Telescope Reviews: Polishing 17" in the wake of my 18"

I see noone really has the faintest idea what I'm referring to; nor has done any flat work, or polished out multi-element blocks.
Anything even remotely resembling a "cleat" or "stop" must -NOT- be used! Said another way, you need to find another way of holding the glass; while polishing, figuring, and testing.
You all have to realise that in finishing the optic, you are talking about measuring tolerances that can be a fraction of a millionth of an inch. All of your efforts, theories, and nice work building the pretty plywood stands, and interferometers will just be wasted.
Contact with -ANYTHING- ajacent to the edge of the optic will haunt you all through your efforts. It will ruin the figure, plain and simple. It will interfere with testing accuracy. Touching anything cannot be allowed, period!
There are several ways of 'skinning the cat'. You can mount the mirror on a pitch button, and allow the thing to cool off and relieve stress overnight. Or you can do what Kevin mentioned in the other thread; by using tape with the mirror resting on a cushion, where the backer was ground to match the back of the mirror. I use a blank that's blancharded flat, and mounted on backers machined precisely flat as possible.
To eliminate astig on the test stand, there are 2 ways that I can think of to solve those issues. First, you could arrange a vertical test, or nearly so. (A slight offset could be introduced so as to prevent dropping anything on the mirror during the test.)
Second idea, which I use, is to have a typical 1"-8thd spindle thread that accomodates a hub fastened directly to the backer. This can be spun off, and simply rescrewed down to a stub shaft of the same thread on the test stand. The stub is mounted on a hinge, with a bolt stop used for adjustment, and a stick is used to prop the hinged affair up so that you can easily thread the mirror/backer to it safely.Naturally, there is also a cushion between the mirror/backer. No pegs, slings, glass blowing off and hitting the floor, no astigmatism induced by test stands, trefoil, etc.
You may think I don't know what I'm talking about, which is your priviledge. But I've never had to do 50- 20" mirrors to figure out how to do a 24",like one reputable mirror pusher, nor had to get involved in all sorts of elaborate tests for astigmatism, nor deal with returned product due to faulty testing/figuring. There's a specific reason why I recommend what I do. I've already been there, or have seen someone else I've worked with deal with the same problems.
I intend for this to cut through the chaff so to speak, and allow the completion of the 17" satisfactorily. Could be it may be upsetting to those who might feel they have a lot of work to modify their procedures. That is not my intent, but just to shed some light on optical procedure that might be encountered in an ISO certified facility. It can certainly short circuit some of the learning curve off of completing a project! Good luck!
Regards, Mark

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Scopes in the works-
Too many for putting down here! Favorites- 8" F/6, 8" F/4.72, 4.5" F/5.4, 14" F/4.455, all completed.

Edited by Mark Harry (09/07/08 05:22 AM)