7.7.1 Absolute Pointing Accuracy

We have looked carefully at a sequence of images to assess the absolute pointing performance that HST delivers to WFPC2. The apertures used in the observations studied were either PC1, PC1-FIX, or WF2. The observed positions of stars on WFPC2 images were measured and compared with the proposed coordinates and apertures. Where necessary, coordinate and proper motion errors were accounted for (with the assumption that SAO catalog coordinates are exact - they form the astrometric basis for the guide star coordinate system). The typical residual pointing error is 0.86", with 1.84" being the largest error seen. This study did bring out several easy-to-make target coordinate errors (which we corrected in the analysis, but which frequently dominated the pointing error), so we discuss these first.

In a number of cases studied, the proposal coordinates were from the printed version of the Yale Bright Star Catalog. One problem is that the equinox 2000 positions in the BSC are given in the FK4 (Besselian) reference system. The proposal system assumes that equinox 2000 and later coordinates are in the FK5 (Julian) reference frame, and that earlier ones are in the FK4 frame. This can be overridden by specifying B2000 instead of J2000 for the equinox in the proposal. The latest digital version of the BSC (BSC5) is in J2000. The 1950 edition of the SAO catalog is in B1950 (FK4), and a digital version is available for J2000 (FK5). An error of up to 1.5" can result from assuming BSC positions are J2000 instead of B2000 in the proposal.

Another common problem with target coordinates is that they lack precision. For example, in the BSC, RA is given to the nearest 0.1s and DEC to the nearest arcsecond. This can cause an error of up to 0.75" in RA and 0.5" in DEC. The SAO coordinates have higher precision, 0.001s in RA and 0.01" in DEC, and should be used when possible.

A common error source is not specifying proper motion or specifying it in the wrong units. It is critical to follow the latest version of the proposal instructions on this. Even small proper motions are significant at the resolution of HST images.

Residual pointing errors (after coordinate errors and aperture location changes) range from 0.26" to 1.84". The average is 0.93" and the median is 0.86". There are no obvious trends in any coordinate system. These are errors which cannot be accounted for by a proposer, being due to guide star position errors, FGS alignment uncertainties, and residual aperture location errors. Using Guide Star Catalog positions may help reduce the error between target and guide stars. Most of the targets used in this study were too bright to have true Guide Star Catalog positions.

In summary, a target with good coordinates (and proper motion) referenced to the SAO catalog can typically be placed within 0.9" of a specified aperture. However, errors of around 1.5" occasionally happen.