Re: ATM beginners basic mirror question

[Bratislav Curcic <epabcc@epa.ericsson.se>]

Stephen Tonkin wrote :

> However, one way of looking at coma in mirrors is that it will
> exist for abaxial objects in any non-spherical mirror, and the degree of
> coma will correspond to the degree of non-sphericalness (non-
> sphericity?). When a mirror is stopped down, what is left approximates
> more closely to a sphere; coma is correspondingly reduced.

This is not true. Seidel coeffitients for coma for paraboloid and
sphere are the same (at least to third order). Sphere will show THE
SAME AMOUNT of coma as paraboloid of same f-ratio. The reason coma
vanishes is simple : coma coefficient depends inversely on square of
f/ratio. Halve the f-ratio, coma drops four times. Paraboloid or
sphere, regardless.

All this discussion is valid only for entrance pupil at primary mirror.
Once we move it, by installing the diaphragm somewhere else, rules
change. In fact, when telescope is stopped down at focus (as is
commonly done with Newtoninans that have solid tubes), coma free
solution is in fact rather strong oblate. Sphere becomes coma free
when stopped at ROC only.

Bratislav