This task calculates the area of a source region used to make a spectral file. The area is written into the header of the SPECTRUM table of the file in the keyword BACKSCAL.
The final value is:
area = geometric_area - ccd_gaps - bad_pixels
In normal use only pixels which lie within the CCD boundaries and not on bad pixels contribute to the total area. The units of area are detector pixels which are square pixels of side 0.05 arcseconds.
If the parameter withbadpixcorr is set false then the pure geometric area will be calculated regardless of where the source region lies. If withbadpixcorr is true then pixels lying off the edges of all the CCDs will be subtracted from the total area. If withbadpixcorr is true and badpixlocation is set to a file containing bad pixel extensions (typically the input event file) then bad pixels lying within the source region area are also subtracted from the total area. By default, area outside the field of view is not included in the backscale calculation. This can be overridden by setting ignoreoutoffov false on the command line.
By default the code allows arfgen to choose the grid pixel size for the calculation of the area. This is currently 0.5" for imaging mode and 0.25" for TIMING/BURST modes. A less accurate value, but faster execution time, may be achieved by setting the parameter badpixelresolution to a larger number, e.g. 2.0.
