RE: Using flat field image

[Tom Droege <tdroege@veriomail.com>]

Mike,

I assume by "scale" in 2 you mean an addition not a multiplication??

Tom Droege

At 12:11 AM 12/19/00 -0500, you wrote:
>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
>