[APML] Reducing Coma

[Roland Roberts <roland@astrofoto.org>]

Last month, I tried some shots with a couple of "new" 135mm lenses I
bought off Ebay.  I had already done focus tests using a Hartman
mask---even the New York City lights don't prevent getting a decent
focus test from something as bright as Arcturus.

I took a couple of test shots at f/4 of Delphinus.  On one camera I
seem to have not tightned down the attachment sufficiently as I have
some obvious flexure that created trails.  On the other I have a
radial pattern of trails that looks like coma.  I think the lens is an
f/3.5 (I'm not at home and can't check right now).  Am I likely to be
succesful at reducing the coma by stopping down the lens?

I don't know why I didn't try it last month.  I also had a new 50mm
f/1.4 and I shot half a roll of Fuji Press 1600 to find how wide I
could open it before aberrations were intolerable (the answer if
f/2.8---I'm happy with that since my f/2 had to be stopped down to f/4
to be usable).

I'll probably give it a try in a couple of weeks while I'm at the
Summer Star Party in the Berkshires.

roland
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Roland B. Roberts, PhD                             RL Enterprises
roland@rlenter.com                     76-15 113th Street, Apt 3B
roland@astrofoto.org                       Forest Hills, NY 11375

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