Re: dawn and twilight sky flats

[Jure Skvarc <Jure.Skvarc@ijs.si>]

Some very complicated devices are being discussed for the production of
flats.  I wonder how many observatories use such things.  On the other
side, although there are many Mark IV's out there, apparently nobody
actually tried some of the simple solutions just to verify if they maybe
accidentaly work.

I make flats almost exactly in the way as Michael suggested.  
Specifically, it goes like this: some time (30 to 45 minutes) after
sunset, I point the telescope to the zenith and start taking the shortest
exposures which seem reliable with our shutter (0.2 s).  After each image
is taken, the central part of the image is checked for intensities.  If
they are at or less than half of the saturation values, the image is
stored to the disk, the telescope is moved slightly and the exposure time
is adjusted so that the next image should have intensities exactly at half
the saturation point.  In this way, the exposure time increases, but the
pixel values remain roughly the same.  The procedure is stopped when 15
images are taken or the exposures become more than 10 second long.  There
are quite a lot of stars in those last exposures but as the telescope is
moved between them, they do not fall on the same pixels.  It is also
possible to reject some of the images if they look too bad.  The duration
of the "sweet spot", as Michael calls it, is only about 15 minutes or even
less.  So, if you are a bit late, you will have less images.  Our readout
time is about 25 seconds, so Mark IV can probably take only 7 twilight
flat images.  Obviously, morning flats would be somewhat more difficult to
make, because the light levels are increasing.

After this we take bias and dark frames.  All of the images are combined
using IRAF zerocombine, darkcombine and flatcombine routines which are so
clever that they take into account different exposure times.

We are not doing photometry with that telescope, so I can not quantify it,
but the images really look flat after correction.  I am quite aware that
the wide field of view of Mark IV may introduce problems not seen on our
telescope, but I think the method is very straight-forward and worth
trying.

				Jure Skvarc