XMM-Newton Star Tracker Blemish Pixels: The New Treatment Strategy

Abstract

The XMM-Newton X-ray observatory mission has achieved ten years of successful scientific observations and spacecraft operations. Due to this outstanding success, the mission has been extended until 2012. This long term objective can only be achieved by maintaining the instruments in a healthy state with the platform able to operate correctly. The spacecraft operates in a strong radiation environment that requires monitoring, taking action according to the strength of radiation to protect detectors and electronics on board. Long term exposure to radiation may degrade the CCD detector of the Star Tracker (STR) modifying the behaviour of the pixels and their response to optical photons. Periodically a survey of the status of the CCD sensor is performed, with a dedicated operational procedure, in order to spot pixels exhibiting anomalous behaviour, here called blemish pixels. During the last survey operation, 6 new blemishes were spotted, increasing the total number to 12. The blemishes result in a number of Single Event Upsets (SEU), loss of guide star and inaccuracy of the pointing. In order to cope with these effects the regions around the blemishes are excluded at mission planning level, rejecting candidate guide stars which would be imaged close to the problematic pixels. This method results in the reduction of the effective area of the CCD, proportional to the number of blemishes. A further method can be used to cope with the blemishes: subtracting the energy of the blemish pixel in the STR software on board, corrects the signal of the pixel. With this method the full effective area of the STR CCD could be used again. The method has not been used so far. This paper will report on a possible implementation and its aspects in S/C operations. It requires testing and validation using the S/C simulator as well as specific tests on the satellite. Special care needs to be taken to avoid saturation of the pixel signal.