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Posted by Robert May on April 7, 2004 19:53:03 UTC

When you look at the problem, the secondary mirror needs to be far enough from the primary mirror so that the focus point is at the side of the tube somewhere. This then determines the diameter of the secondary as the diameter of the light cone (plus the FOV angle) needs to be fully or near fully reflected to the focuser.
Programs like NEWT25 (runs under windows) can do all of your calculatioins and the program is freeware readily available on the web.

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