Subject: Dome Flats with the DSC

From: Gordon Richards

Submitted: Tue, 16 Jan 1996 03:10:43 -0600

Message number: 16 (previous: 15, next: 17 up: Index)

        Since Xiaohui and I were pretty much clouded out, we took a slew
of dome flats.  We attempted to battle the shutter timing problem and the
rotation problem.  

       To solve the shutter problem we put a 1% ND in front of the quartz
lamps to lengthen the exposure time for a flat field.  This worked pretty
well.  We used the ND for r,i, and z, and found the following exposure times
gave an average number of counts near the half filled well value:

r       30 sec
i       25 sec
z       35 sec

No ND was used for the g, and u filters, adn the exposure times were 15 sec
and 300 sec, respectively.  (Note 300 gives nowhere near the half filled
well value.)

        Now more important is the rotation problem.  Specifically the   
illumination is uneven, and you need to rotate the DSC when taking flats to
get an even flat, or so we were told.  In fact, we took 3 images at each
of 6 rotations (0,60,120,180,240,300), and median co-added them.  The result--
an image that looks like you rotated the instrument six times.  Thus our
advice:

DON'T USE ONLY DOME FLATS WITH THE DSC!

even with rotation.  Definitely get some twilight flats!  If anyone wants
to see one of our resulting flats, let me know.

Gordon Richards
Xiaohui Fan
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