RE: Using flat field image

[Michael Gutzwiller <deepsky@fuse.net>]

Hi Michael,

I don't know if you are doing this or not but it made a big difference in my
flat fields generated from sky images.  I do the following steps:

1. dark subtract each sky image
2. scale each image from step 1 so that the median pixel value in the image
is the same
3. create the sky flat as the median at each pixel of the images from step 2

I found that the residual stars disappeared quite readily when I added step
2 since the background does vary over time.

Thanks,

Mike G.


-----Original Message-----
From: owner-tass@listserv.wwa.com [mailto:owner-tass@listserv.wwa.com]On
Behalf Of Stupendous Man
Sent: Monday, December 18, 2000 9:00 PM
To: tass@listserv.wwa.com
Cc: mwrsps@rit.edu
Subject: Re: Using flat field image



  A note on Mark IV data from disk15 regarding flatfields:
I tried to use the images of the night-sky on the disk to
generate flatfields.  The result was less than satisfactory,
and I want to warn others of the problems.  Tom programmed
the camera to take a series of images:

             15-sec  of field 1
             150-sec of field 1
             15-sec  of field 1
             150-sec of field 1

             15-sec  of field 2
             150-sec of field 2
             15-sec  of field 2
             150-sec of field 2

             etc.

  This arrangement is great for many purposes, but making
flats is unfortunately not one of them.  The 15-second frames
have too low a signal in the sky to be used in creating a
flatfield.  The tracking is good enough that the two 150-second
images of each field show stars in the same place -- which
tends to increase the chance that residuals due to stars will
leak through the flat-creation process.  I decided to combine
a single 150-second exposure of each field to generate a master
sky flat.  There were only about 8 fields on the disk, which
I combined via pixel-by-pixel median.  The result shows some
very faint, but still visible to the eye, residual spots
which are due to stars in the original frames.

  Just a caution that 15 or 20 or 30 different night-sky
fields may be necessary to get rid of stellar residuals
in night-sky flats.

  On another technical note, I've been trying to do a very
rough photometric solution for the frames on disk15.
I've used the Tycho catalog to generate analogs to the
V-band (based on Tycho V) and I-band (based on Tycho V, Tycho B
and Arne's color transformation).  I match up detected stars
against these Tycho stars, then throw all the frames during
the night together and make a big solution for

               a) a zero-point (shared by all frames)
               b) a color term (shared by all frames)

  Yes, I know that the frames aren't all at the same airmass,
but they are close enough (range of airmass is about 10%) that
this should suffice for a first cut.

  Okay, the problem is this: my solution shows that MOST of the
matched stars (detected in images and in the Tycho catalog)
share a common solution, just as one expects.  Good.  However,
there's a small fraction -- maybe 5% to 10% -- which are
conspicuously FAINTER (about 0.4 mag) in the Mark IV images than in
the Tycho catalog.  There is a clump of fainter stars in both
the V and I solutions -- and the same stars are fainter in
each.  I've made obvious checks: they have a range of colors,
they aren't saturated, they don't always fall in the same
part of a Mark IV image.  The only hints I can find are

               1) they are at the faint end of the stars
                      in the Tycho catalog (but some stars of
                      equal brightness do agree with the
                      photometric solution); this is still far,
                      far brighter than the average star in a
                      Mark IV frame

               2) many of them appear in just one or two frames
                      out of the 14 or so I included in the solution

  Passing clouds don't appeal to me as a solution, as some of
the stars in the suspect frames are exactly where they ought
to be in the solution.

  I can supply more numbers and diagnostic plots later; right now,
I'm typing this from home, and can't generate pictures or WWW pages
easily.

  Any ideas for this set of recalcitrant stars?

                                         Michael