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RE: Using flat field image
Mike,
Just to clarify this - are you rescaling the *sky flats* only?
(You shouldn't be rescaling "science exposures".)
Cheers,
Doug
On Tue, 19 Dec 2000, Michael Gutzwiller wrote:
> I really do mean multiplication after the dark field is subtracted. The
> idea is to compensate for the change in overall brightness of the field.
>
> Thanks,
>
> Mike G.
>
> 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
> >
>
>
>
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Douglas L Welch | Office/voicemail (905) 525-9140 x23186
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