Can I use arfgen for spectra extracted from regions of arbitrary shape?

• the PSF correction in arfgen assumes that the source is located at the center of the extraction region. If your extraction region is arbitrarily complex, this is probably not the case. If this happens, the encircled energy correction in arfgen may be largely wrong. In order to ensure that the PSF correction is properly calculated for extraction regions of arbitrary shape, one needs to specify explicitly the coordinates of the source centroid, through the parameters: withsourcepos, sourcecoords, sourcex, and sourcey, as in the following examples:

  
  arfgen spectrumset=my_spectrum.pi withsourcepos=yes sourcecoords=det sourcex=31.5 sourcey=-236.0
  
  

  or

  
  arfgen spectrumset=my_spectrum.pi withsourcepos=yes sourcecoords=pos sourcex=25872 sourcey=25774
  
  

  You should be aware that arfgen is forced to use the HIGH mode PSF to calculate the encircled energy correction if the source shape is other than a simple circle, annulus or rectangle. This mode is less accurate than the default EXTENDED mode (cf. the PSF watchout item for more information on this subject).

• if your spectrum is extracted from an annulus, make sure that the center coordinates of the two circles exactly coincide. Otherwise, arfgen considers these as two separate circles, and applies again the HIGH mode PSF