From: "Bob May" 
To: "atmlist" 
Reply-To: "Bob May" 


Jack, Coma is the appearance of a focused image that looks like a comet
with two tails.  Is this what you really see?  The image below is what a
rectangular array of ligit beams looks like for coma.  The other thing that
you may be seein is astigmatism and that is where, as you focus, the image
goes from a line in one direction to a line in the other direction.  There
will be a point where the image is round but not quite in focus.  Most EPs
will show this at the edge of the FOV to some degree or another.  If this
is what you are seeing, then you probably have some curvature or other
error on your secondary or the primary is pinched if the astigmatism is
about the same across the whole FOV - edge effects are generally from the
EP more often than not.
Bob May
http://nav.to/bobmay
bobmay{at}nethere.com
NEW! http://bobmay.astronomy.net


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