[ATM] Grinding questions

["John Stephen" <atmlist.net@johnstephen.org>]

I'm grinding my first mirror, an 8" F4 on 3/4" plate glass, and have some questions.  This mirror is to replace the primary mirror in a Meade Schmidt-Newtonian which means that I won't need to parabolize, but it also means that the focal length needs to be pretty close in order to match the existing optics.  My question is how close is close enough?  I'm using a spherometer to measure the curvature instead of measuring sagitta.  I'm nearly finished grinding with the #220 grit and at this point my focal length is 31.5" whereas my desired FL is 32".  Do you folks think this is close enough or should I try to get it even closer?  I'm not sure how much this difference will make in my ability to focus or in the effectiveness of the Schmidt corrector plate.

I've also noticed that the mirror temperature seems to have a measurable effect on the curvature -- is that really the case or am I just getting loopy from all this grinding?  If I let the mirror sit for 30 minutes or so, the focal length appears to decrease as much as 0.5".  I wouldn't have expected this much change from thermal expansion, but I'm new at this so I don't really know what to expect.

Thanks for any advice,
- John
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