WFPC2 Instrument Science Report 95-02:
The Geometric Distortion for the WFPC2 Cameras
by Roberto Gilmozzi, Shawn Ewald, and Ellyne Kinney
Abstract excerpted from the report:
The WF/PC-1 and the WFPC2 are both subject to a geometric distortion effect which stems from optical and mechanical design constraints (wide field of view and small optical bench volume) and off-axis optics. In both cameras, the distortion is exaggerated by the telescope's spherical aberration. The distortion ranges from a few tenths of a pixel in the center of each chip, up to 2-3 pixels at the edges of each chip. The plate scales for the WFCs are approximately 0.1 arcseconds/pixel, so the distortion effect can cause positional errors anywhere from 20 mas to 300 mas. For the PCs, which have plate scales of 0.043 arcseconds/pixel, these errors range from 8 mas to over 100 mas.
For both scientific and operational purposes it is desirable to have more accurate measurements. An example of a scientific use of the distortion correction would be to derive relative positions of stars in a star cluster. On the operational side, a major use of the distortion correction is for WFPC - assisted Target Acquisition for one of the small aperture SIs such as FOS or HRS. This strategy is especially useful for acquiring faint objects in crowded or high background fields. A WFPC image is taken beforehand, and is used to select a brights and /or isolated object near the fainter one. An accurate offset between the tow objects is derived. The small aperture SI then peaks up on the brighter target, and then the slew to the intended target is performed. The WFC is usually used since the PC, which having a higher pixel resolution, has a FOV that is insufficient for the most offset pointing operations.