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Hubble Space Telescope
Summary of WF3-FIX Problem

- by Colin Cox

NOTE: As of May, 1998, these problem datasets have been fixed!!!

A problem has been noted with the data headers for observations using the WFPC2 UWF3-FIX aperture between 4/15/94 and 3/27/95. Specifically, the RA and DEC values placed in the headers are incorrect by approximately 2 arc seconds. The incorrect values appear in the CRVAL1 and CRVAL2 header keywords, which contain the RA and DEC, respectively. This problem does not affect images taken in 2x2 sum mode using the WF3-FIX aperture. We note this problem impacts only these two values in the image headers, and that there are no problems with the images themselves or the telescope pointing.

Occasion of Problem Introduction:

At the time of a calibration update for all apertures of WFPC2, post obervation processing (PODPS) received incorrect values for the WF3-FIX reference V2,V3 position. Prior to 4/15/94 the position of aperture WF3-FIX coincided with that of WF3.

Along with normal calibration refinements, the non-fixed WF3 aperture's reference point in detector space was moved from pixel position (416.5 424.5) to (436, 424.5) in order to avoid a bad column on the WF3 chip. This change implied a change in (V2,V3) from (0.69, 46.70) to (-0.69, 48.06) in arcsec, a shift of (-1.38, 1.36). This shift was incorrectly propagated to the fixed aperture V2V3 values supplied to PODPS for header reporting purposes but not to the S/C system for pointing. The reference point in pixel units remained for the WF3-FIX at (416.5 , 424.5) during these calibration updates.

The discrepancy was uncovered during a periodic review of all WFPC2 apertures for calibration refinement purposes.

General Correction of the Problem:

The only effect is that for observations which specified WF3-FIX during this period, the reported values in the header of the group parameters CRVAL1 and CRVAL2 were in error. This applies to all four groups. Specifically, the delta corrections (in degrees) to be applied to the header values are:

delta_RA  = [-1.38*cos(roll) + 1.36*sin(roll)]/(3600*cos(DEC))
delta_DEC = [ 1.38*sin(roll) + 1.36*cos(roll)]/3600

Where DEC (contained in keyword CRVAL2) is expressed in decimal degrees. The roll angle is given in the keyword PA_V3.