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5.2 Extraction and Mask Regions

In the ideal case of an isolated point source with zero background, selection of an extraction region is simple--the larger the region the better. In real observations however, where sources just above the background are inevitably studied, selection of an extraction region is of course a trade off between maximizing the source's signal and minimizing the background's signal. Additionally, one may need to avoid overlapping PSFs, treat extended sources, or deal with complex background.

AE constructs polygon extraction regions which approximate contours of the Chandra-ACIS point spread function (PSF). A PSF contour is selected such that the fraction of the PSF enclosed approximates a target PSF fraction value specified by the observer. In a multi-observation reduction, an extraction region is constructed independently for each observation.

A circular mask region is also defined for each source in order to support the construction of a background data set (see §5.5) for each observation. The radius of the mask region is nominally chosen to be 1.1 times a radius that encloses 99% of the PSF.


next up previous contents pdf.png
Next: 5.3 Source Positions Up: 5 Algorithms Previous: 5.1 Point Spread Function
Patrick Broos
Penn State Department of Astronomy
2015-07-07