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2. FOS DATA PREPARATION

The IDL testbed version of the standard FOS pipeline reduction routine, CALFOS, is used to uniformly process each individual cycle 4 calibration observation with the most recent calibration files. This standard processing includes the following steps, which produce the net count rates that are required for sensitivity computation. The cycle 1 through 3
pre-COSTAR FOS calibration of Lindler & Bohlin (1994) followed the same prescription, except that the GIMP correction is now done in the instrument, rather than by ground software.

  1. Conversion to count rates. The raw .d0* counts are corrected for the dead diodes and converted to true count rates (Lindler & Bohlin 1988).

  2. Background subtraction. A particle background and dark rate is subtracted using a background model that varies with geomagnetic latitude (Lyons et al. 1992).

  3. The flat field correction uses the flat fields currently installed in PODPS by Keyes and decouples the flat field effects from the absolute calibration, according to the procedure of Lindler et al. (1993).

  4. Residual background and scattered light is subtracted for the modes which include a wavelength region where the response to the dispersed light is zero. The scattered light correction is especially important in computing the calibration in wavelength regions of low sensitivity. As the sensitivity approaches zero, the scattered light becomes a large fraction of the observed light. The scattered light estimate is the average count rate in regions of zero sensitivity (Kinney and Bohlin 1993).

  5. Wavelengths are computed using the dispersion coefficient table presently used by PODPS (Kriss, Blair, and Davidsen 1991).

  6. In order to monitor for possible photometric error associated with the centering of the spectra on the diode array in the direction perpendicular to the dispersion, data are collected for each observation at three different y-deflections of the diode array, one at the nominal y-position and two offset by 0.156 arcsec above and below the nominal position. The count rates for the lower and upper y-steps are normalized to the center y-step, and all three spectra are averaged.

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