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rgsspectrum creates an OGIP-compliant spectrum of counts for a single RGS exposure. By accounting for pixel-to-pixel and node-to-node variations in exposure while histogramming the events, rgsspectrum is able to produce a more precise spectrum than is possible when exposure-correction is handled as a separate step (see xmmselect). However, due to its alternative approach, this task is unable to benefit from a number of low-level SAS facilities. For instance, the user is limited to a single, hard-coded expression for selecting the events to be included in the spectrum: the ``canonical'' spectrum for a given source and reflection order. Further, no attempt is made to document the selections as a data sub-space within the output file. In these respects rgsspectrum is inferior to xmmselect and evselect. There is some possibility that these deficiencies will be addressed as the SAS continues to evolve. Nevertheless, rgsspectrum is the preferred spectrum-generator for RGS, and must be used for compatibility with rgsrmfgen and rgsfluxer.
Three types of spectrum are available, indicated by the value of the header attribute, HDUCLAS2:
| NET | background-corrected source spectrum |
| TOTAL | the source spectrum without background-correction |
| BKG | the background spectrum |
Note that although the spatial selection region for the background spectrum is not source-specific, the energy selection region is source-specific, and for this reason the background spectrum is technically source-specific. By default the TOTAL and BKG spectra are produced together as a pair. These two have the same format, which includes the new AREASCAL and BACKSCAL columns as specified under HDU Version 1.2.0 of the OGIP Spectral File Format (OGIP Memo OGIP/92-007). This format is supported as of LHEASOFT Version 5.1 (XSPEC Version 11.1). The background-corrected spectrum omits the BACKSCAL column, which is not needed, and instead provides the STAT_ERR column.
In High Time Resolution (HTR) mode the entire cross-dispersion dimension is collapsed into one row, inextricably mixing source and background events together. There is no background region in this mode, and so no background corrections or background spectra. Requesting a NET spectrum will yield a TOTAL spectrum instead, along with a warning message.