
* It's not an actual hill, of course, but an ever-so-slightly flatter central area with a slightly longer (radius of curvature) focus. In the 4th image, the Ronchi apparatus has been gently nudged a wee-bitty further from the mirror in order to find that longer focus --and Lo: it "blinked", indicating that it was fairly spherical. (The surrounding area could be blinked as well, at a slightly shorter focus.)
**At 75.5 hours: At 74 hours I'd proceeded on the gnotion that my (MOT) 1/6th to 1/4th "W" stroking was slowly marching a flatter spherical curve outward, to over-run the existing, peripheral spherical curve --taking out any vestigial TDE in the process.
Nope, it didn't happen. The "hill" area got weaker, then stronger --but it stayed put. I next used a 1/3rd W stroke --which once got rid of a choppy surface and produced a nice smooth sphere (but left some TDE). This time I got a more pronounced hill (again: a central shallow area, actually).
* My next step was to tear into that stubborn hill with a 4/5th W stroke (which always gets results) for a solid half hour, with medium pressure, rouge, a cradle, and the new 166 minute pitch lap. Although apprehensive that I'd end up with a badly over-corrected surface, all I did was remove that pronounced "hill" and produce a perfectly spherical surface (and just a tad of TDE).
~ Due (surely) to my polishing room starting out colder than normal --plus my long strokes, a few flakes of pitch snapped off and smeared up the mirror. It took 3 cleaning sessions with lighter fluid to get it all off. This has happened before with harder pitch. I'm glad to report that it doesn't seem to mess up the mirror's figure.
* I roughly measured a center to periphery RC difference something like 0.035" for this 6" x 50.25" (f/8.38) mirror. I tried measuring that difference again with masks, but ended up returning to rocking a Ronchi "blinking" bar back and forth.
* The "star test": We had fair sky with stars (also with cold, wet and dog poop). I ran the power as high as 196x on a bright star (Cygnus Alpha/Deneb at mag-1.3), since Polaris seemed too dark for my unsilvered 6" mirror --but golly: the same(?) mess to either side of prime focus didn't tell me much.
Hopefully, this "blink tested" tip-of-the-hat to parabolizing is gud-enuf for a 6" x f/8.4 telescope.
* The next order of business was to religiously clean up the mirror. As mentioned above, I ended up having to grind off crusted-on cerium oxide from the beveled edge, using a carborundum stone. After thoroughly washing away the grit from that, I washed the mirror 3 times with Fells-Naphtha soap (using a stiff little brush on the back and sides, face resting on a clean, soft, terry cloth towel) while wearing surgical gloves. Next I washed it with alcohol, followed with a good rinse, a finish rinse with distilled water, and blot the mirror dry with clean, soft cotton towels.
* The second issue: how best to pack and ship my precious mirror? (Answer: ask the coating service how they want you to package and ship it --so they don't blame-shed on you for any mishaps.)
The packaging not only has to cushion the mirror from blows, but the cushioning/bracing must not rub against the front of the mirror in transit --to or from the coating service.
* One solution --and what seems to be the generally preferred solution, is to wrap and tape up the mirror tightly in packing tissue (but maybe with a piece of a soft cotton dish towel or tee-shirt next to the mirror?), such that if the mirror moves, the wrapping moves with it --no mirror abrasion --then pack the wrapped mirror in styrofoam or typical plastic cellular foam sheets --cut to fit. (See the instructions on that at:
(scroll down to their instruction photos)
> www.opticwavelabs.com/products.php (click on "Customer Education" in the drop-down menu).
In Texereau's book, he suggests using four wooden wedges, against which rests the fragile edge of the mirror's face. Should the package take a hard blow, that would magnify and concentrate its force upon the part of the mirror most vulnerable to chipping. However, Texereau wrote this before protective coatings were common, and a fresh coat of aluminum takes weeks to skin over its own protective coating of aluminum oxide.
* Paul Zacharias, owner of "Spectrum Coatings" (at the time: $50 for a standard coat with a silicon oxide overcoat), suggested packaging similar to this:
* That white stuff (from the UPS Store) is messy. Plastic cells break off and blow around. Spectrum Coatings suggests using some "blue or green foam from Home Depot or Lowe's", which must be available from other building materials stores.
~ I asked Majestic Optical Coatings for their suggestions as to shipping, packaging and costs (none of which could I find at their web site --which might be due to my fault/connections) and their Jeff Decker responded, suggesting that we wrap the mirror in any suitable material (perhaps bubble wrap), snugly pack it with more material into a smallish box, seal it, then pack that box into a larger box --such that if there's to be any slipping and sliding, it will be between the boxes and packing. (Jeff also assured me that their coating process leaves no support or "fixture/rail" marks on the coated surface.)
* Here's my mirror, back from OWL --and it was less than a week for them to turn it around. The coating looks perfect and that perforated center spot is accurately placed. Too bad about the 3 support indents.
The mirror was supported upside down in the coating chamber by 3 round ears, which left those 3 bare spots, one of which intrudes about 3/16 inch, so it's not fully covered by the 1/8" trim ring I'll be using --per:
That blue thing is the trim ring --sort of a square cross-section, 1/8th inch O-ring that I had laying around, which really wants to be a black, round cross-section, O-ring. It's cut a tad long, so as to hold itself "roundly" in place. At 50.25 inches of focal length, my Dogson then becomes an f/8.74.
What I ended up using for the collar was a simple strip of black poster board paper, onto which I sprayed two coats of artist's varnish. Then I Scotch taped it snugly to itself --such that I could still pull it up to the height of the O-ring, at which point I taped it to the bottom/edge of the mirror in 4 places. (I'll hang onto that embroidery hoop for bench testing future 6 inch mirrors with trial peripheral masks.)
The trim ring's purposes are to cover the ragged beveled/champhered edge, perhaps a wee bit of turned down edge (most mirrors have some), and to present a smoothly defined aperture to the incoming light --per Gary M. Bloom's web pages titled: "The Perfect Telescope" (which, unfortunately, is no longer posted, but you can find it via the "Way Back" web site).
** Ohdear! The above storage idea went bad. My telescope had gotten only a dozen trips outside over a period of months --and just look at the crud on its face:
I can only guess at what went wrong. Possibly, the plastic box, that coffee can cover, or the inch of cellular plastic the mirror's back is resting on --out-gassed platicisor. So here's what I did next:
Beneath the mirror is a folded sheet of plain stationary paper --on top of two sheets of ordinary paper toweling, and then the same one inch of cellular foam as before. --Will see how this arrangement works out.
* My mirror cleaned up nicely, by the way, and I reactivated the desiccant packets. (335 deg F for 30 minutes.) (Hmmnnn: better go for an hour --to be sure, and turn them at 30 minutes.)
If done right, the diagonal shouldn't be coated alongside the primary (as can be done with standard coating). Due to it being used at a 45 degree angle, one would think the coatings need to be 70.7% as thick.
~ There's an interesting article discussing such
considerations posted at Oldham Optical:
> http://www.oldhamoptical.com (among several other subjects, like diagonal quality). It points out that the Hubble Telescope mirror was not given enhanced coatings, and that major observatories often use bare aluminum (which takes about 6 weeks to skin over with a transparent layer of protective aluminum oxide). When it comes time to chemically strip and recoat the mirror, a protective overcoat (let alone a stack of undocumented enhanced coatings) might make the process spotty and less predictable --perhaps even chewing up some of the glass surface as well. (Thanks for these articles, Oldham Optical!)
~ I gather that nearly all coating services routinely apply a protective overcoat, usually of silicon dioxide or magnesium fluoride. Again: the overcoat presents no special thickness problem.
