[ATM] Re: ATM Digest, Vol 2, Issue 15

[wa4guu at verizon.net (Jerry)]

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-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of artbianconi@blast.net

Now we are ready to suspend that divinely precise hunk of glass and 
suddenly, out of now where comes this zealous preoccupation with 
?sagging?!  Why? 
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While I think that I may agree with what I believe is your intended point, you have included in the stew too many things of unstable dimensions that do not cause mirror "sagging". The mount boxes that are open on three sides, the high expansion coefficient aluminum truss in wood tube ends and other such components of the scope may have dimensional instability but the instabilities do not bend the mirror. You seem to be including every part of the scope that might flex or change size or shape into things that cause mirror sagging.

Many ATMs do consider flex in grinding, polishing, figuring and testing. There are ATMs who can put that glass wheel on the plywood bench and figure it and not polish in significant astigmatism. For some problems the solution is incredibly easy. And then other problems are harder. I don't think many ATMs will feel sufficient motivation to go to vertical testing. But I can imagine many thousands of man years of work on various schemes to rigidly support the mirror while freeing it from all external forces that would flex it from it's native zero g parabola shape. It would seem that would be easier to do for the fixed testing setup than in the telescope itself.

What would have to be simpler would be to take some of that FEM stuff like what PLOP uses for designing mirror cells and rearrange the program to calculate the shape and desired zone readings your perfect parabola will have when stood on edge in that lousy two points of edge support like what you will never have it in real use mirror holder used in your test setup.

I see it in others and myself too. And sometimes I even know when I am doing it. It is an affliction that goes with the territory in ATM. If 1/4 wave is ok then 1/10 is better and if 1/100 is possible then 1/50 is terrible. Well my 12.5" numbers run through one to those program variants of Texareau is 1/21.4305 wave. Just a bit better than the goal I had set at 1/21.43 wave. After I had it coated I started to wonder if I had made a mistake not trying to get it just a bit better. I think I could have gotten to 1/21.4309 or maybe even 1/21.4311. Well, it turns out it is just as well that I stopped. I realize now that I need to totally redo the mirror because I was trying to make an f6 and I forgot to take the beveled edge into account. It is really a 12.23" diameter mirror so to be an f6 the fl should be 73.38" and I got 75.00". I should have searched the archives. I'll bet that mistake has been made by someone on the list already.

I guess a good name for the affliction would be Hyper Accuracy Syndrome (HAS). I figure my taxes on a scientific calculator for high precision. I wouldn't want to make an error on my taxes so I want all the precision I can get. I'm sure glad the old days of the slide rule are over. I'll bet the IRS is too.

Jerry