Extracting Spectra using Sky Coordinate Regions
Last updated on 1997 Jun 17
Please note that there is a conceptual problem with extracting spectra using sky coordinate regions.
When the region is specified in detector coordinates it is relatively easy to calculate which map pixels lie outside the selected region and hence should be set to -1 (which downstream s/w recognises as a flag). However, when the region is specified in sky coordinates things are not so easy. Because of the spacecraft pointing jitter there is no simple relation between the selected region in sky coordinates and a region in detector coordinates. What the extractor does is to set all map pixels to -1 and then accumulate into the detector map the events which pass the sky coordinate selection criterion. At the end of the accumulation any pixels with values not equal to -1 are assumed to be part of the selected region and their value is incremented by one. The catch is that if a pixel within the selected region has no accumulated events then it will incorrectly be registered as lying outside the region. This could cause problems in downstream software. In particular, the effective area calculated by ascaarf could be underestimated.
Note also that if the spacecraft pointing position changes during the observation and data is combined from multiple spacecraft pointings then ascaarf will not work correctly.
We are considering ways to improve this situation, probably by using the attitude file to determine the detector pixels within a sky coordinate region.
The best work-around we have at present is to do the following. Use your sky coordinate region to extract a new event file of events within the region. Make a detector coordinate image of this new event file. On the image find the region which encompasses the events. Return to the original event file and use the new detector coordinate region to extract the spectrum.
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