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Orbit Variation of SXT Dark Signal

Image name: orbit_dark_variation.png (click image to enlarge)
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Date submitted: 13-Jun-1994

L. Acton 10-Jun-94, corrected 13-Jun-94.

This plot demonstrates the change in gross SXT dark signal through a
daylight pass. The diamonds (Al.1) were taken over 3 passes during an XBP
campaign when I made a mistake on the sequence table and took far too many
dark frames. The crosses were AlMg dark images taken the next day with the
table corrected to make fewer dark frames. SAA data at the end of each
pass have been rejected--which explains why the data only extend to 50
minutes. The gap between 18 and 28 minutes is caused by BDR overwrite.
All data were taken with DPE=27 (10.7 sec) exposure.

I was surprised to see that the AlMg dark signal is nearly 4% larger than
the Al.1 dark signal

Note the sharp drop in dark signal at the beginning of each orbit day. I
believe this to be due to recovery of the CCD from the morning UV flood.
The remaining slow decrease of 1.2% over one hour in the dark signal is not
understood at this time. Perhaps this, too, is recovery from the effects
of UV flood.

For this case the orbit began with sunrise over KSC and ended with sunset
in the SAA. The onset of dark signal increase at the boundary of the SAA,
not shown here, was quite sharp.

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