The background area needs to be large but constrained within a narrow range of off-axis angles (so that a single vignetting correction may be applied to the data). The best shape to satisfy these criteria is an annulus at a reasonably large off-axis angle (e.g. r=0.6o - 0.7o). To produce such an annulus with sources and window support structure removed, use the following procedure:
xrtsub may then be run on the source and background spectral images. The output spectral image can be compressed into an image using project. Alternatively it may be projected into a spectrum for spectral work. NB: DO NOT compress the background spectral image into a spectrum before background subtraction as this will result in the spatial information being lost and the program will be unable to normalise for the box areas.
The above procedure is fairly robust against spatial variations in the background. One problem is that beyond the central ring, it becomes harder to resolve point sources from the background. This means that faint sources may be included in the background region when they would have been ignored in the middle. This can cause a slight over-subtraction and be a problem for very accurate work. No data should be used beyond a radius of 50 arcminutes due to inaccuracies in the vignetting function at these large radii.