File:ABELL 2147 83074000 Rad.gif
The background (and sigma of the background) was iterative determined by stepping inward from the end of the profile and averaging (using a three sigma clipping algorithm). The extraction region was chosen to be the radius at which the radial profile reached 5*sigma times the background level.
The radial profile is shown below. The red line is the background level. The blue line is the 5*sigma level. SIS extaction radii were set at 0.70 times the GIS extraction radius.