If the spatial selection has been specified in sky coordinates (X/Y) then arfgen needs to internally convert these into detector coordinates. The default conversion uses attitude information taken from the header of the newly created spectrum. On rare occasions this can cause a problem and so the option exists, (useodfatt=yes), to use the raw attitude files stored in the ODF directory pointed to by the environment parameter SAS_ODF.
The extendedsource option tells arfgen to produce the ARF relevant for an extended source. The distinction between the ARF needed for a point-like and extended source is rather subtle. In practice, for an extended source the ARF has no correction for encircled energy, the mirror vignetting is averaged over the whole source area and area lost due to bad pixels and chip gaps is not convolved with the Point Spread Function (PSF). All of these effects are toggled by the extendedsource parameter.
To calculate the effective area curve arfgen averages over a grid known as a detector map, which by default has five by five elements. In certain cases, such as a narrow annulus or large complicated region, this is insufficient to sample the region and can result in a null ARF or BACKSCAL value being returned. To avoid this the number of bins may be specified on the command line using the parameters detxbins and detybins.
See the arfgen documentation for further details.
