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Дата изменения: Fri Jan 10 22:27:47 2003
Дата индексирования: Sat Dec 22 12:27:20 2007
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Поисковые слова: annular solar eclipse

XMM-Newton Science Analysis System


rgsspectrum (rgsspectrum-1.4.2) [xmmsas_20030110_1802-5.4.1]

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Algorithm

The following pseudo-code summarizes the computation of the spectrum. Some definitions are required. fg and bg refer to source and background respectively, though it must be noted that a BKG spectrum is computed as a TOTAL spectrum using the background selection region as the fg spatial region. Within each channel the following statistics are accumulated for each pixel and summed over the cross-dispersion limits of the spatial region: Pixels, Counts, Exposure and Mass (Exposure weighted by Counts). Only events within the PI limits of the energy selection region are considered. Rebinning (parameter rebin) is performed during the accumulation phase, so that the Exposure is actually an average over the exposure map under each rebinned pixel. A more rigorous explanation of these formulae is provided by Andy Rasmussen's memo, RGS-COL-CAL-00008.

Originally Exposure and Mass were computed according to the detailed exposure as recorded in the exposure maps. This approach was found to be conceptually incorrect, but a new approach has not yet been fully worked out. In this version a virtual exposure map is used instead, in which every pixel is fully exposed to the maximum exposure for its node. Spectrum channels that overlap more than one node are discarded with QUALITY=2 (dubious) because this approach does not provide for such cases.

FOR EACH channel:
  IF ( fgExposure > 0 )
    fgT = fgCounts ? fgMass / fgCounts : fgExposure / fgPixels
    COUNTS = fgCounts
    AREASCAL = fgT / EXPOSURE
    IF ( HDUCLAS2 == NET )
      IF ( bgExposure > 0 )
        IF ( bgCounts )
          scale = fgExposure / bgExposure
          correction = scale * fgT * bgCounts / bgMass
          COUNTS -= round( scale * bgCounts )
          STAT_ERR = sqrt( fgCounts + bgCounts * correction^2 )
        ELSE
          STAT_ERR = fgCounts ? sqrt( fgCounts ) : 1
      ELSE
        STAT_ERR = fgCounts ? sqrt( fgCounts ) : 1
        QUALITY = 2
    ELSE
      BACKSCAL = fgExposure / EXPOSURE
  ELSE
    COUNTS = 0
    AREASCAL = 0
    QUALITY = 1
    IF ( HDUCLAS2 == NET )
      STAT_ERR = 0
    ELSE
      BACKSCAL = 0