[ATM] what causes coma?

[bobmay at nethere.com (Bob May)]

Coma has its error in the angle of the beam so refocusing won't fix it at
all.  When you look at a comatic image, you will see that the light from the
center area is all pointed at the tip of the coma where the comet would be
and, as the light comes from further out on the mirror, the light goes
further up the tails.  This makes it difficult to find any particular point
at which the focus is best although the focusing to the center of the mirror
produces the sharpest of the comatic shape.
Bob May
http://nav.to/bobmay
bobmay@nethere.com
NEW! http://bobmay.astronomy.net