From: "Robert J. Modic" <rjmodic
Date: Sun, 5 Mar 2006 10:41:53 -0500

I'd like to know what kind of lenses other observers are 
using for wide field CCD photometry.  I've experimented 
with simple achromats (from old finders and eyepieces) as 
well as lenses made for 35mm film cameras .  Both have
their strengths and weaknesses.  The achromats seem to
be easier to flat field but its necessary to refocus 
for each color when doing BVRI work.  Camera lenses give
a better quality image and may be a bit more "apochromatic" 
than the achromats, but flat fielding is trickier.  I have 
to position my lightbox very close to the front element 
to get good flats.  Too far away and the flats look mottled.  
Some camera lenses have a little play in the focus ring and 
I have to be careful not to disturb it during an imaging 
session.  With the achromats, I focus once and then lock it 
down.  Focus is then good from one night to the next.

Bob Modic (MRV)


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