~ The price for enhanced coatings on a 24 inch mirror was recently (December, 2017) quoted at about $600. At 25 inches, it (must be that it) takes "the big vacuum chamber", since the price jumped to $1375. The regular price for a "standard coating" on my 6 inch mirror (two years ago) was under $100 --which included return shipping, packing and $10 extra for "center spotting" (applying a little triangular target to help with collimation). If one is willing to wait an extra week, OWL will also throw in a free precision mirror figure evaluation.
~ My practice with the Dogson (here near the salty air coast) is to always take out the primary mirror at the end of an observation session (it has a door and a single thumbscrew for that purpose), then store it (face down) with desiccant --for which its peripheral "trim ring" and collar worked out nicely. The "SnapWare" brand 18 cup (size) sealable plastic storage keeper is just right for my 6 inch mirror --and available in the kitchenware department of a store near you.
What (if any) optical tube assembly ("OTA") finishes^ should one apply?
* The inside, of course, has to be flat black (leaving threads and bearing surfaces bare) --but what paint to use?
~ I don't like breathing the stuff --or "over-spraying", so I went for a good bristle brush and a can of real paint.
~ When I tap a spit wet finger on the OTA's tempered HDF, it soaks right in --so I bought oil base flat black (assuming it's better for oil/solvent to soak in, than the water of "latex" paint --right?).
~ If you turn the rough side of your HDF panels inward, that self-adhesive black "flocking material" (from ScopeStuff) won't stick --but the rough side painted flat black seems pretty good as is.
* Most everyone tells me that the best protective coating is urethane spar varnish ("Helmsman" brand), and my HDF needed 3 coats before it at least looked varnished. This way the outside of the 'scope looks like what it is: tempered HDF and wood. My inside and outside coverage won't be 100%, but some should be better than none. (As soon as I'd finished doing that, my 2nd oldest friend assured me that I should have went with "Deft" exterior varnish, since a coat of that lasted 20 years on the propeller of his air boat.) (Oh well.)
In the next photo you see what I bought.
* I opened up the varnish, stirred (didn't seem to need stirring), and poured it into a quart Mason jar --since I don't like or trust resealing metal cans. Each time I decant, I wipe the jar's lip and the lid/ring areas dry with a paper towel, so the lid will come off easily next time.
* I decant an inch or so into another Mason jar for use, the lip and lid of which also gets a good wipe when I'm done. While that minimizes air exposure and prolongs the life of the varnish, my experience is that varnish, once opened, will skin over in a few days anyway. Just cut/pull it off and use what's left.
* That 3rd jar has an inch of mineral spirits for cleaning the brush. First brush out as much varnish (or paint) as you can on one of your crummy (and getting crummier) shop walls, clean some more with a paper towel, lightly dip the brush in mineral spirits (don't swish or stir --keep your spirits clean), brush it out some more, paper towel again --and your brush is ready for another session.
* That white thing is chalk, for filling the many Phillips head screw divots --so as not to fill them with varnish, but I've since decided that a sharpened "Crayola" (brand) crayon works better.
* That ugly round thing in the OTA is where I plugged a hole. (Some of my HDF was salvaged from another scope project.)
^ As you can probably tell, I know very little about painting and otherwise "finishing" wood --except that you shouldn't apply water base latex/acrylic type paints over oil base type paints and finishes --and it also seems a bad idea over oil based primers or sealers. I have my doubts as to whether paints, lacquers, varnishes and oils actually seal wood/HDF, but I think they at least slow the ingress and egress of moisture.
--is fairly simple for a long focus mirror. Whereas with an f/4 telescope you have a 1.4mm diameter "sweet spot" to center up (meaning: at best focus, coma doesn't detract from a perfect mirror); an f/8's sweet spot is 11mm wide, and an f/10's is a whopping 22mm.
I can do a fair collimation during construction without the actual primary, by using a dummy 6 inch mirror --
Actually: that section of top panel is a door that slides off for installing & removing the mirror, which is held captive with a thumb screw. This seems a safest way to go for the first approach to collimation, instead of putting the real primary at risk of a dropped tool or fastener.
* I made a collimator eyepiece as well and it seems to work:
Using a junked out Barlow, I masked the field end with red plastic--
1) Center the (spotted dummy) primary with a ruler, vertically and horizontally.
2) Verify that the ocular focuser is centered. (It does focus a wee bit, due to the thread.)
3) Verify that the curved strut makes right angles. and that the secondary mount dowel piece is drilled straight.
4) Center the strut's top end --vertically and horizontally, with a ruler.
5) Install the collimator and make sure it's snug --no slop.
6) Snug up all the adjustment screws, while still being able to turn and position stuff.
7) While aiming and re-aiming the secondary --so as to keep the collimator's center point on the primary's center spot, introduce and remove (as necessary) the white poster board sheet and contrasting color backgrounds behind the diagonal --in order to see and distinguish the secondary (just the secondary) --and to concentrically center it within the collimator's mask (custom made for this purpose).
Depending on how well you planned, cut out and drilled out this scope, you might have to enlarge the strut base block's location holes in the OTA bottom panel, then add fender washers to span the holes (--like I did.)
8) That having been accomplished and the various screws tightened down (without disturbing your collimation), it's time to adjust the primary's 3 locating screws. (There are no "locking" screws nor mirror retaining clips in the Dogson design, just 3 rather tight (nylon T-nutted) thumb screws tipped with soft electronics solder.)
~ In the photo is a length of green #10 or #12 gauge electrical wire. I use it to reach down and tug on the (dummy) mirror a little bit to see which screw I want to turn which way. This helps a lot.
When the reflected image of the diagonal is centered up, you're almost done.
9) Finally install the actual primary mirror, bring it to infinity
focus, exchange the eyepiece for the collimator, and tweak the primary's
3 locator screws as might be necessary.
For a more informed and universal guide to collimation, see (and save) Don Peckman's page at:
* At first I hoped to discover a stroke pattern --or a pattern of strokes, which would surely lead to a fair spherical figure --as one would logically expect, and where the "proper application" of the venerable "W stroke" is often said to lead.
* My fall-back goal was to assemble a small and simple armory of strokes which, used one to mitigate the other, will definitely and prescriptively produce the same result. (I've only selected, defined and restricted these strokes --all of which have long been variously practiced and advocated by others.)
* Unfortunately, my methods (unless found to be more widely applicable by others) will probably have only limited application --to the making of a traditional, shallow (f/8 or so), small mirror, from a thick blank, by means of a "cradle" (at least in the polishing stages), using a lap made of discrete tiles and the hardest pitch which is practical to use (about the same as Gulgolz-64 at 68 degrees Fahrenheit) and keeping the mirror on top ("MOT").
* Normal stroke speed:
~Slow: the mirror averages about 4 inches per second (varying sinusoidally)
~Normal: 8 inches per second
* Normal rotations:
~The mirror is turned 45 to 90 degrees CCW each minute (maybe more --to distribute slurry).
~My stance moves about 30 degrees CCW around the tool at 5 minutes (avoiding
alignment with the lap channels) through 150 degrees, completing 180 with my
return to the starting position (that drain mat work surface cut-out).
~ Each session (30 minutes) the tool/lap is turned 90 degrees CW.
* One minute R&Rs (important for short and parabolizing sessions):
~The mirror is turned 45 to 90 degrees CCW each minute.
~My stance moves about 30 degrees CCW around the tool ALSO at 1 minute
intervals (avoiding alignment with the lap channels) through 150 degrees,
completing 180 with my return to the starting position.
* Stroking pressure, rudely defined:
~Minimal: only the weight of the mirror and the cradle (about 3 pounds, 2 oz/squinch).
~Medium: the weight of hands and forearms --5 pounds forward, 4 back (5oz/sqinch).
~Normal: Conscious pressure, which adds about 5 pounds for me (7.5oz/squinch).
~Heavy: Adds another 5 pounds for me (3 + 15 pounds (10 oz/squinch).
** The following strokes were mostly explored with much harder pitch than what I've ended up using. I'm gradually revising my comments here to reflect results with the current 166 minute (test) pitch lap.
