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Two CCD star sensors are available for position sensing of guide stars. The optical axes of the star sensors are tilted by with respect to the optical axis of the XRT. The field of view of each sensor is a rectangular area of with an overlap of about ; the overlap region of the two star sensors also overlaps with about half of the PSPC and HRI fields of view.
Bright objects must be avoided by the star sensors; this particularly applies to the Moon and the Sun-lit Earth's limb which - while in the field of view of the star sensors - prevent any position sensing by the star sensors.