7.7.2 Updates to Aperture / Coordinate Systems

On 11 April, 1994, an update was made to the spacecraft database which tells HST where to place targets relative to the FGSs. This update affected both the location of targets in the WFPC2 field-of-view, and the position reference frame in the image headers. The nominal (or intended) pixel locations of the apertures in the WFPC2 focal plane did not change. Only the (V2,V3) coordinates of the apertures changed, as their locations relative to the FGSs became better known. For example, PC1 and PC1-FIX are designated to be at pixel (420,424.5). Before April 1994, this aperture was thought to be at (V2,V3) =(4.95",-30.77"), which, using the most current information, was actually located at pixel (X,Y) = (459.8,377.3). Since April 1994, the aperture in the spacecraft's database has been at (V2,V3) = (1.87",-30.96") or, assuming the current best estimate is exactly correct, at (X,Y) = (414.9,428.1). Thus, for the same coordinates and aperture, the pixel position of a target in an image taken before April 1994, could be nearly 3" different from its position in later images, due to aperture updates. Similar corrections apply to all WFPC2 data taken before this date.

This update also affects the position information placed in the image headers, which maps sky coordinates onto each individual CCD. Observations taken before April 11, 1994, have preliminary plate scales, rotations, and reference pixel locations in their image headers. Thus, the sky coordinates associated with a given pixel will be different for otherwise identical images taken before and after April 11, 1994, due to improvements in the aperture locations. The change is primarily an approximate 3" shift, as well as a small rotation. There is a 0.8° rotation for WF2, and smaller rotations for the other chips (0.28° in PC1, 0.46° in WF3, and 0.06° in WF4). We note that the On-The-Fly Calibration System initiated in 2000 does not correct for these offsets, since the pointing information is set upstream of the pipeline calibration; the On-The-Fly Reprocessing System installed in May 2001 does, however, correct the pointing offsets.

The STSDAS tasks METRIC/INVMETRIC and WMOSAIC use this header information; hence, images taken before April 11, 1994, required header updates in order for these tasks to produce optimum results. In this situation, observers were advised to run the STSDAS task UCHCOORD, to update the headers, prior to running METRIC/INVMETRIC and/or WMOSAIC.

The On-The-Fly Calibration System (OTFC), in place from Dec. 1999 to May 2001, did not correct for these offsets. Observers submitting requests to the archive prior to May 16, 2001 received data processed through OTFC; this data would benefit from running UCHCOORD.

As of May 16, 2001, however, the On-The-Fly Reprocessing System (OTFR) is in place and OTFR data does contain the most up-to-date header information.

UCHCOORD should not be run on OTFR data.

OTFR data can be identified by the presence of the keyword PROCTIME in the header. Please see On-The-Fly Reprocessing Systems for more details on OTFC, OTFR, and the use of UCHCOORD.

We also note, that in April and May 1996, two updates were made to the (V2,V3) coordinate system. This update should not affect observers. The purpose was to remove a slow drift in the position of WFPC2 in the HST focal plane; the largest change was 0.6". (See Table 3.15 for details.) An additional update of 0.2" was made on December 1, 1997. All the apertures are now thought to be correct to within 0.3", and future updates should be small. Please also see the section on the ORIENT Anomaly.