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4 IRAF/PROS implementation

The ROSAT Science Data Center (RSDC) at SAO has developed scripts to assist users in evaluating individual OBIs and performing the operations required for de-wobbling and alignment. The scripts are available from our anonftp area: Then change directory to pub/rosat/dewob.

An initial analysis needs to be performed to determine the stable roll angle intervals, to check for any misalignment of OBIs and to examine the guide star combinations. These factors together with the source intensity are important in deciding what can be done and the best method to use.

4.1 OBI by OBI method

  If the observation contains a strong source ($\ge$ 0.1 counts/s) near the field center (i.e. close enough to the center that the mirror blurring is not important), then the preferred method is to dewobble each OBI. The data are thus divided into $n \times p$ qpoe files (n = number of OBIs; p = number of phase bins). The position of the centroid of the reference source is determined and each file is shifted in x and y so as to align the centroids from all OBIs and all phase bins. The data are then co-added or stacked to realize the final image (qpoe file).

4.2 Stable roll angle intervals

For sources weaker than 0.1 counts/s, it is normally the case that there are not enough counts for centroiding when 10 phase bins are used. If it is determined that there are no noticeable shifts between OBIs, then it is possible to use many OBIs together so long as the roll angle does not change by a degree or more.

4.3 Method for visual inspection

On rare occasions, it may be useful to examine each phase bin visually to evaluate the segments in order to decide if some should be deleted before restacking for the final result. We have found it useful to do this via contour diagrams of the source. This approach can be labor intensive if there are a large number of OBIs and phase bins but scripts we provide do most of the manipulations.

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