Re: [ATM] Updated web site: mirror testing and Draper machine

["Mike Spooner" <spoon@PageAmerica.net>]

Hi Ric,

For smaller mirrors (<10") it probably doesn't matter a lot but for larger 
mirrors it makes a difference. Even in the small sizes, overcorrection of 
any amount is a problem so my preference is also to have slight 
undercorrection even with these. I often test (autocollimation) with my shop 
temps dropping 2 to 5 deg F per hour and recall a high end 20" mirror I 
refigured that was grossly overcorrected when I first received it. Testing 
in the morning while warming the shop showed a fairly smooth null but when 
cooling it grossly overshot (my guess was 3/4 wave). It was not a smooth 
figure at that point either. This performance had been verified in the field 
and the mirror had been declared "within spec" when sent back to the 
original optician. I saved .jpgs of the before and after. Some few small 
pyrex mirrors are almost as bad as plate glass but for the most part the 
majority do well. If the mirror can be kept close to ambient then close to 
full correction is desired. Guessing how to correct for good correction in 
actual use is problematic but ATMs have the star test available as a final 
judge. Most of my personal scopes have low expansion substrates and I find 
the testing and figuring a joy using them (but not so joyful for the 
pocketbook). A well annealed piece of pyrex is also pretty good.

Best,
--Mike Spooner

.
>
> There is one point he made,that really has me
> perplexed,the statement,just below the image of mirror
> B were he says one needs to keep pyrex mirrors
> slightly undercorrected -since they cool off all
> night.
>
> I wonder if this is the professional opinon,or just
> Roger's view?This topic has been covered a lot on this
> list and I for one got the feeling that the group view
> was to correct the surface to the best figure one
> could.
>
> Ric
>


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