Instrument Science Report 2002-04:
An Analysis of WFPC2 Filter Positional Anomalies
Shireen Gonzaga, Sylvia Baggett, John Biretta
July 15, 2002
In March of 2001, a routine analysis of FR533N VISFLAT images revealed an anomaly: the filter wheel appeared to be offset by about 0.42 in some images. This offset angle is close to 0.5 degrees, which corresponds to one "filter step," (i.e., one increment in movement of the filter wheel by the filter electronics). These offsets are not a recent phenomena, and have been found in data as far back as mid-1994. This document describes a more exhaustive analysis of the filter wheel rotation anomaly: flatfield images for all available filters in the archive were inspected to determine if they exhibited similar behavior. Only filters that projected unique features (such as pinholes) onto the image were useful because these features were needed to detect the offsets. In addition to the previously-reported problem with FR533N, filter wheel rotation offsets have also now been found in these additional filter configurations: F375N, FR418N, FR533N18, FR533N33, FR533P15, FR680N, FR680N33, FQCH4N, FQCH4N15, and FQCH4P15. The offset angle was found to be 0.42 +/- 0.06. The occurrence rate of the offset error ranges from 0% to 40% for these filters (based on samples of 20 images or larger). It is also apparent that some filters (e.g. F160BW) show no rotation errors. In general, the errors will have little impact on GO science though photometric errors can reach a few percent in worst-case scenarios. There is also some hint that the problem may be getting worse with time. Currently, the cause of the anomaly is not completely understood. Cycle 10 and Cycle 11 calibration programs will continue monitoring these filter wheel offsets.
The Complete Paper (PDF 0.34 MB) is available.