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The simple answer to that is yes Pyrex is normally a better mirror substrate than BK-7 but only slightly. The pyrex mirror will cool slightly faster that is all, when the size and thickness are the same. I have an 18" Obsession with a pyrex mirror and a 10" GSO dob with a BK7 mirror and the smaller BK7 mirror cools infinitely faster than the larger pyrex mirror.
The 10" BK7 mirrors in the GSO dobs are very good, mine is actually outstanding for a mass produced mirror. Base your decision on which scope to choose on factors other than the mirror substrate.
CS-John B
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