Re: coma, coma, coma
HOFFMAN@wmich.edu
Tue, 02 May 1995 13:03:10 -0400 (EDT)
The coma is inherent in the design, gets worse faster with
faster f/ ratios.
A parabola forms a perfect image from light rays that are
PARALLEL to it's axis of symmetry. Therefore, since parallel
rays are focussed at the center of the image field, that part
is perfect. All the light rays from stars are parallel, but
only a star that is on-axis gets a perfect image. The farther
(angularly) from the axis, the farther from the center is the
image, and the worse the image. A fast f/ ratio is just as good
ON-AXIS, but since it can cover a wider (angular) field, people
look at the edge of the image and find the distortions called
coma.
Richard Hoffman