[vsnet-chat 1707] Re: CCD's Lumpy flat field?

[aah@nofs.navy.mil]

Lew is finding out some of the 'fun' aspects of CCD
observing!  I commented when I posted the HL CMa
field photometry that Sirius was scattering into the
field, and Lew confirms that. :-)
  The best way to handle such scattering is to offset
the variable in the field, pushing the bright star
as far away from the CCD as possible.  As long as you
have decent flatfields, placing the variable at the
center of a frame or at its edge should have no effect
on the photometry.
  Working near the moon is difficult at best.  Not
only do you have a bright background with a gradient,
but dust gets illuminated differently and does not
subtract out.  This is probably the cause of the
lumpiness in your image.  The solution here is to avoid
any field within 10-20 degrees of the moon, and to use
as red a filter as possible (since the atmospheric
scattering is predominately blue).
Arne

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