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Clive Gibbons
Mostly Harmless
Reged: 05/26/05
Posts: 13420
Loc: Oort Cloud
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Re: Prism Diagonal
09/29/08 09:34 AM
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Further testing with ED doublets has shown that star diagonal prisms using BK7 glass typically have a greater benefit with regards to improving color correction in many ED doublet refractors.
It's usually the less expensive star diagonal prisms which use BK7 glass.
More expensive prisms, such as the Baader T-2 model (Zeiss) and the Takahashi 1.25" star diagonal, use a higher index glass (BAK-4, or analogous). This results in more complete "total internal reflection" when used in a very fast optical system (f/5 or faster). However, since most refractors aren't of such a short f/ratio, it's not necessary to use a high-index prism. BK7 glass will work just fine.
But, MOST IMPORTANTLY, the high index glass in more expensive prisms doesn't correct the prevalent color error (red defocus) which so many ED doublets display.
In conclusion, it would appear that the optimal type of prism for use in many of today's ED doublet refractors, is one using BK7 glass.
Inexpensive 1.25" star diagonal prisms by Celestron, Meade and Orion all use BK7 glass.
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"Semper ubi sub ubi"
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