* A 1/6 to 1/4 "I stroke" (ie: straight, center
over center), applied with normal pressure:
~Rapidly eliminates a TDE with hard pitch.
~Raises a central hill (even a hill within a hill with hard pitch).
~Results in a very choppy surface if prolonged (ragged Ronchi lines).
* A 1/3rd and greater "I stroke", applied with
minimal pressure and 1 minute R&Rs:
~Reduction of TDE and any central hill is gradual.
~Tends to result in an oblate spheroid.
~Tends to leave the surface mildly choppy.
* A 1/3 wide by 1/4 high "Tangential stroke",
applied with medium to normal pressure toward one side of the tool:
>This stroke is carried over the edge until "pinch" resistance/drag is just felt (or just short of that).
>The mirror is rotated CCW every 60 seconds.
>One's stance is moved 30 degrees CCW every minute.
>Tool/lap rotation is only needed if stance rotation is less than 180 degrees.
~This significantly reduces a central hill in about 15 minutes (Pyrex) --and
~Appears to reduce a TDE as well. (With more rotations, Jerry reduces a TDE this way,
but with TOT.)
~Produces an ever more choppy surface, though not nearly as bad as a short "I stroke".
* A near 1/2 wide by 1/3 high "Tangential W stroke",
applied with minimal or medium pressure:
>This stroke is also carried over the edge until "pinch" resistance/drag is just felt, then returned
>The mirror is rotated CCW and one's stance is moved 30 degrees CCW every minute.
>Tool/lap rotation is only needed if stance rotation is less than 180 degrees.
~So far, this produces a spherical, TDE-free figure with roughness if medium pressure is used.
I was hoping for smooth results with minimal pressure, but the roughness persisted through an extra hour of polishing. Also: the mirror remained slightly oblate, so I had to aggressively attack the center of the mirror (which resulted in a rough surface again).
** A 1/6 to 1/4 "W stroke", applied with medium
(or minimum) pressure and normal rotations (or one minute R&Rs for
finish and figuring stage):
~ Smoothes the mirror, with less tendency toward a TDE.
~ Tends to produce a "hill" (a flatter central spot with a longer RC).
** The standard, 1/3 "W stroke", applied with
medium pressure and normal rotations:
~ Smoothes a mirror, but with a tendency for TDE to creep in.
~ I saw little reduction in the radius of curvature during many hours of polishing out #500 pits^ with this stroke.
~ Working with a cradle and a 166 minute (test) pitch lap, this stroke showed no tendency to remove a "hill", once it was established with a long period of 1/6th to 1/4th W stroking (used to get after a TDE).
** The classical, 4/5 (wide and high), parabolizing W stroke --applied with medium pressure. This is the only stroke (for me, and using the cradle) which turns out a paraboloidal surface. My initial impression is that medium pressure, and perhaps even minimal pressure --results in a small, but unacceptable degree of surface roughness. There's posted talk on the Internet about needing to actually lift off some of the mirror's weight (MOT) in order to get the best results. I hope that's seldom necessary --it wasn't for me, using Pyrex glass.
~ My most recent experience with this stroke (30 minute session using rouge, a cradle, medium pressure and 166 minute pitch) was that it removed a pronounced "hill" (large, central shallow area) and produced a perfectly spherical surface. My polishing room was a tad cold, however, so will have at it again --warmed up.
^ Those #500 pits were deliberate --and the last fine grinding stage, since scratches are most common during grinding with "micron grits". Also: old timers didn't have micron grade grits, so I wanted to see how tough it was to polish them out --and with Pyrex to boot.
(The remainder of this page is under edit. Find some poorly organized Dogson-1 information mixed in.)
The Dogson Telescope
Some of the following appeared in the Fall 2008 Issue #59 of Amateur Astronomy magazine.
(Last worked on: 6/3/2016, but lotsa older stuff here yet.)
This telescope, without my knowing how it would end up, went through design changes for 59 years --since I was 14 years old. I think it's finally matured into a satisfactory 'scope. The only things which haven't changed since my initial equatorial mount effort is that it remains a long focus Newtonian instrument, embodied in a closed tube (the "OTA"/optical tube assembly) --and a rectangular one these past 20-odd years. The accompanying photos reveal extra holes and scars which attest to mistakes, redesigns and modifications --in the Dogson's long struggle to be born.
Now that the design has finally stabilized, best I'd recognize/admit that this is only the second "roll-out" in 13 years --by calling it the "DHS-2A1" (and, as ever: just "Dog-2").
The "DHS" part recognizes and appreciates Mr. Holcomb's and Mr. Sellers'
priority (see below).
The most pleasant way to personally take in the night sky is probably with a good pair of binoculars, either while leaning back in a lounge chair (using a counter-balanced binocular mount) or comfortably seated at a sturdy table (your short tripod mounted binoculars fitted with angled oculars, your charts and etc. at hand). Next best would be some kind of a (light concrete) table mounted telescope, perhaps in the style of a Questar. 3rd best (hopefully) will be one of my forthcoming "Dogson" telescopes --perhaps even a "Dogstar" version.
* In general, I suggest you'll be happier looking at familiar objects (perhaps some variables that you follow for the AAVSO) and using low to medium magnification --not much more than 60 power, since that's about as much as average "seeing" supports with excellent contrast, visual resolution, a nice field of view, a comfortable exit pupil and eye relief.
For observing with a telescope, one is tempted to go for a 45 degree "erecting prism", such that sky charts, aiming and moving the scope are intuitive --rather like a binoculars experience (but without the sore neck). We're cautioned that such an "Amici" prism "won't work with DOBs and other Newtonians" --for lack of enough inward focus travel, but that it should work with most refractors and Cassegrain or Maksutov-Cassegrain telescopes.
A William Optics Amici prism should also work with a Dogson-2 telescope, due to its traveling cell sled, and provided it has an adequate secondary mirror. However, at this point I only intend to use an Amici (if at all) for terrestrial observing, and future Dogsons might forgo this option in favor of more focal length (f/8.66+). (I've read that even the best Amici prism will add a horizontal spike to bright stars.)
* I also suggest that you'll be least happy when trying to do imaging (photography), unless you keep your approach really simple and fast (like: a camera aimed at the sky and: *click*). I'm really torn on that issue, since I do much better actual "astronomy" by analyzing photos/astrographs --than I do in "real time" at the eyepiece. (For more on that approach, see my (and others') "barn door drive" experiences). Never-the-less, I fantasize about doing direct astrometry, photometry and sketches --in real time --at the eyepiece. (Ha!)
* In my humble opinion: if you live in or near a city, a 6 inch f/8-10 (however configured) might be all the telescope you want or need^, and something much smaller (like binoculars) is what you'll likely use the most --provided that you're familiar with and can navigate with your instrument, you know what objects/fields it works well with, and you have a well configured observation setup/setting --including a well behaved telescope or mounted binoculars, a nice chair, an ample work table, notes, adequate illumination, warmth and charts near to hand.
* Yes: "adequate illumination", meaning white light --perhaps as a headband mounted LED lamp. Adequate "red light" is no better than white at not knocking down dark adaptation (IMO), and fuzzy "night vision" is only good for logging and bragging on having noticed faint "DSOs". I want to study, count, measure and describe stuff.
* The nebulas in Orion, of course, are reason enough to purchase or build a larger 'scope. Our Coulter 13" really did a job on that and other such objects, but I think you need reasonably dark skies --and it is not fun (IMHO) to drive to some distant spot, set up, fight sleep, and then drive home half dead (unless you go through the greater inconvenience and grunge of turning it into a fursluginer camping trip).
