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On June 12, 2003 CAP 843 was used to cool the ACA CCD from -12C to
-13C. This cooling is the third step in a ten week process of
reducing the temperature from -10C to -15C. Feasibility analysis was
done by RAC and is posted in a
17-Feb-2003 report. This report details the intermediate
results.
Monitor windows selected to track specific regions of the CCD have been scheduled into perigee pass observations since the APR2803 week. Below are dark current histograms of pixels in an 8x8 region of the CCD centered on y-angle=2306, z-angle=-1290 (row = -254.5, col = -494.5). The observed shift corresponds to a 30% average decrease in dark current for this region over the entire cooling process.
Also of interest are the temperatures of the ACA and CCD, as well as the TEC setpoint. The plots below are constructed from the monitor window data during the weeks immediately before and through the cooling process. A sharp increase in the ACA housing temperature is observed starting around day 161. The immediate cause is unknown, though it may be due (at least in part) to the dual use of IRU-1 and IRU-2 beginning on day 157. Some discussion will be required before determining if it is safe to proceed with the cooling process.
No large scale effect on the spacecraft bus current was expected, as the increase in power consumption is very low (a few Watts). This has been verified. The bus current is essentially unchanged. The next step in the cooling process (from -13C to -14C) is planned to proceed on Wednesday, Jun 25th. |
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Email: Brett Unks |