SIM drift

Apparent SIM drift during observations

Summary

During science observations the apparent location of the science instrument focal plane can drift by an angular distance approaching 10 arcsec. This is seen as a drift in the measured fid light locations.

Over the mission the magnitude of the drift during observations has steadily increased, indicating that the root cause is most likely thermal in nature. A full description of the phenomenology and possible contributing factors is deferred for a later draft of this report. At this time we only discuss correlation of apparent fid drift with the change in ACA housing temperature during observations.

Correlation with ACA housing temperature

The plots below show the correlation between the measured centroid drift (Y, Z-angle for fid-0 during science observations) and the ACA housing temperature (AACH1T). The data were collected and processed as follows:

Select science observations longer than 50 ksec
Trim 1 ksec from either end of the Kalman interval
Fetch 5-min average data for AACH1T (ACA housing temperature)
Fetch 5-min average data for AOACYAN0, AOACZAN0 (fid Y, Z centroids in slot 0)
Subtract the mean value during the observation for each quantity
Plot d_AOAC{Y,Z}AN0 versus d_AACH1T

Discussion

The plots above show a clear correlation between the ACA housing temperature and apparent SIM drift, particularly for the ACA Y-angle direction. Further analysis of the ACA mount geometry, material properties, and other nearby thermal telemetry is needed to determine if this correlation indicates an actual causitive factor.

A strong argument in favor of this explanation is the abrupt change in ACA alignment (via observed mean fid positions) when the ACA CCD setpoint temperature was lowered (see below and see fid drift). Lowering the setpoint required more heater power and there was a corresponding increase in AACH1T. This change was seen to be ~8 arcsec in Y and ~4 arcsec in Z. This 2:1 ratio matches what is seen in the observation data plotted above.

Analysis data

The scripts and data used in this analysis are in /proj/sot/ska/analysis/sim_dy_dz

Last modified:06/26/11

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