Re: [ATM] undercorrecting?

["Mel Bartels" <mbartels@bbastrodesigns.com>]

> mirror (well placed fans of sufficient capacity)  the effect is so large
> that it is necessary to leave a large mirror undercorrected. Did I miss
> something?


Yes, the suspicion that people are making excuses for incomplete mirror
figuring by rationalizing that undercorrected incomplete mirrors are somehow
ok.  I've made studies of figure change versus temperature drop at the
eyepiece using the star test.  Believe me, you don't want an undercorrected
mirror.  If nothing else, what do you do about the early morning hours when
the air can begin to warm?

The change in star test is noticeable with dramatic mirror temp changes
(bringing to outside cool night air from inside warm house) but not enough
to affect the image in any material way.  Most of the image degradation is
due to the interaction of the mirror with the air of cooler temperature and
not due to change in shape of the mirror.  You can test for this by turning
off/on a fan aimed at the mirror.

Mel Bartels


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