^ Most observers (especially modern ATMs and ATBs) disagree with me, saying that bigger is always better (aside from any portability and set up inconvenience). My opinion is influenced by living very near the Pacific coast under an urban, often hazy/cloudy, light polluted sky. I jointly own a good 13" reflector with my friend Chuck. I've wheeled it out and tested full aperture, then with a 5" (oval off-center) aperture mask, looking nearly straight up at something like M-13, --and seeing little advantage in the full aperture.
I'm also biased in favor of the people-compatible dimensions, weight and the relative simplicity of "eyeball" collimation possible with a 6"x f/8 to f/9 instrument.
However it was to come out, I wanted my telescope to be simple, stable, humbly practical, and easy for others to build, but perhaps it's mainly "easy" in that one doesn't have to worry about weight distribution.
I'd been inspired by the obvious stability of F. J. Sellers' 1920s design, which our regional discussion and construction group: "The Toledo Scope Works", took a look at some time ago. (David, Chuck, Jerry, Tom, and Ted of the Scope Works were the reason and the resources by which I was able to resume my old passion for astronomy and telescope making. They've been supplying me with ideas, software, encouragement, and components for years. David made the mirror and Chuck made the Crayford focuser for Dogson-1. I've yet another 6" mirror on hand from Jerry --"Dog-3"?)
** A newer fellow at the Skopewerke, Mel Bartels (who goes way way back with the Oregon ATM scene, plus he has a great presence among our literature and web sites --including his own) --alerted me that this type of mounting goes back much further than Sellers. It was invented by the first manufacturer of telescopes in the United States: Amasa Holcomb, who you can read about at:
Holcomb was a self-taught American Renaissance man who lived a long life and made great use of his time. He started making telescopes later in life, purely for the love of doing so, but ended up filling many orders from private and institutional clients. There are a number of observatories which still bear his name.
Sellers, a professional astronomer, considered alt-az
mounts superior to equatorial for visual use. He discarded
several designs before settling on this one. (Yes: he carried
along that little board in back for smooth azimuth guidance.)
My 'scope became a "Dogson" (with a humble tip of the hat to John Dobson's fine telescopes, spirituality and philosophies) after I started trying to figure out how to add a foreleg or two:
(04) Modeling design variants and trying
to get a "feel" for how it would handle.
To make a long trial and error story short, my first Dogson ended up looking like this:
That flimsy stool is only for holding my clipboard
and whatever's attached: stop watch, a good clock,
microsette recorder and a headband light for seeing
stuff at night. My poses were to stand, stoop, or go down
on a knee. The short leg was used under 30 degrees of altitude
(ie: not often). Those rubber cane tips come off to be replaced by --
--a cut down plastic caster which swivels to catch the dirt and allows the pole to be smoothly turned.
I need a lot of light to see its little display, but this affordable --
--Zire 21 and the Planetarium program supply the information I need
to aim my scope. Each of those stars was at the center of one of my
"Observation Areas" --which I watched and photographed for years
in hopes of at least getting to know a few small parts of the sky.
The Dogsonian design eliminates bearings, counterweights, weight and thinking^.
^Well, some. It takes a bit of thinking to relate sky program or planisphere co-ordinates to one's plan for the evening and night. What I'm trying to say is that it takes a lot less thinking to turn out a Dogson --a 'scope which doesn't need balancing, and which has cell sled focusing with a good range of adjustment. It's easier for an amateur astronomer to build and to transport this one-piece design (plus kneeling pad, which stows on top of Dog-1).
Yes: that's an ordinary hardware store or paint shop "pole sander block" and extension tube (30 to 60 inch) --which attaches with an Acme thread --which, when turned, gives slow-mo on the altitude. Dogson-2 uses a much longer thread, but operates the same way.
There are cheap through professional extension poles to choose from. That aluminum pole was easy to
cut down and it adjusts in steps. Dog-2 uses a custom made pole--
--and adds limits to the pole's lateral
excursions, such that the 'scope can't easily fall over.
This might be altogether too much work. Next time I'll try to simplify the wooden extension pole
fore-leg, then screw and lock the top of it into the U-joint of the commercial sanding block I used
for Dog-1, but I'll still need to somehow brake and limit the leg's lean-over travel.
An early image at left (sans leg excursion limiting) and a recent image of the leg snap set.
* Originally, the foreleg was going to be articulated (folding in half by steps), thus the "dog" name.
* With the scope set up for stool sitting eyeball height (at 53 degrees altitude), I found that I could turn the thread and track in altitude from 49 to 62 degrees --and lean the scope in azimuth through 18 degrees.
You might ask: "where does one attach the setting circles?" Well the "DDNU" goes right here:
This early Dobson-Dogson Nav Unit attaches with
a single wing nut. The most recent version (2016)
has 4 feet and attaches with a single brass flat-head screw,
and is meant to stay atop the telescope's OTA.
The DDNU's altitude readings are not dependent upon
the scope being leveled --until you're near the zenith,
then I keep an eye on that extra level (below image).
With this early DDNU version, you start by
getting the current alt-az of Polaris (or maybe a radio
tower), sight Polaris with that disk, calibrate the circles
to read true, then go after your quarries --back-sighting
on Polaris for the azimuth. (Arghhh: I went to a magnetic compass.)
(10) That second level is for navigating
at higher altitudes and/or finding somewhat level ground.
And finally: a magnetic compass --along with a swing-out glow finder (seen here on Dog-2).
* A helpful addition to the problem of "finding"/aiming has been a new 32mm Celestron Omni eyepiece --affordable and as much field of view one can squeeze into a 1-1/4 inch focuser. Its light gathering is somewhat crippled by my 1-1/16 inch diagonal, so it might be 50% dimmer at the edge, but the 40 power 1.25 degree actual field is all there. My old 25mm Plossl (Surplus Shed "500" series) only passes a 0.84 degree field (75% illumination at the edge).
There's a little cover for the glow finder's lens, which otherwise dews up quickly and I've rounded the sharp edges. In use, the glow finder usually ends up above my head when deployed and I've yet to so much as bump it. This configuration seems natural to use --just turn my head and look.
During a recent session (the night of 6/4/2016 UTC), the spirit level altitude indication was within 1/3rd degree (and is usually quite accurate) but my compass was off by 8 degrees on one object (Jupiter). Nearly all the hardware in the vicinity of the compass is brass. Maybe I need to degauss what's not (mainly: the diagonal strut).
The sky-ball approach (using just the device on the left):
--requires a fairly heavy ball and seems hard to work with in the dark. (More)
* While the level based altitude indication is accurate enough, finding the azimuth with any of these devices has been difficult, so why not simply use the imprecision of a magnetic compass (with a minimum of ferrous metal screws and whatnot in its vicinity)?
* Yes: something like a GPS, a Celestron "SkyScout", or a "Safari" ap'd i-whatever would probably work well, but I'm trying to use low-tech methods --that I understand, can make, and maybe improve upon.
The paint got replaced by "glop" film from "Scope Stuff",
which is 100x better. This is similar to a 1952 design by Stanley B.
Rowson, but also in the vein of "points at" type
finders made by my friend Chuck.
--and just below it in the above photo was a finder scope with a 6 degree field made from a binocular objective, a rectangle of front surface mirror, and a 20mm Plossl ocular with a home made copper wire "points at" reticule:
At left is the finder's Plossl with a #24g wire
pointer. At right is a 25mm Kellner with a straight-
across wire reticule, for timing the passage of stars and
extended details (which has since become an "X" of two wires).
With the 6 degree finder as an in-between step, it was pretty easy
to nail an object in the sky, but Dogson-2 has just a glow finder (with
the ability to swing it around to the other side --for guiding the scope
while a guest is observing). What made that work is a 32mm Plossl ocular
with a 1.25 degree field. It will be about one magnitude dim at the edge
(due to the one inch diagonal), but that's plenty good enough for bringing
the quarry to center.
