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Space Telescope Imaging Spectrograph
Correction of Errors in APD

Correction of Errors in Aperture Description Table

The aperture description (_apd) reference file contains information on the size and location of the apertures needed for the pipeline data reduction. This information is used by calstis to calculate the offset between wavecal and external exposures, and between the wavecal and the reference wavelength calibration spectrum. In December 1997, the default location for targets in slitless apertures were changed to make them concentric with the standard 0.2x0.2 aperture, and a new apd file was delivered to reflect these changes. Unfortunately, in early 1999, a version of this reference file was delivered which contained only the pre-1997 values for the aperture offsets. For slitless spectra this caused zero-point errors in the wavelength solution calculated by calstis equivalent to two CCD pixels. New apd files have now been delivered with the correct aperture offsets for each epoch. Smaller updates (<0.3 CCD pixels) to the positions of other apertures have also been included.