Subject: short report of DSC runs Aug 26/27 and Sep 03/04 1995

From: Stupendous Man

Submitted: Tue, 5 Sep 95 11:47:50 EDT

Message number: 3 (previous: 2, next: 4 up: Index)

  I used the 3.5-m plus DSC for three hours each on Aug 26/27 and
Sep 03/04 1995.  My plan was to scan at 0.2 sidereal (exposure
time 96 seconds) and look at distant Abell clusters in the SDSS r' band.

  Images taken on the first run suffered from huge halos around
bright sources.  Bright stars resembled automobile headlights
viewed at a distance through a thick fog.  I tried to measure
the radial profile of some stars, and here's what I found:

        radius (pix)       radius (arcsec)      fraction of light(*)
           6                   1.7                  14%
          10                   2.8                  31
          20                   5.5                  61
          50                  13.8                  60
         100                  28                    79
         150                  41                    98
         200                  56                   100  (by definition)

The last column is "fraction of all the light which falls within
a circle of 200 pixels, contained in a smaller circle of N pixels".

  The seeing was about 8 pixels FWHM = 2 arcsec.  Some clouds
sneaked in and out near dawn.

  On Sep 03/04, I again had three hours.  Once again, I used
the r' band and a drift rate of 0.2 sidereal.  This time, the seeing
was better (about 1.5-1.6 arcsec), and stellar images did NOT appear
to suffer from the "fog halo" effect.  I have not yet reduced
the data, but preliminary inspection doesn't show it.

  The big problem on this run was that the telescope drives 
threatened to act up.  Tim McKay's run on Sep 02/03 suffered
from the following problem.  When he issued an "offset" command
(just before starting a drift scan), the drive would fail.  Here's
his comment, forwarded to me by Karen Gloria:

>Couple of notes that may be useful tomorrow. First the halting of the
>axis. This happened every time I executed a telescope offset using the
>"tcc offset inst/pabs" command with a DC offset. So to drift across my
>object I had to move to where I wanted to start the drift and use "tcc
>offset inst/pabs 0.0,0.0,-0.00041666667,0.0", i.e. only the 0 DC
>offset allowed it to work. Don't know why this is a problem, it never
>has been before.

  So, as Tim suggested, I just figured out where I should point the 
telescope to place it just to the west of my objects, and then scanned
past them, as normal, with 

    tcc offset inst/pabs 0.0,0.0,-0.00083333,0.0

And this worked out fine.  The weather was very nice on this run, 
the sky beautifully clear.  I look forward to examining the results.

                              Michael Richmond
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