Re: ATM cass vs. greg

[Anthony Stillman <atmer@flash.net>]

Rod Brackenridge wrote:
>I think this needs correcting.  The gregorian secondary is an ellipse...


Oops.  It seems I dropped a minus sign.

For my own edification.

First
The conic constant of the secondary of a classical cassigrain or gregorian is:

   - ((m+1)/(m-1))^2

where m is the magnification factor, positive for a cassigrain, negative
for a gregorian.

Second
The relationship between the conics and conic constants are:

Hyperboloid                            K < -1
Paraboloid                             K = -1
Prolate spheroid (ellipsoid)      -1 < K < 0
Spheroid                               K = 0
Oblate spheroid                        K > 0



Anthony

PS I still know of no easy axial tests for an oblate spheroid.