Re: ATM Cass Question

[Aplanatic@aol.com]

The ray from the eye reflected off of the primary must pass through the 
center of curvature of the secondary if it is to be reflected back to the 
eye.  A simple drawing will show this.  Letting the distance from the primary 
vertex to the eye be O, we must then have our old friend:

1/O = 1/F - 1/(Dps + Rs)

where F is the focal length of the primary, Dps is the primary-secondary 
spacing, and Rs is the radius of curvature of the secondary.  Using your 
numbers I get O=303 inches.  The above formula is, of course, only strictly 
correct for paraxial rays.  The image will not be perfect since the 
asphericity of the primary and secondary are not correct to cancel spherical 
aberration at this focal location.

Dave Rowe
Torrance, CA
Medium Format Astrophotography:
http://members.aol.com/aplanatic/photos/astro.html