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.
The FUV plate scale given here only applies to filtered FUV imaging. For unfiltered (25MAMA) FUV imaging configuration, the plate scale is 1.003 times larger (more 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 program) will rectify the CCD, NUV-MAMA, and filtered FUV-MAMA images to the corrected mean pixel scales given in Table 14.38. For the FUV-MAMA unfiltered configuration, the adopted scale is larger by a factor of 1.003.
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 values of the distortion coefficients, and the full analysis used to derive them, can be found in Walsh, Goudfrooij, and Malumuth (STIS ISR 2001-02). These image mode geometric distortion corrections have been applied in the STIS pipeline since January 7, 2002.
|
Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |