On May 8, 2003 CAP 843 was used to cool the ACA CCD from -10C to -11C. This cooling is the first 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 pre-cool and post-cool histograms of warm pixels (>100 e-/s) in an 8x8 region of the CCD centered on y-angle=2306, z-angle=-1290 (row = -254.5, col = -494.5). The shift in the curve corresponds to a 10.8% average decrease in dark current for this region.
Also of interest are the pre- and post-cool temperatures of the ACA and the CCD, as well as the TEC set point. The plots below are constructed from monitor window data during the weeks immediately before and after the cooling (note the short time scale). The first set of plots represents the pre-cool data while the second set represents the post-cool period. No unexpected changes are evident, but the TEC does appear to need about 2 days to ramp up to a stable level.
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 checked and verified. The bus current is essentially unchanged.
The next step in the cooling process (from -11C to -12C) is expected to proceed on Tuesday, May 27th.
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Email: Brett Unks |