[ATM] what causes coma?

[burrjaw at earthlink.net (Jim Burrows)]

At 2004-02-13 11:43 -0500, lance clarke wrote:

>Aside from that, I'm particularly interested in the references made to an 
>aplanatic gregorian.  One thing though...if a classical greg has a 
>parabolic primary and an elliptical secondary, how can an aplanatic have 
>both primary and secondary that is elliptical?  Different parts of the ellipse?

Texereau sez, p. 148, "One could also (if it were worth the trouble) 
similarly calculate a Gregorian aplanatic (R2 and p negative)."  Eq. (42) 
on the previous page says the primary is indeed elliptical.  It would be 
fun to substitute into Eq. (43) for a practical case and see what the 
secondary figure is.  It wouldn't be a "different part of the ellipse", 
just an ellipse with some b> -1.

         -- Jim Burrows
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