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Refractor Plans

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Posted by Robert May on October 25, 2001 21:26:56 UTC

Unlike the designing of a reflector, a refractor is a whole different ballgame. The problem is that glass has a different refractive index for each color of the spectrum. What makes things really fun is that these changes in index aren't mirrored for each different glass. On top of all that, to get the lens to where it's repeatable, you have to recalculate the curves regularly as the various glasses that you get all have minor changes in their indexes with each production run. This makes for real fun and is why expensive refractors are so expensive - they use odd glass types that are expensive.
For a quick shot at very basic refractor design, you may want to go to my website http://nav.to/bobmay and pick up a copy of GSUM from it and also get the Schott glass catalog program from their website and start playing with doublet designs. The program will show you some of the problems that happen with refractor design.
FWIW, you can get a set of glass from Newport Glass Works in a kit that will build a decent scope. I'd modify the radii of the kit's spec depending upon the actual results of the GSUM program but those radius changes shouldn't be too much.
If you are seriously interested in doing refractor work, get several of the books available for lens design and also a program like OSLO to run the designs through and optimize them for the glass.
I might also add that a triplet will allow for better correction of the color error than a doublet will and more elements will also improve upon that but beware as the improper selection of glasses will do nothing but make things more difficult, if not impossible, to get good results.

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