Quote:
Originally Posted by glend
All you have to do is google BK7 vs Pyrex and there is heaps to read on the subject, including an interesting IIS thread where Mark raised the that claim originally.
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I think it is reasonable to discuss the optical figure on these mirrors- without a discussion of that - discussion becomes distorted towards fan vs no fan and doesn't take in to consideration the original figure.
I think the Crown glass equilibrium issues make it harder to nail the figure during manufacture, as they do not really have time to leave mirrors sitting over night to fully equilibrate after polishing and handling in the final stages .
All three 16" GSO /Meade mirrors I measured came in at around 2/3 wave wavefront under-corrected . I suspect they leave them undercorrected to counteract the bending `bi-metallic strip' effect a mirror undergoes when there is temperature difference between the centre and the thicker edge.
This under-correction may help to give a sharper image earlier in the night but not later. Undercorrection is better than over-correction at the eyepiece, so if time and available material is an issue it is better to leave a mirror under-corrected.