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Space Telescope Imaging Spectrograph
Updates to MAMA Distortion Corrections

Cross-dispersion plate scale corrections

The cross-dispersion plate scale values used in the OTFR pipeline were initially set using pre-launch estimates and were never updated to reflect the more accurate values subsequently determined. As of 2004-Nov-23, we have updated the values that appear in image headers to reflect our best estimates of the actual values, as measured using on-orbit data. The cross-dispersion plate scale values for first-order spectroscopic modes have been revised as follows:

                             Old       New
                             (arcsec/pixel)
All first-order CCD spectra  0.05     0.05078
G140L                        0.0244   0.0246
G140M                        0.029    0.0293
G230L                        0.0244   0.0248
G230M                        0.029    0.0296

For the MAMA modes these numbers give the plate scale for low-res (1024 x 1024) format data. For files in high-res format (2048 x 2048), the cross-dispersion plate scale will be half the above numbers. Note that the differences with the previous values are small (of order 1-2%).

The revised numbers will change the values contained in the PLATESC and CD2_2 header keywords. Users may now also find that the WAVECORR step in CALSTIS gives slightly different values for the image shift in the cross-dispersion direction (SHIFTA2 keyword). The calibrated fluxes given in 2D-rectified spectra (x2d or sx2 files) will also change slightly due to the revised definition of the pixel scale (i.e., by 1-2% depending on the mode), as will the DIFF2PT keyword values.

New distortion coefficient (sdc) and aperture description (apd) reference tables were delivered to the OTFR pipeline to support these changes.

Adopted plate scales along the dispersion direction remain unchanged.