The STIS cameras have significant geometric distortion which 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 modes 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 plate scale at the center of the field for each detector. For the CCD the scale is 
. For the FUV-MAMA (unfiltered) the scale is 
in the x (row) direction and 
in the y (column) direction. The plate scale is 1.003 times larger (more arcsec per pixel) in the filtered modes. Portions of the NUV-MAMA calibration were lost due to an on-board computer problem; the data that were obtained yield a slightly less precise solution. The NUV-MAMA plate scale (with the F25CN270 filter) is 
in x and 
in y. The errors quoted 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 program) will rectify the CCD images to a uniform pixel scale of 
per pixel, and the FUV-MAMA unfiltered and NUV-MAMA (all) modes to a uniform pixel scale of 
per pixel. For the FUV-MAMA filtered modes, an additional scaling by a factor of 1.003 is required.
The geometric distortion for the STIS cameras has been characterized in the same way as for WFPC2, with a cubic distortion solution, which relates the true x and y positions of the stars, xt and yt, in pixel coordinates from the center of the image, to the observed positions x and y also in pixels measured from the center of the image:
The coefficients are listed in Table 14.37:. The initial analysis for the CCD and the FUV-MAMA are described by Malumuth and Bowers (1997 HST Calibration workshop, page 144). The coefficients for the FUV-MAMA were changed since then (and since the Cycle 8 Handbook) to rectify the image to the same plate scale for both axes.The NUV-MAMA solution was derived by comparing an image through the F25CN270 filter to a rectified unfiltered FUV-MAMA observation of the same star field.
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