* Extended comments 1: > While that copper tubing strut mount for the secondary painted nicely and presents smooth edges, I found it ever harder to bend and work with. I consequently went to a 3/16 inch steel strut, but this time presenting a curve to the optical path (to eliminate "spikes"). It's mounted from the bottom of the tube, so as to keep it away from the DDNU's magnetic compass over the top panel.
> Dogson-1's oversized square tube worked out quite well with room for convection vents, wiring (but no more battery stuff for Dog-2 and beyond), a mirror cell sled arrangement and a long threaded rod for moving the cell sled back and forth (when trying out cameras, different oculars and focusers). I used Dale A. Keller's "Newt" program to place and size the baffles. For Dog-2 I decided that I can get away with just 2 baffles. There are also a pair of top vents, so as to not force convection currents over the entry baffle and into the optical path.
> The top side of the second baffle below the secondary really catches the light and should be treated to something like a black velvet or a flocked black covering, but the stuff didn't stick to my HDF board. Consequently, all of the tube's interior relies upon the rough backside surface of HDF and a good coat of flat black oil based paint (brushed on).
> I originally decided upon an f/9+ telescope because a 6 inch mirror can be finished sphere and still be about 1/10th wave away of perfection. (A 6" f/8 sphere is about 1/8th of a wave from paraboloid.) However, when David made the mirror for Dogson-1 he troubled to fully correct it :-) Since giving this scope away (to become the "Buddsonian"), I've been given another 6" mirror plus I purchased 6" blanks for making Dog-2, which was also figured paraboloid. I've been persuaded that even at f/9, on a really good night, a spherical 6 inch mirror can noticeably fall short of one that's been figured. Also: the height of the eyepiece was too tall for kids and shorties at f/9, so the aim point became a more traditional 6" x f/8 (or so) scope.
* Another consideration is the "sweet spot", and how critical collimation becomes with a short focus scope. f/9+ is really to be preferred, but I think f/8-plus is still manageable with "eyeball collimation".
Comment 2: The plastic half-lens' optical
center is level with the pointer and top of the target block, but is cut
off 1/8th inch higher, such that the phosphorescent pointer's image (projected
at infinity onto the sky) easily clears your line-of-sight over the block.
The blunt point is about 1/3rd of a degree wide --as projected. Mine is
made from amazingly bright and long lasting glow film that I got as a sample.
It's otherwise to be had from ScopeStuff
as their "Glop" product. The next versions will probably use a "pointless
arrow". See here for the view through it, and
for a drawing and prior art.
One of the nicest features of Dogson-1 and 2 is having a back door for easy access to the
mirror --which gets removed after each session to a "SnapWare" sealed container with desiccant
(and a monitoring hygrometer). (The lesser door covers a hole through which a fan once injected outside air .)
**Click here** to see Dog-2's slide-off access panel.
The mirror cell sled is moved by turning the
threaded rod stock --seen on the floor of the tube. The
sled carries the rear-most baffle and positions the mirror
via the usual thumb screws in back (mirror is held against
them only by gravity), as it rests on two one inch dowels. The
dowels are held in place by screws which are about 1/8th inch
eccentric, such that the mirror can be centered in the tube by
turning the dowels. Spring clips locate the sled as it moves.
* Dogson-2 also has centering dowels, but a wooden threaded rod.
This view of the bottom of the scope and the
back of the mirror cell reveals that a wooden puck
is glued to the mirror's back with dabs of silicone
rubber. There's a 1/4-20 T-nut in the other side of
the center hole which accepts the thumb screw which
holds that canning jar cover in place--
--and thereby holds the mirror captive to the cell
(This cell-sled design isn't beefy enough.)
The secondary's copper tubing strut
mount at least looks nice and has a smooth
profile in the beam --
--and it took paint nicely, but I've gone to a doubly curved
3/16" steel strut for Dogson-2. Also: that universal
joint has been eliminated and it's just the knob now
(for turning the threaded rod which moves the
mirror cell-sled), and it got moved to the left corner.
Later on those pretty brass corners got replaced with sensible corner
baffles. In this photo the elbow light is shining light pollution down into the
tube, providing some background light for my non-illuminated crosshair
reticule --for when all that baffling worked too well and normally ample our city light
pollution and haze cover was insufficient.
I've officially designated the current Dogson project "DHS-2" in recognition of the resourceful guys, Holcomb and Sellers who originated this type of telescope. (Holcomb's was a diagonal-less Herschelian).
Dog-2's diagonal is supported by a positionable float
block and the whole of it gets involved for collimation.
I doubt that secondary vibration will be a problem, but
that gap between the nuts carries a dampening washer --
--and you might like to try this experiment to see
how effective a loose washer can be at stopping
vibrations. (Far better than a dangling loose chain).
Here's the unfinished interior of Dog-2 (only two baffles this time).
--and here's what you see:
For more about these tests, see here.
For more about my approach to making a small pitch lap (ala Jean Texereau), see here.
8/17/2015: A seemingly inevitable "hobby killer" which we all face --the more so as average amateur mirror sizes and shipping costs increase, is getting a finished mirror aluminized, or recoated. I've thought about this at length (and haven't we all) in search of alternatives, but (as yet) to no avail.
Cleaning the mirrors
Have on hand: (Blue text is for something like a Coulter cell and mirror unit.)
* A plastic shopping bag --large enough to cover the mirror's cell (if duct taped to mirror).
* A bottle of 70% rubbing alcohol, and pour out some into one of the bowls.
* A bottle of distilled water --and pour out a pint into one of the bowls.
* A quart jar of large pharmaceutical grade cotton balls
* A box of unscented facial tissue.
* A number of paper towel sheets (drying hands, spills, mirror cell, possibly for blotting, but not rubbing the mirror).
* Dawn dishwashing detergent.
* Two clean mixing bowls or kitchen graduates.
* New surgical/examination gloves (to be worn pretty much throughout).
* Good, bright lighting.
* Black electrical tape and 3M Scotch tape
0) Remove the diagonal/mount (if it needs cleaning --usually not), primary cell and mirror from the OTA. (Separate them --or don't --if it's all duct taped together.)
1) Lay out all your materials onto a nearby table that's covered with paper toweling.
1b) If the mirror is held in place with clips, This is a good time to make sure the clips aren't too tight.
2) Cut off the top of the shopping bag squarely (so as to eliminate the handles).
2b) Open and place the bag on a level surface. Lower the cell onto it (mirror up) and raise the bag up to the mirror's edge, pulling it around the cell (assuming it's duct taped to the mirror --or that you just don't want to remove the mirror from the cell) and snugly fitting the open end around the periphery of the mirror, using short pieces of "Scotch" tape to secure it, with the excess folded over and taped down to one side (about halfway up the mirror's edge).
3) Finish up by sealing the bag in place with one overlapped turn of black electrical tape. Take care not to run the tape too close to the aluminized upper edge/face of the mirror. (If the mirror is held with clips, be sure to dry any water that gets under them after the final rinse and blotting.
4) Use a laundry tub or a large enough sink, with a hand sprayer. Scrub and rinse it --spray head and faucet handles as well.
4b) Place a plush towel into the bottom of the tub such that it's folded over where the mirror will rest upon it. The idea is to somewhat over-hang the mirror's face or to lean it back a bit, allowing water to freely run off. Be sure the drain is not blocked. You might choose to simply hold a small mirror --like my Dogson's 6 incher.
5) Pour out a cup of alcohol into one bowl, mix a quart of water (ambient tempperature) and about a teaspoon of Dawn into the 2nd bowl. Pour a pint of distilled water into the 3rd bowl. Set out your cotton balls, wads of facial tissue and a handy waste receptacle. Have a few sheets of paper towels stacked and ready to grab..
6) Put on your rubber gloves and lower the cell-mirror assembly sideways into the tub, such that the mirror's surface is vertically on edge --resting on the folded over portion of the towel and securely leaned back, such that you can see it's surface with good bright lighting.
7) Play/spray ambient temperature water onto the face of the mirror, dissolving and pushing off what dirt it will remove. (Not very much, in my experience.)
