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To improve the execution time of the task the source region is divided into a grid which is used to check whether pixels lie on a CCD. The width of each grid element is set to be 10 detector pixels ($\sim0.5
$ arcseconds) for small regions (region half-width less than 2 arcminutes) and 20 detector pixels ($\sim1$ arcsecond) for larger regions. It has been chosen like this to give good coverage of CCD gaps where they make a significant contribution to the total area while keeping reasonable execution times for larger areas. This results in an execution time of $\sim$1 minute for a circular source region of 1 arcminute radius and $\sim$5 minutes for a circle of radius 5 arcminutes.

XMM-Newton SOC/SSC -- 2005-08-16