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Message: <blockquote data-quote="henry link" data-source="post: 2183051" data-attributes="member: 6806">Not too likely, Brock. SK15 is only 1.79 times as expensive as cheap lowly BK7 in the Schott catalogue from the back of Rutten and van Venrooij. BTW, "costly" BAK4 is exactly 1.02 times as expensive as cheap lowly BK7. I imagine the cost of the raw glass for the prisms differs by less than $10, $20 at the most.I'm more curious about why such glass is being used at all. It's Abbe# falls between  Bak4 and BK7. It's dispersion is a little higher than BAK4, so it could be useful for maintaining TIR if the objective is very fast, but otherwise it wouldn't be "better".Maybe in this case "SK15" isn't glass at all, but just another marketing term for a prism mirror coating.Henry</blockquote>