SBIG ST-8300M frame temperature reported inconsistently


#1

I have noticed, just lately, that the camera temperature reported in my SGP file names is not consistent with the set point of the SGP temperature controller, varying as much as ± 5c. I was concerned that I might have a camera problem so I did some extensive testing today (prior to calling SBIG for a repair return authorization) to try to isolate the problem.

Since it is extremely hot here in Central Texas, I move the camera into my workshop, where the ambient is about 27C, to run the tests. I used both SGP and APT in order to isolate the software. The tests consisted of running 5 to 10 frames at various exposure times and temperature set points and monitoring the CCD temperature as displayed by the software. The observed temperatures were compared to that reported in the file name.

To make a very long (and boring) story short, here are the results (test set point at +5C, 300 sec exposure, 5 frames):

  1. The instantaneous CCD temperature, as reported periodically by both programs, was correct (4.8C to 5.2C).
  2. SGP file names reported temperatures of 4.0, 4.5, 4.9, 5.3, 9.3 (APT was 6, 5, 5, 5, 6).
  3. MOST IMPORTANTLY, THE CCD TEMPERATURE IN THE FITS FILE WAS CORRECT IN ALL CASES!

It appears, from looking at the SGP log (https://www.dropbox.com/s/8bzit0snltywlo9/sg_logfile_20160803113141.txt?dl=0) that SGP grabs a temperature at the beginning of a frame and stuffs it into the file name. I notice that there is a sharp change (sometimes up, other times down) in the instantaneous CCD temperature shown on the SGP screen immediately after frame download and this is probably what is being grabbed for the next frame temperature. I wonder if there is something in the SBIG driver or SGP that is affecting cooling during download. I did not see this in APT (which used the same SBIG driver as SGP).

At any rate, my camera doesn’t have to go visit Uncle Bill and I have confidence that my FITS headers show the correct temperature for calibration. Just wanted to pass the word in case others noticed the file naming problem.

Clear (and hopefully cooler) skies.


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