* Some advise to "soak" off dirt and grime, but I don't think it's a good idea to prolong this or the other steps beyond a few minutes. I get the impression that the reflective coating can be a bit hygroscopic, especially if the overcoating is no longer intact. In any event, you should attempt to keep the mirror fully wet or fully dried.
8) Keeping the water running, the spray head deployed and handy, completely wet a large cotton ball in the Dawn detergent solution, then lightly wash the mirror with it, using a gentle, backward rolling stroke/swipe. Discard the ball before you completely turn it over-around and start in with another.
* It's critically important that you initially roll the ball in a direction opposite the ball's travel, such that removed dirt is immediately and continuously rolled up and away from contact with the mirror's surface. Subsequent cleaning might be done with one "side" of a cotton ball per single stroke. (I get 3 such "sides" per ball.)
9) Immediately rinse off the mirror with the spray head.
* Does it look clean already (or as clean looking as it's likely to get)? If not, repeat steps 8 and 9. Any grime or greasy "smear" should come off. Caked on deposits might take 3 soap sessions --and you'll probably have to gently swab (not scrub) to get the mirror clean. (You might read elsewhere about "using only the weight of the cotton ball" or some such. That's not likely to do the job.)
9b) Grab a fresh cotton ball, wet it well with 70% alcohol, and swab --which might get any remnant crud which answers to alcohol solvent.
9c) Do another wash with a Dawn soap solution soaked cotton ball. (Alcohol is otherwise very clingy and stubborn to dry directly, whereas distilled water beads up nicely and mostly runs off a very clean mirror.)
10) Completely wet a fresh cotton ball in distilled water and very lightly rinse the mirror with it. Do this several times with fresh balls.
11) At this point, and holding the mirror with one hand, pull off the wet glove from the other (with your teeth), then dry by blotting/pressing with wads of facial tissue, using your dry exposed hand. Quickly follow with fresh tissue wads.
* Spots of water can be tenacious. Designed to be absorbent facial tissue is essential. Gently wipe/brush to dry only as you have to --but be quick enough about it that droplets are not allowed to dry by evaporation.
11b) Rest the mirror on its back, remove the other glove, grab some paper towels and start blotting/wiping off the side and back of the mirror. When dry, rest it on its back and place a sheet of fresh stationary over its face.
12) A clean dry mirror accumulates static electricity and dust --like
a magnet draws filings. Don't fret. Simply use a small squeeze-bulb air
puffer (not your breath) to blast it off (which, of course, imparts more
static electricity). A camel's hair lens brush also leaves static
electricity --plus it tends to smear and sleek older mirror crud.
(In the bad old days we could buy a radioactive darkroom photographer's
negative brush, which would discharge the electricity. I believe
they used polonium --arghhhh.)
* The physical moves of dropping in and removing the primary (to its sealed and desiccated box) has gone swimmingly.
* Presumably, such an approach, or maybe an arrangement for placing
an air tight desiccated cover over the primary of a larger telescope, could
preserve a silver coating indefinitely. The Reverend Ellison (pages 102-103
of ATM-1) reported such results using lesser efforts. (The cost of silver
nitrate is quite high now, but so is the cost of shipping and coating a
larger mirror. However, a 6 incher ships and coats for well under $100.)
* As to mirrors which remain exposed: the advice in Sidgwick's Handbook, sections 7.2 and 7.4, runs opposite our common practice of only washing a mirror when it's become ungodly dirty. The recommendation is to preventively wash an aluminized mirror with detergent ("Dreft") --maybe every 2 months --especially if near the ocean --and even if it's a silicon dioxide over-coated mirror.
Again, here's the mirror/cell compartment access:
--and how the mirror is held in place by one thumb screw (dummy mirrors in these photos). The current cell sled is far more robust, but the mirror is still located and collimated by gravity versus 3 solder-tipped thumb screws and 2 eccentric support pegs:
--and here's where it's kept when not in use --along with a handy pair of white cotton gloves --but the mirror is placed face down (now) and that dark inner lid isn't used:
I bought a bargain Meade 26mm Super Plossl "LP" 4000 series eyepiece from the Surplus Shed and it turned out to be an oddball, specially made for ETX telescopes. The barrel fits loosely in my focuser, measuring 1.231 inches in diameter (will add some tape), compared to 1.240 inches for the other two standard 4000 SP eyepieces I have. More importantly, it's probably the only "4000 series" eyepiece that isn't par focal. I had to back it out 5/16" (8mm) to focus on an object that I had looking through my Meade 9.7mm SP-4000. Other dimensions (barrel, body) are different too.
I'm wondering, however, if the "LP" isn't the better eyepiece anyway. I'll have to repeat my comparisons (and hopefully against other eyepieces) to be sure, but in my "first light" tests on Jupiter, there seemed to be a halo --or cone-- of halation around the planet with the standard lens, which wasn't there when using the "LP". (Update: confirmed that again with the new mirror -6/5/2005.) I also saw quickly skittering ghosts --thinking they were meteorites at first. (Update: not sure of that now.)
** It's important to note that I have an epiretinal membrane in my right eye, vitreous floaters and deterioration in both, so it takes me some while to sort out just what I'm all looking at. I don't use much less than a 1mm exit pupil on account of my eye problems. Eventually my friends will confirm what I seem to be seeing.
Here's how the two 26mm and my 9.7mm eyepieces compare in a casual test of their anti-reflection coatings:
By turning this threesome a wee bitty: the standard 26mm shows a bright, full scale reflection flash. Must be a plano lens surface in there someplace (uncoated?). I can see the same flash looking in from the field lens too, but I couldn't see any such flash in the other two, fore or aft.
---for comparison and found them to perform fairly well. The 25mm is at least as bright, free from halation, and sharp as the Meade 26mm LP, but it does have a faint ghost on bright objects (like Jupiter, don't see it otherwise) which moves across center in the opposite direction of the primary image. I didn't think their 10mm Kellner was nearly as good as the Meade 9.7mm I have (which doesn't have any halation or ghosts). I'm tempted to order the Surplus Shed 25mm Plossl, as well as two more 25mm Kellners --which have a nice field stop landing for home-made reticules.
* The alt-az Sky Compass is the type which got used (with a better compass and spirit level) and it worked fairly well --especially for the altitude setting. It needs that 360 level for steering to high altitudes. It (of course) also needs a sky program application and a computer --if only as some sort of a hand-held digital device.
* The ball type steers to the local hour angle and declination of an object, such that it might be used with a sidereal clock, and without resort to a computer^. (Just any clock set to sidereal time for the night will do, since it would lose only 10 seconds per sidereal hour.) ^These equatorial sky compasses plus the 'scope amount to a torquetum: an analog computer.
* For a good introduction to eyepieces, see Doug Tanaka's Web
> http://bpastro.org/astronomy/amateur-astronomy/selecting-eyepieces --^
^ However, I feel compelled to rewrite and extend his: "But medium-powered eyepieces produce exit pupils that range from 2 to 4mm in size. Choose one that produces a 2mm exit pupil. This exit pupil most closely matches the resolution of the human eye: about 60 arc-seconds. The resolution of a telescope is 4.55 arc-seconds divided by its aperture in inches. For optimum resolution we need to find the magnification that will produce an image of 60 arc-seconds, which computes to roughly 13x per inch of aperture. This will produce an exit pupil of 1/13 inch, close to 2mm."
Try instead (and I'm being somewhat repetitive to my "choosing" paragraphs):
"20/20 vision" amounts to a resolution of about one (1) arc-minute --or 60 arc-seconds, typically with an eye dilation of about 2mm under good lighting. Amateur astronomers are familiar with dividing a telescope objective's inch aperture into the number 4.55 in order to get its theoretical resolving power in arc-seconds. While 2 millimeters (about 1/13 inch) of eyeball pupil diameter, divided into 4.55, gives nearly 60 arc-seconds, and while most observers are said to gravitate toward toward such eyepieces, the lens of your eye might perform better at a pupil diameter closer to 3mm, depending on how good your vision is. Find out what's best for you by spending a lot of time with your telescope.
