I've researched coma more fully. Here are the two best sites I found that explain the concepts:
http://www.opticalmechanics.com/about_coma.htm
and
http://www.astromart.com/articles/ar...?article_id=50
The best quote is:
"Using this method to specify field position, all telescopes of a given focal ratio have the same amount of off-axis coma, regardless of aperture. For example, a 10” F/4 has the same amount of coma as a 20” F/4 at the edge of a 30-mm diameter fov."
The formula is:
Linear diameter of coma is 3r/(4N)^2 where r=off axis distance in mm, and N=focal ratio. No mention of focal length is made.
Just thought people would like to know.
Don