Space Telescope Imaging Spectrograph Instrument Handbook for Cycle 14

14.6 Image-Mode Geometric Distortion

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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 are given in Table 14.38.

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
CCD	0.050716	0.000072	0.050735	0.000087	0.050725
FUV-MAMA	0.024328	0.000025	0.024608	0.000086	0.024468
NUV-MAMA	0.024526	0.000120	0.024829	0.000126	0.024677

The MAMA plate scales given here only apply to filtered imaging. For the unfiltered (25MAMA) imaging configuration, the FUV plate scale is 1.0031 times larger (more arcsec per pixel) and the NUV plate scale is 1.0008 times smaller (less arcsec per pixel). The quoted errors are formal random errors derived from the uncertainties in measuring the positions of the sources.

The geometric distortion equations given below account for the plate scale differences along rows and columns, as well as higher-order distortions. Application of these equations (e.g., using the STSDAS drizzle task) will rectify the CCD, NUV-MAMA, and filtered FUV-MAMA images to the corrected mean pixel scales given in Table 14.38. For the MAMA unfiltered configurations, the adopted scales are larger by a factor of 1.0031 for the FUV detector and smaller by a factor of 1.0008 for the NUV detector.

The geometric distortion for the STIS cameras has been characterized in a similar way to that used for WFPC2, with a cubic or quartic distortion solution, which relates the true x_i, y_i positions of the stars with the observed positions x, y using:

where x_r and y_r are the coordinates of the central pixel.

The values of the original distortion coefficients and the full analysis used to derive them can be found in Walsh, Goudfrooij, and Malumuth (STIS ISR 2001-02). Maíz-Apellániz and Úbeda (STIS ISR 2004-01) discovered that the coefficients for the NUV-MAMA had been incorrectly implemented into the 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 and the CCD is planned using calibration data that will be acquired in Cycle 13.

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