From the above paragraph and Tanaka's article, you can see that using a power of about 13x per inch of aperture yields a 2mm exit pupil, and that 8.5x per inch yields a 3mm exit pupil. (Also, the focal ratio of your telescope divided into the eyepiece focal length gives the exit pupil.)
To be clear: an exit pupil of 2mm with (say) a 6 inch (150mm) telescope aperture yields 75 power --a bit more than you need to boost good arc-second sky seeing to arc-minute eye resolution --but that's fine. An exit pupil of 2mm with a 12 inch telescope aperture, however, yields 150 power --which you might need to comfortably sort out detail and split double stars.
* A confounding (but intuitively true) rule-of-thumb: To get full performance out of (say) photographic film (or a modern sensor, or the retina of your eye), the lens should be 2 to 3 times better. But to get full performance out of that lens, the film has to be twice as good. And for either the lens or the film to hit their specifications, the imaged resolution target has to be twice as sharp --a "vicious circle".
Unfortunately, when our eyes open up wider than 2 to 3 millimeters, more light does get in, but the sharpness of the retinal image usually falls off, which might even result in star magnitude loss as their point images spread out into dim, blurry little blobs.
From The Rule-of-thumb, we see that, on a "good night" of arc-second or better "seeing", one might lay on not 60x of power, but 120x of resolved power (which requires 10 to 12 inches of well figured aperture) --in order to realize a throughput of arc-minute resolution onto your retina. (While a crisp 60x image is visually pleasing, your goal might be to milk out all the available sky resolution.)
In my humble experience, on nearly any cloudless night, I've been
delighted with the brightness, field richness, clarity, contrast, ease
of finding and tracking when using 40x to 60x of power --through most any
telescope. A 6 inch by f/8 telescope with a standard one inch (25mm) Plossl
eyepiece yields a well defined, one degree (two moons wide) field. Moreover,
in the form of a reflecting telescope with a full length "closed" tube,
that's as much telescope as I care to carry around. (My Dog-2
'scope weighs 30 pounds.)
* The following chart assumes:
* The use of either 1.25" barrel eyepieces (maximum field stop: 27mm) --or
* 2" barrel eyepieces (maximum field stop: 46mm) --as needed.
I had the opportunity to closely study an 8 inch by f/6 commercial Dobsonian telescope with a scant round tube of 9 inches inside diameter (should have been a tad larger). It would have been possible to use to use a low profile 1.25 inch focuser if the lowest power ocular is 25mm by 50 degrees. My impression is that any telescope larger and shorter than a 6 inch x f/6 should be fitted with the equivalent of a 2 inch focuser.
* A diagonal that's sufficient to support the chosen eyepiece (which might legitimately be significantly "undersized"),
* Either affordable Plossl eyepieces (aka: "oculars") with about a 50 degree apparent field of view --or
* "Panoptic" type oculars with a 68 degree apparent field of view, which work well at f/6 and higher.
* Longer ones are made, but 1.25 inch Plossl oculars with an apparent field of 50 degrees start clipping a maximum 27mm field stop at 32mm of focal length, Panoptics clip at 24mm. For 2 inch barrel oculars, that's 55mm and 39mm.
* A 6mm widest exit pupil. (The secondary shadow might take
up 15% to 33% of that diameter, occulting a large
part of your eye's best, 2 to 3 millimeters of core vision. (I downplay reaching your eye's limit.)
* Medium and high power oculars are out of play here (and would always be 1.25 inchers).
* I've disregarded Newtonian "get the ladder" focal lengths over about 1500mm.
* It's been suggested (by Doug Tanaka & others) that most
of us can only critically look at about a 50 degree wide
field of view at one time. Jean Texereau was doubtful about the utility of apparent fields wider than 60 degrees.
* Bolded values indicate some advantage for the added expense (though you might need a larger diagonal).
* The listed eyepieces don't necessarily exist in the given focal lengths, so go for the next longer or shorter.
**Please advise me of any errors here --in my math, awareness, understanding, something missing --whatever.
Big Note: The "Plossl" listings under F/3 are only be good for theoretical comparisons and about what's do-able. You need eyepieces designed for an f/3's steep cone of light, as well as something like a TeleVue coma corrector.
* The best information on short focal length "richest
field telescopes" (RFTs) is on Mel
Bartels' web pages, along with a wealth of other resources.
Mel offers an interactive utility for diagonal sizing and performance,
as well as a telescope planner utility with which you can plug in scores
of real world eyepieces.
If you like the simple "Newt" program and are
trying to go RFT, you'll love Mel's utilities. (Be sure to check out his
short focus, wide angle, very rich field of view, 6 inch aperture telescope,
with which he's been drawing impressions of immense filamentary structures
--which might otherwise have gone unobserved. Some of these may well be
The you-need-a-ladder combinations have been left blank.
The diagonal --itself:
Per Oldham Optical's presentations at:
--one might use a rather humble piece of plate or even "double strength" window glass for the diagonal.
* It might still have to be worked in order to minimize a coarse polish, the "digs and scratches" with which window glass is afflicted, plus whatever other atrocities the hardware store or you may have inflicted (say: while grinding or core drilling out an elliptical shape). However, we see in Ingalls' "Amateur Telescope Making" books that old timers used selected pieces of plate glass just as it came.
* Oldham's reasoning about surface tolerances doesn't hold up (and thanks for pointing that out, Mark). His illustration of surface depth departures (and Oldham does state that such divot steps aren'r real world) aren't (IMO) the main consideration --it's the surface slopes which result in such depth departures.
* The Dog-2 project on this page ended up at a focal ratio of f/8.74, which placed the secondary/diagonal only 10% of the way back from prime focus to the primary mirror. Reason and intuition then suggests that as much as a wave of error at that point would only contribute what 1/10th wave of error in the primary's surface would produce.
Surface roughness (lack of polish, "scratch and digs"), depending on their degree and dimensions, might contribute much more or less in terms of light dispersion and contrast loss. "Wave" error dimensions are more about the average surface.
* I think there's more prospect in cutting down an existing front surface mirror --by donning cotton gloves and carefully cutting out a rectangle of the right 1-to1.4 size, then "nibbling" the corners off as well. (You'll get diffraction spikes if the diagonal isn't smoothly elliptical --and its supporting strut curved, but some astronomers like and make use of such spikes. Alternatively, a somewhat over-sized wooden mounting plug could also be used, such that the primary sees its shadow instead of the diagonal's --and at very little obstruction cost.)
Amateur Telescope Making Book One has a good description on page 55 of how to go about deducing which of three prospective pieces of glass are flat (but I suggest that you use small paper spacers and minimal pressure, rather than to "wring" pieces of glass together ==like a machinist's set of Johanson blocks).
** Then there's the thorny issue of sizing the diagonal --!
~ For a general purpose telescope, the diagonal is often made "undersized", which means that it limits the light to the edge of the lowest power field of view. Obviously, this depends a lot upon the eyepieces you intend to use, and how large an exit pupil your own eye can make use of (7mm if you're young, maybe only 5mm if you're old).
~ The luminosity profiles from Mel's web site shows the trade-offs for diagonals which might be fitted into an old 13.1 inch, f/4.5 Coulter (brand) "Odyssey" telescope. The smaller diagonal delivers a bit more light to the 1/2 inch core of a 1 inch (and 1 degree) low power field because it blocks 2% less light. The outer portion, however (which is 75% of the field by area), falls off by 15% or so. Were you to be visually going after wide fields of faint nebulosities with this scope, you might prefer the flat-across brightness of the larger diagonal.
