The STIS cameras have significant geometric distortion that not only affects astrometry, but also in principle affects photometry (because the extended sources used to generate flat fields have an induced change in the apparent surface brightness). In the CCD the image distortions are less than one pixel across the whole detector, and can often be ignored. For the MAMA the distortions are larger, approaching 3 pixels at the corners of cameras.
Geometric distortion and plate scales for the STIS imaging configurations have been measured on-orbit by observing star fields shifted to different positions in the field, following a procedure similar to that used for WFPC-2 (Holtzman et al., PASP, 107, 156). The geometric distortion data also allowed a determination of the mean plate scale at the center of the field for each detector. These plate scales, for unfiltered CCD exposures and filtered MAMA exposures (see
ISR STIS 2001-02), are given in
Table 14.1. The quoted errors are formal random errors derived from the uncertainties in measuring the positions of the sources. Maíz-Apellániz has determined that the plate scale for unfiltered
FUV-MAMA images (25MAMA) is 1.0031 times larger than for filtered images (F25QTZ and F25SRF2); the plate scale for unfiltered
NUV-MAMA images (25MAMA) is 1.0008 times smaller than for filtered images (F25QTZ and F25CN182).
Distortion solutions have been implemented in the IDCTAB reference files (see
ISR ACS 2001-08), to be applied by the
STSDAS drizzle task. The equations which relate the true x
i, y
i positions of the stars with the observed positions x, y are as follows (
STIS ISR 2004-01):
where xr and
yr are the coordinates of the central pixel.
Maíz-Apellániz and Úbeda discovered that the coefficients for the NUV-MAMA had been incorrectly implemented in the original IDCTAB reference file and produced a revised analysis which is the one currently used for that detector. A similar re-analysis for the
FUV-MAMA is planned. The latest IDCTAB for the
FUV-MAMA contains only a linear transformation in each coordinate, using the number of arcsec/pixel given in
Table 14.1. The plate scales imposed on rectified images produced by
calstis and by
drizzle using the latest IDCTABs (column “SCALE”) are shown in
Table 14.2.
