The brightness of the mirror depends on the coating, not the substrate. Cost aside, Pyrex is a better material for a mirror due to it's lower coeffecient of thermal expansion compared to BK-7. It's less than 1/2 of BK-7's (3.25x10^6 vs 7.1x10^6 for BK-7).
Word is that BK-7 is very cheap in China and Pyrex like glass rare and expensive, so Chinese mirror makers are using BK-7. It's not a bad choice at all and a good mirror can be made from it.
Philip
Word is that BK-7 is very cheap in China and Pyrex like glass rare and expensive, so Chinese mirror makers are using BK-7. It's not a bad choice at all and a good mirror can be made from it.
Philip