~ However: if this were intended to be a "planetary" scope, used at higher powers, the smaller diagonal would be ample for the field of a higher power eyepiece. It's smaller obstruction should also allow of better contrast and resolution.
~ There's yet another important factor/trade-off: the diagonal's shadow to your eye. Used with (say) a 32mm (47x in this Coulter scope) eyepiece, you get a 7.1mm exit pupil --consisting of an image of the primary mirror and the diagonal's obstructing shadow. If you went for the 3.1 inch diagonal, its shadow is (3.1/13.1) x 7.1mm = 1.7mm wide --and that's the best part of your eye's core vision. (Some say your best eyepiece for critical observing is one delivering a 2mm to a 3mm exit pupil.)
* This last factor favors traditional "long focus" (f/8+) telescopes, since diagonals can be smaller: maybe only one inch in a six inch aperture f/8 (although such a scope can't practically reach a 7mm exit pupil, with affordable 1.25" barrel eyepieces).
* If (say) a 6" refractor is within your resources (to build or to buy), then the eyepiece/field stop, and any ("star") diagonal can be as large as might be convenient. There's no "secondary shadow" as a consequence nor any secondary support spider spikes.
As to affordability and ATM-ability, an achromatic doublet objective is probably the "working class" limit, and that means long focus --perhaps as long as f/15 --and a "long tall sally" OTA, tripod or peer. Fortunately, that doesn't mean standing on as tall a ladder (ala an f/15 reflector), and while OTA rigidity is nice, it's not critical, since the prime focus "sweet spot" is huge.
Be my guess: the ATM might consider building the low power ocular as well (using recycled 35mm film camera lenses?) since I'd expect an off-the-shelf huge eyepiece to be hugely expensive.
* One of the show stoppers for such a project would be getting anti-reflection coatings on the 4 surfaces of the objective. But might that be skipped? Previous to World War Two, artificial^ AR coatings were considered to be a military secret, so even professional astronomers didn't have them. Once upon a time I shaped and sold uncoated lenses for the stereoscopes I made (later with Luther Askeland) and as accessory lenses for stereo cameras. Neither I nor my many customers noticed reflections. (I did blacken the lens edges for better contrast.)
* A killer show stopper would be met in trying to grind, polish and test the 4 surfaces. Although each of those surfaces are spherical and much less critical than a reflecting telescope's primary mirror, you can't rough up the "back side" as you grind and polish the second side, the 4 optical centers/vertexes must closely agree when you're done, and although you'd carefully ground the 2 surfaces of each lens parallel at the start, good luck on them staying that way.
* The last price I've seen for buying the 2 blanks and 4 tools was in the $500 to $600 range (Newport). The last price I've seen for purchasing a fully coated f/15 achromat, already mounted up in a cell: $500 to $600 (Istar Optical).
* Then there's getting your refractor rig out into the field and using it. Best you first go to a star party or go out observing with a friend who owns a big (6"+) long focus refractor and see how it goes. Unless the owner has gotten the focuser close to the altitude (or equatorial) bearings with counterweights, the eyepiece is really going to move around.
* On the other hand, there are plenty of affordable 70mm to 100mm refractor scopes and binoculars to be had which can deliver wide fields and exit pupils.
I note once again that, when you reach a 5mm to 7mm exit pupil (depending on your age), there's nothing more to see --at that power. Let's say you're at 10x with a 5 degree field and a 6mm exit pupil. That's a 60mm objective (against maybe a one inch eyepiece in a binocular). By going to a 120mm objective, nothing is gained --unless you double the power and halve the field with a normal 1/2 inch eyepiece. (One might try a wide angle eyepiece, but at considerable cost.)
Mel Bartels is doing some of his best "IFN" work tracing faint tendrils of gas and dust with a very short focus 6 inch reflector and wide eyepieces. (I think he'd do even better with a 6 inch refractor, but I'm wondering if he could do just as well at lower power [same exit pupil] and [say] a 4 inch refractor?)
However, for serious power, resolution and stellar magnitude reach, a longer focus reflecting telescope, built or purchased, is hard to beat.
* If I haven't talked you out of building a refractor by now, I suggest that you make the OTA square --somewhat like the plan in Richard Berry's book (Build Your Own Telescope), but of lighter construction and with fewer baffles.
^ After many years, camera lenses were sometimes known to develop a natural patina with anti-reflection properties. Such lenses were highly prized among professionals. Also, at least one German company sold cameras with AR coated lenses before the war.
* I've gone to a retired Motorola cell phone (a compact "flip" phone, but old enough to have a pull-up antenna) which has a very nice voice memo utility, displaying it's sound files with time-date stamps. This particular model charges through a standard mini-USB port and shows the time through its flip cover whether on and illuminated or not (via its high contrast passive display).
Newer cell phones don't need a contract or prepaid minutes to be very useful. They tell the exact time, WiFi onto the net, alarm, memo, take photos --on and on. You can pick them up new for as little as $10. Our TracFones (LG, Huawei, and an older Kyocera) and the (was sold by Verizon) Motorola are always in synch with the WWV shortwave time signal --as long as they can connect with a cell phone tower --but none of them display seconds.
** I think it's also worth your while to see
what your cell phone camera can do as an "afocally coupled" telescope camera.
You should be able to get the photos out of your cell with a $5 adapter
cable to your graphics computer. (If you have no personal computer, explore
doing astronomy with i-Phone or Android applications like "Safari" (something
I have next to no experience with).
** Below is the Swedish "Precision Planet and Star Locator" planisphere which, besides showing tonight's sky map, would spit out the declination and right ascension of objects, as well as your local sidereal time (especially if you had the one made for your approximate latitude). This is also known as the "Kennedal planisphere" --for the man who designed and manufactured it.
You can't get these anymore (killed off by digital sky programs), but a regular David Chandler, double-sided "The Night Sky" planisphere is a very nice item to have instead.
* That piece of glow tape (3) is double-stick taped to the compass and shines through the drafting compass --as the hand compass hinges up on 3M brand "V" plastic (#8 --for stopping drafts through door and window jambs) --and gets held at a given setting by spring tension (#4 --made from 1/16" brazing rod).
* The small level (6) (which might have to be salvaged from an inexpensive hand level) sets the 'scopes altitude with good accuracy, although that small 360 degree level (5) is needed aa you approach the zenith --and to find somewhat level ground. Both are adjusted against a small dab of "UHU" brand office tackie clay, then secured with "Duco Household Cement".
* All-in-all, the best way to find your way around the night sky is to learn it well enough (with the help of a planisphere, star charts, and/or sky programs) that you can "pick" your way around by "star hopping" with a good finder at low power. The Sky Compass works, but getting and transferring your updated co-ordinates is slow going.
* Another alternative (for VSOing, say) is to photograph (astrograph) in the general direction, then find your way and pick out your stars later.
The Sky Compass, as mounted on Dog-2 (with its glow finder in the stored position, and here shown without the initial open sights):
* The Backyard Stargazer (2005) by Pat Price
** Scientific American's "The Amateur Astronomer/Scientist" from the Surplus Shed
The Telescope Handbook and Star Atlas (1967) by Neale E. Howard
The Guide to Amateur Astronomy (1988) by Jack Newton and Philip Teece
Burnham's Celestial Handbook (1978) by Robert Burnham, JR.
How To Make a Telescope by Jean Texereau (1957 --there are later additions)
** Amateur Astronomer's Handbook (4th Edition) by J. B. Sidgwick (revised by James Muirden)
Amateur Telescope Making (4th Edition -1974) edited by Albert G. Ingalls
Making Your Own Telescope (1947) by Allyn J. Thompson
Astronomy (1913) by Harold Jacoby
* Astronomy by Sir Fred Hoyle
"Keep Looking Up"
(Depiction of an old-time mariner finding his position by the stars.)
* The original content on these pages is unencumbered by copyright.