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Because of the astigmatic nature of the grating system, at long wavelengths the dispersed image is signficantly defocussed and broadened in the cross-dispersion direction. In order to minimize the amount of background included in the extracted spectrum, a bowtie-shaped spectral region is used during data processing, as shown in the figure below. Extraction efficiency varies from roughly 90% near zero order to more than 96% at the longest wavelengths. Effective areas used during standard processing, and provided on the LETGS Observer Info page and in the CALDB, are based on this extraction region.
This bowtie region was implemented in Automated Processing version R4CU5UPD8 on 2000/08/28. Prior to that time, a rectangular extraction region was used, which needlessly included more background at short wavelengths. All data taken before that time have been reprocessed with the new extraction region.
A MARX simulation of a flat spectrum illustrating the
broadening of the LETG+HRC-S profile in the cross-dispersion
direction, and showing the "bowtie" spectral extraction window.
Comments and questions to: Jeremy Drake
Last modified: 06/14/02
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