How can I measure accurate relative positions from WFPC2 images?
The IRAF task xy2rd can be used to transform pixel coordinates into RA, Dec. If high precision (better than 0.5 arcsec) is desired, we recommend using the task METRIC which accounts for known geometric distortions in the pixel coordinate system. Note that this will still offer an accuracy of about 0.5 arcsec, due to the uncertainties in the actual coordinates of the guide stars, but the relative precision of the position of different points in different chips should be better than 0.1”. One possibility for your image alignment is to use the task wmosaic, which will rebin the HST image to a common frame for all four chips, removing known geometric distortions and relative rotations of the chips. This image should not be used for accurate photometry or astrometry, but can be used profitably for cross-identification and other, non-critical tasks. Please note that wmosaic will degrade the PC images severely, since they will be resampled at WFC resolution.
