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How do I calculate the BACKSCAL keyword for my EPIC spectra?

The BACKSCAL keyword in a spectral file indicates the area of the spatial region, from which the spectrum has been extracted. In the EPIC spectra, this keyword is expressed in units of natural pixel size (i.e.: 0.05 × 0.05 arcseconds² if the spectrum is extracted from an image in sky coordinates).

The value of the BACKSCAL keyword can be calculated via the SAS task backscal. This task must be run on any spectral files created by evselect. Please, be aware that the on-line help of evselect still refers to an obsolete input parameter calculatebackscal, which no longer exists.

If you have created the spectral file with xmmselect, however, you may not need to run backscal on it. By default, xmmselect runs automatically backscal upon extraction of a spectrum. An item to disable this option can be found under the Products menu in the main xmmselect window.

The BACKSCAL value can be at any moment modified through, e.g., the FTOOLS fparkey, or grppha