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As can be deduced from the following considerations this specification requirement is not completely satisfied. The PSPC and HRI perform in an identical fashion as far as the accuracy of sky position determinations for on-axis X-ray sources is concerned. For 61 suitable bright PSPC sources we studied the deviations of the determined positions from the known optical positions (the latter are accurate to 1''). For the HRI we only managed to find six suitable sources. This is due to the reduced sensitivity of the HRI and the smaller number of HRI observations carried out so far. A comparison of both distributions revealed the same scatter of the source positions around their corresponding optical positions of 6'' (i.e., after considering the appropriate boresight offset of the respective detector).
The cumulative histogram of the deviations of the sources from their optical positions is shown in the figure. It can be seen that 7% of the sources deviate by more than 10'' from their optical positions. Less than 50% of the photons from these sources arrive within the 10'' radius of the specification. The number of pointing targets which satisfy the specification requirement (68% of the photons within the 10''-Radius) is even smaller (see Section D.2).
Since the main error source for the attitude inaccuracies could not be spotted the large number of the performed studies resulted in only a small improvement of the attitude solution. However, these efforts constitute an essential contribution to the optimal calibration of the X-ray system. It is in particular the detailed knowledge of the properties of the attitude variations that makes it possible in certain cases to develop suitable correction procedures which can be applied in the scientific evaluation of the X-ray data. The details of these empirical procedures depend on the scientific goal to be achieved. The following section summarizes the performed studies.