[ATM] what causes coma?

[mdholm at telerama.com (Mark Holm)]

Hugues.Laroche@ses-astra.com wrote:

>Hm, good observation. I think the key word is entrance pupil.
>
>If the mirror is the entrance pupil like in a Newt, then
>the explanation applies to paraboloid and spherical surfaces
>which should both exhibit coma.
>
>Now in Schmidt (with corrector) or a "lensless" Schmidt
>(with a kind of iris placed at the COC of the spherical mirror),
>the entrance pupil is the corrector plate or the iris. And
>then the spherical surface has no definite optical axis anymore,
>and each bundle of light focuses w/o coma.
>  
>
I wondered if that might be the explanation.  It is a long time since I 
read anything about Schmidt theory.  In effect, what you are suggesting 
is that the entrance pupil causes the Schmidt to behave like a whole 
family of telescopes placed side by side, each with it's own optical 
axis.  Each "individual" telescope in the family works only (or 
primarily) on axis and each forms an image of a slightly different part 
of the sky.  I wonder if, for complete coma cancellation, the entrance 
pupil would have to approach zero aperture?  If so, then the Schmidt 
isn't really coma free, just coma limited.  The size of the entrance 
pupil would determine the amount of residual coma.

Anybody up on their Schmidt theory?

Mark Holm
mdholm@telerama.com