Quote:
Originally Posted by ausastronomer
There are a lot more things to affect optical performance in such a scope, than worrying about what substrate a 10" mass produced mirror is made out of.
CS-John B
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Thanks John, for that detailed treatment, I hadn't really noticed that the emphasis of the original thread was particularly about 10" mirrors, so I stand corrected in that case.
One needs to bear in mind that cooling issues are even more significant in the manufacture too. Pitch polishing generates heat which bends the shape even more than normal use. For the limited time these mass production mirror have to be figured , one can imagine that the final figure is something that the optician guesses at because they simply don't have the time to sit around and equalise properly during figuring. I find when putting the finishing touches on large mirrors that they need to be left overnight and not touched to reveal their final figure.
Its easy for the uninitiated to extrapolate comments to apply to all sizes of mirror so I'm emphasising that as the volume of glass escalates fast in the larger sizes the figuring and using will become far more of a challenge. . I was thinking more about low expansion vs. high expansion larger mirrors. My experience figuring and using a 50mm thick 16" mirror was very poor and B270 crown ( as distinct from BK7 Crown ) glass of the larger Chinese mirrors is around 2.9 X the expansion of Pyrex.
Mark