An alternative method is to use background from within the central ring. Although much of the area may be contaminated by sources, there is usually a reasonable amount of blank sky available. The method for producing such a background file is very similar to the annular background described in the 'Extended source' section.
Never xrtsort directly to a circular or annular spectral_image. The pixels at the edge of the circle will be only half filled and there is no way of subsequently correcting for this in the software.
An example of this method is shown in Figure 3.
A restriction of this method is that a single vignetting correction has to be applied to the whole background file. This means that the range of off-axis angles that can be used with this method is limited. A circular background region with off-axis angles 0-12 arcminutes or an annular box with angles 10-17 are acceptable.
When removing sources from the background region, careful consideration must be given to the point spread function of the instrument. Very high and very low energies are the most affected by photon scattering and so if sources are inadequately removed the spectral characteristics of the background will be strongly biased. A description of the PSPC PSF is given in Hasinger et al. 1992. Faint sources (< 100 counts) can be adequately excluded with a circle of 1-2 arcminutes radius, centered on the source. Brighter sources should be treated individually. Isys can be used to obtain a radial profile to ascertain the distance from the source at which the source counts fall below the level of the background. Photons should be excluded from a circle of this radius centred on the source. Its often a good idea to smooth the image, before extracting the profile.