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Originally Posted by MiG
As far as mirrors go, BK7 is hardly better than plate glass. Its light transmission characteristics are better but that is irrelevant for a mirror. Its thermal characteristics are only a bit better.
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Thats not quite correct either. In terms of thermal properties BK7 falls between pyrex and soda lime (plate or float or whatever you want to call it). In addition to having marginally better thermal properties than plate glass, BK7 is harder and can take a better polish than plate glass. Also its co- efficient of elasticity is lot lower than plate glass therefore it is less susceptible to flexure from physical stresses (poor cell design etc). Also its surface deviation is significantly lower than plate as the mirror expands or contracts due to temperature changes. In other words a non temperature stabilised BK7 mirror will not change its figure as much as plate during the stabilisation process.
Most importantly of all however BK7 has a significantly higher consistency in terms of its quality than float glass. Float glass with poor annealing can be subjected to flaws and internal fractures and stresses and particularly air bubbles which are a pain in the ars_ to a mirror maker. There is nothing worse than spending countless hours figuring a mirror only to find an air bubble and go back to square 1. This doesn't happen with BK7.
Simple fact is BK7 is more expensive than Plate glass and if it wasn't better in many ways than plate the manufuacturer wouldn't be using it. Same with pyrex, it is more expensive than Plate and BK7 and if it wasn't better than the others the premium mirror makers wouldn't be using it.
The above having been said, there have been some exceptional mirrors made with all 3 substrates. The early 8" Starsplitters with Zambuto mirrors are actually plate glass and they are as good as it gets. So the mirror substrate shouldn't really be a determining factor in deciding which scope to buy, although for thermal reasons pyrex is the best, keeping the exotic unaffordable substrates out of the equation.
CS-John B