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Introduction

The procedures described in this section are still under development. Users with problems or suggestions are urged to contact us: rsdc@cfa.harvard.edu. The scripts described below are available from our anonftp area: sao-ftp.harvard.edu. cd to pub/rosat/dewob. A paper containing further discussion and examples is in preparation (Harris, Silverman, and Hasinger, 1998, to be submitted to Astron. & Astrophys.). Throughout this discussion, we use the term ``PRF'' in the dynamic sense: it is the point response function realized in any given situation: i.e. that which includes whatever aspect errors are present. We start with an observation for which the PRF is much worse than it should be; caused by residual wobble errors or other causes. We seek to improve the PRF by isolating the offending contributions and correcting them if possible. Spatial analysis of ROSAT HRI observations are often plagued by poor aspect solutions, precluding the attainment of the potential resolution of about 5''. In many cases (but not all), the major contribution to the degradation in the effective Point Response Function (PRF) comes from aspect errors associated with the ROSAT wobble. This work is similar to an analysis by Morse (1994) and provides a set of routines to minimize these effects. The procedure in section 13.2.4 will also fix most cases of ``displaced obis''. Occasionally a multi-obi observation consists of two quite distinct aspect solutions. A recent example showed one obi for which the source was 10 $^{\prime\prime}$ north of its position in the other 17 obis. The 'model' assumes that the star tracker has some pixels with different gain than others. As the wobble moves the de-focused star image across the CCD, the centroiding of the stellar image gets the wrong value because it is based on the response from several pixels. If the roll angle is stable, it is likely that the error is repeated during each cycle of the wobble since the star's path is over the same pixels (to a first approximation if the aspect 'jitter' is small compared to the pixel size). What is not addressed is the error in roll angle induced by erroneous star positions. We have reason to believe that this error may be rather large so that the centroiding technique with one strong source 'fixes' only that source. The method given here assigns a 'wobble phase' to each event; then divides each obi into a number of wobble phase bins. Each bin is used to create an image and the centroid of the source is measured. The data are recombined after applying x and y offsets in order to ensure that the centroids of sub images are aligned. What is required is that there are enough counts in the source to obtain a reliable centroid. Variations of this method for sources weaker than approx 0.1 count/sec involve using all obis together before dividing into phase bins. This is a valid approach so long as the nominal roll angle is stable (i.e. within a few tenths of a degree) for all obis. We have seen misalignment of 5 to 8 arcsec between obis and when different combinations of guide stars are used in the aspect solution. It is important that the phase of the wobble is maintained during the observation. This is ensured if there is no 'reset' of the space craft clock during an observation. If your qpoe file has a begin and end time/date that includes a reset, it will be necessary to divide the qpoe file into two with a time filter before running qp-phase. Dates of clock resets are provided by MPE: http://www.rosat.mpe-garching.mpg.de/$\sim$prp/timcor.html. Currently they are:

Year     Day
-----------
90     151.87975   (launch)
91     25.386331
92     42.353305
93     18.705978
94     19.631352
95     18.169322
96     28.489871
97     16.069990
98     19.445738