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Apertures can be indicated for the source spectrum and the background spectrum separately, listed on the same line. Since only ratios are used no measured values must be given, e.g., in the case of identical apertures it only matters that the values app_s and app_b are the same. This is useful, when different extraction regions for source and background were used during the reduction of, e.g., Chandra grating spectra.

Jan-Uwe Ness