7.8.2 ORIENT Anomaly

We note that a minor anomaly was discovered in the data header values pertaining to image orientation (i.e. rotation about the target aperture) for data taken prior to September 15, 1997. Specifically the header keywords PA_V3 and ORIENTAT were affected. During long visits their values were incremented by up to 0.05 degree per hour whenever the telescope pointing was changed, when in fact these header values should have remained fixed. Observers requiring highly accurate image orientations should check values in the so-called jitter files (*jit.fits and *jif.fits), which were not affected by the bug. Data extracted from the HST archive using the On-The-Fly Reprocessing system implemented in mid-2001 is automatically corrected for this problem.

Figure 7.11: Example of ORIENT and POS TARG Selection. (A) A galaxy with major axis at PA=60° is to be placed across WF2 and WF3. (B) The aperture is specified as "WF3" and the nucleus is placed near the outer edge of WF3 using "POS TARG +20,0." To place the major axis across WF2 and WF3 "ORIENT 5D TO 25D" is specified. Note that "ORIENT 185D TO 205D" is also feasible.