Subject: notes on using the DSC, Mar 29, 1996

From: Stupendous Man

Submitted: Sun, 7 Apr 1996 14:57:30 -0400 (EDT)

Message number: 29 (previous: 28, next: 30 up: Index)

  I recently had a couple of short slots on the APO 3.5-m and tried
to use the DSC to look at a very faint, distant SN.  I noticed
a few items while reducing the data that might interest other
DSC users.

     1. There is a change in the bias level when the camera is
        attached to the telescope.  Before my time on the 3.5-m,
        I took a series of 300-sec dark exposures; then the camera
        was hooked up to the telescope, and I took a series of 300-sec
        exposures; and afterwards, when the DSC had been removed
        from the telescope, I took another series of 300-sec darks.

        The bias levels (in the pre-/over-scan areas of the chip)
        varied roughly as follows:

               before telescope:  7196 DN
               on telescope:      7148-7128 DN (decreasing with time)
               after telescope:   7200 DN

        I do not know if the pattern of the bias changed, since I 
        did not take any bias or dark frames when the camera was
        attached to the telescope.  Perhaps, next time, I shall
        (even though I'll lose time to observe my targets).

     2. The 300-sec dark exposures reveal non-uniform levels across the
        chip; there was a general pattern with amplitude about
        30 DN from lowest to highest like so:

               | low      med     high |
               |                       |
               |                       |
               |                       |
               | med         highest   |
               |                       |
               |                       |
               | high             high |

         I suspect that some of this is due to charge generation in the
         chip ("spurious charge"), and some is due to scattered light;
         the highest area has a roughly circular shape, which leads me
         to suspect a light leak.  I compared dark frames taken before
         my observing time to those afterwards; there was a difference
         of about 4-5 DN, which showed the same pattern as above.
         This change in properties over the course of only 2 hours
         again seems consistent with a light leak, if the camera
         was positioned slightly differently in its stowed position.
         The later darks had LOWER counts than the earlier ones, and
         the moon was LOWER in the sky at the later time ...
      3. There is a pair of bad columns not far from the center of 
         the chip (cols 695-696), so one should place the target 
         slightly off-center, on the side opposite to these columns.

      4. I made small offsets between each of several exposures of a
         field in order to be able to create a clean image by taking
         median of the series.  I discovered that offsets 

                 a) should be more than 10 arcsec in size (maybe 20 arcsec)
                 b) should be chosen carefully so that no exposures 
                       allow the same bad column to line up with itself

         This means that one may have to plan the offsets between 
         exposures carefully, in advance; I was certainly unable to 
         adapt to the situation at 4 AM.

  I hope that these hints might help other users of the DSC.

                               Michael Richmond

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