14.6	Image Mode Geometric Distortion

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 14.38. 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).

Table 14.38: Mean pixel scales for STIS detectors.

Detector	X scale (arcsec/pix)	X error (arcsec/pix)	Y scale (arcsec/pix)	Y error (arcsec/pix)	Corrected mean scale
Unfiltered CCD	0.050716	0.000072	0.050735	0.000087	0.050725
Filtered FUV-MAMA	0.024328	0.000025	0.024608	0.000086	0.024468
Filtered NUV-MAMA	0.024526	0.000120	0.024829	0.000126	0.024677

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 xi, yi 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 14.38. The plate scales imposed on rectified images produced by calstis and by drizzle using the latest IDCTABs (column “SCALE”) are shown in Table 14.39.

Table 14.39: Plate scales of rectified and drizzled images.

Detector	IDCTAB	Plate Scale (arcsec/pixel)
		Clear	Filtered
CCD	o8g1508do_idc	0.05072	0.05075
FUV-MAMA	o8g1508eo_idc	0.024395	0.024468
NUV-MAMA	o8g1508fo_idc	0.024604	0.024678