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Scott Milligan
super member
Reged: 09/08/05
Posts: 114
Loc: MA
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I'm a little surprised that no one has mentioned the (to me anyways) obvious ideal use for such a blank; make an autocollimating flat. Used indoors in a temp controlled environment, the larger thermal inertia will be of little consequence, and the figure as used will be less sensitive to the details of how the flat is mounted.
Scott M. 
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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I think a couple people already suggested making a flat (on the first page of the thread).
Considering the problem with the weight, though, can a good flat be made working strictly tool-on-top, and if so, can anybody describe how to go about it? This is interesting to me, as I could work up a 20" flat on fused silica to any thickness I'd care to order, but the A-B-C method also calls for swapping the parts top and bottom, does it not?
Best,
Mark
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3128
Loc: Northeast
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"can a good flat be made working strictly tool-on-top, ..."
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Yup, certainly can. Done many of those. ABC is used when you don't have any good reference pieces available.
M.
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So many projects, so little time!
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JohnH
professor emeritus
Reged: 10/04/05
Posts: 734
Loc: vancouver near the wilds of B...
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I didn't really want a huge flat out of it, as I would need 2 other pieces of similar size. I may need such a flat at one point but not at the moment.
I put the blank away on a bottom shelf for the time being, as I don't have time right now to start on something else. Good thing too, as my back is acting up again now and I need to use it to get through work for the time being
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Scott Milligan
super member
Reged: 09/08/05
Posts: 114
Loc: MA
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Re-joining the conversation again after a couple of weeks (a bad habit of mine, sorry Mark). While I think the most reliable method for polishing flats is via a well-adjusted CP machine, very few of us will ever have access to one of these. I myself have limited experience making flats, although what little I have suggests that working MOT can become necessary when the work is especially prone to bending under the weight of the tool (ex. backside of an already perforated Schmidt corrector being reworked), but is otherwise less of a requirement when working a thick and un-or-minimally perforated substrate.
Working TOT, both stroke parameters and lap diameter can be varied to keep the figure simultaneously smooth and flat, provided that the lap backing is made stiff enough to prevent appreciable flexing during the stroke excursion, otherwise predictable figure control will be elusive.
The ABC problem can be avoided using the Ritchey-Common test, where the concave sphere is, of course, self-checking. ABC also suffers from self-weight deflection issues in diameters above 8 inches, not to mention a statistically significant increased risk of myocardial "events" when trying to separate the plates safely. Another workable test method is to check against a known good paraboloid in auto-collimation, although this method suffers the drawback that the errors in the flat are viewed single-pass, while residual errors in the paraboloid are, of course, impressed twice.
Quite helpful is a "pressing plate", polished flat to within a fringe or two, and of a diameter large enough to match the largest lap size you will use (typ. 80-85% of the workpiece diameter). Hard laps work best I've found, provided they can be "persuaded" to press out before dinner is ready.
Even better would be two such plates, one about a fringe convex, the other a fringe concave (I think there is an article in ATM II extolling the virtues of this luxury), but I suspect many reading this will not want to take the trouble to go this route unless making flats becomes an ala carte addiction (or else perhaps a business proposition).
In fact, I am currently making a 14.5" flat for a client using TOT + R-C test, and thus should be in a position to comment further shortly. Meanwhile, if Mark H. is still following this thread and finds a loquacious moment, I'm sure he could add more detailed and varied wisdom to the subject.
Best Regards,
Scott
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3128
Loc: Northeast
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I believe the ABC method is highly underrated. I made larger flats by using smaller 2-3" reference pieces. It has to be kept in mind that a reference half the size of the flat being worked shows only 1/8th the error across the worked flat. But usually the accuracy can be assessed pretty closely to 1/4 wave or a twitch better, with very good smoothness using careful deliberate work. Experience using contact IF is valuable.
We used to have planetary polishers at work, but with exposed gears, they weren't exactly "legal" with OSHA- so as a comsequence they were 'filed'.
What we generally used after that, for a certain size flat, was a specified size polisher used to effect little to no change with a set stroke of the polishing machine. As the surface got smooth and regular, we made slight changes to correct for a bit of power very slowly over a period of perhaps several hours. If careful, and with greatly thinned compound, you could get some beautiful pieces.
The polishers were a whole science to themselves. Could write a book just on pitch polishing for flats alone. Suffice to say, flower-power polishers was used extensively with production, but I found it a bit too hard for microfinish specification. I personally like used #73 with a high quality compound for most glasses. Quartz needed an agressive compound to make any significant changes.
M.
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So many projects, so little time!
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Speak on, I'm now saving these posts for reference. 
Best,
Mark
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 12230
Loc: Los Angeles
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Over 3" is going to be a problem for thermal equilibrium, but it can be put in a scope with fans both underneath and on top.
I start the one rear-mounted fan on my mirror (2-3/16" thick) about 1/2 hour before sunset and let it run. By 1/2 hour into the twilight, the mirror is +2.5 deg. above ambient. By the end of twilight, usually +1 degree above ambient. Because the temperature falls quickly where I observe, the mirror rises to +4 degrees above ambient an hour later and then, because the temperature drop where I observe always levels out, the mirror is down to +1 degree above ambient a couple hours later and stays there till dawn. Yes, I do run the fan continuously.
Now if I had 3 rear fans, the mirror would stay close to ambient and it would cool faster. With top fans to blow away the boundary layer (they could even be run periodically), I see no serious problem with a 3"+ mirror.
And, if steps were taken to prevent the mirror from reaching too high a temperature (mine is stored in the coldest place in the house, and the mirror averages 56-59 degrees when I pull the scope out), some carefully-placed fans would work fine.
The advantages of a full-thickness blank have been stated. Yet another advantage would be a low COG on the scope, so the scope might be quite a good performer.
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Don Pensack
12.5" Truss Dob, 5" Maksutov, Fujinon Binos
Sustaining Lifetime IDA member
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charles genovese
super member
Reged: 02/04/06
Posts: 132
Loc: Madisonville Louisiana
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Cutting a 3" or so central hole would probably dramatically increase cooling
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 12230
Loc: Los Angeles
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If he uses a 4" secondary, the "umbra" of the secondary is only 2" or so, so a 3" perforation would be too large.
Also, removing the center would make collimation more difficult later.
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Don Pensack
12.5" Truss Dob, 5" Maksutov, Fujinon Binos
Sustaining Lifetime IDA member
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JohnH
professor emeritus
Reged: 10/04/05
Posts: 734
Loc: vancouver near the wilds of B...
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I had wanted to go the route for a big Dobsonian. I am currently just about to go the 25 15 9 5 micron grits on it. When I bought this blank in 2006, it was partly ground with an off center depression. After doing some 100+ wets with 46/70 and 80 grit with a weight of 20+ pounds, it made me think about going any larger with a full thickness blank.
I looked through most of the large Dobsonians at Table Mountain in 2007 and 2008, and found that the limiting factor was the seeing. I came to the conclusion a 16" mirror would gather plenty of photons, could be kept under 120 pounds and under really steady skies, get close to its potential more often.
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