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The Chandra project has decided to postpone indefinitely the bakeout of the ACIS instrument in September, 2004. The purpose of the bakeout is to remove much or most of the contaminant believed to be present on the ACIS optical blocking filter. The major effect of the contaminant is to reduce the ACIS effective area at lower energies.
There are two reasons for the delay. The proposed bakeout intended to use additional heaters on the Science Instrument Module (SIM) to heat the ACIS aperture in the SIM and the top of the ACIS collimator to improve the effectiveness of the bakeout. A safety review of the use of these heaters revealed a concern that the heaters may not work as expected and may pose a threat to the other heaters on that circuit. The heaters in question were designed to be used on the first day of the mission in case the shuttle needed to return to earth with Chandra in the payload bay. They have not been used since the first day of the mission. But some of the other heaters on the circuit are used during normal operations. Second, further analysis of the rate at which the contaminant(s) have built up in the centers and edges of the OBFs indicates that the contaminant(s) might be less volatile than what was assumed in the simulations of the bakeout. Further analysis is planned to investigate this suggestion.
The Chandra project will continue to investigate the feasibility of a bakeout of the ACIS instrument. At this time it is impossible to predict if this effort will result in a decision to bakeout the instrument.
Last modified: 07/23/04
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