Given that some of the ARF components vary as a function of position, the spatial variation of a source should be considered in the generation of an ARF. This information can be passed to arfgen by specifying a dataset containing an image of the source distribution (a so-called `detector map'). This is most useful for extended sources.
arfgen computes a weighted ARF based on the variation of the ARF for each pixel in the detector map, using the values in the detector map as a weighting. Ideally this map should represent a model of the incident source distribution, convolved with the telescope PSF, but with no other instrument effects applied
The detector map also specifies the spatial points at which arfgen extracts ARF information from the CAL. These points correspond to the centre of each pixel.
For extended sources the detector map can also used to calculate the fraction of flux which is lost due to chip gaps, bad columns and bad pixels by setting the parameter badpixmaptype=dataset. In this case the task finds the fraction of counts found in the image which coincide with bad pixels and reduces the final ARF values by this fraction. For this reason it is not correct to use an image from the same instrument, where the bad pixels and gaps will have no counts, to make this correction. However, an image from a different instrument, e.g. a MOS image applied to an EPIC-pn spectrum, may be used.
By default bad pixels are corrected assuming a flat spatial distribution badpixmaptype=flat.
NB: The detector map image may be created in detector or image coordinates (before arfgen 1.94 only DET coords were allowed) and may be from any of the EPIC instruments or from another satellite such as ROSAT or Chandra.
