Re: ATM Coma Question??

[Aplanatic@aol.com]

Chuck:

> Is it possible to BUY a mirror 16" F/4 without Coma??

Yes, a spherical 16" f/4 mirror with an aperture stop at the radius of 
curvature will have no coma.  It will also be totally useless for astronomy 
because of the spherical aberration inherent in a fast sphere.  We then use a 
full aperture corrector plate (Schmidt, Maksutov, Houghton, Baker, etc.) to 
correct for the spherical aberration present in the mirror.  These systems 
will produce very fine images across very wide fields without coma or 
spherical aberration, and are called aplanatic.  If astigmatism is also well 
controlled (by design) across a wide field, then the optical system is 
stigmatic (point-like).

Dave Rowe