Quote:
Originally Posted by Satchmo
Hi Astrojunk
Could you elaborate there ? The relative mechanical dimensions of a glass blank are the same regardless of its temperature. Are you referring instead to the practise of undercorrecting a plate glass mirror to allow for possible overcorrection condition as the ambient temperature falls and the optics can't keep up, when in use under the night sky ?
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You're right, I suppose the critical factor is the even-ness of temperature and it was probably cheaper to keep the workshop evenly cold rather than evenly warm where I grew up!
As you have reminded me, it was always said to better to under correct mirrors - and back then we used to grind everything to f8 anyway.