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In order to extend the life of the CCDs and DEA Analog-to-Digital converters, ACIS monitors the state of the spacecraft controlled Radiation Flag, and can command the DEA to switch CCD biases on and off based on the flag's state.
The Spacecraft contains a radiation monitor which is controlled by a ground configurable threshold set point. Whenever the measured radiation exceeds this threshold, the spacecraft tells ACIS that the monitor's threshold has been exceeded. When the radiation level subsides, the spacecraft will tell ACIS that the condition has subsided. This information appears to the ACIS software as a single radiation flag.
During periods of unexpectedly high radiation, the radiation monitor's level will trip, and the ACIS radiation monitor flag will be asserted. The software will then issue a command to the DEAs to disable power to the CCDs and terminate the active science run. Meanwhile, the ACIS software will continue to send any backlogged science data and bias information, and will produce DEA housekeeping data that indicates that the boards are powered off. When the radiation environment returns to acceptable levels, the flag will be de-asserted. ACIS will issue a command to the DEAs to allow power to be restored to the CCDs and command the DEA to re-calibrate its A/D converters.
If the radiation monitor is asserted while the instrument is booting and remains asserted until the instrument is completely initialized, the instrument shall act as described above. If the radiation monitor is asserted while the instrument is powered off, or does not remain asserted until the instrument has completed its initialization, the instrument will not react to the monitor.
The ACIS software shall periodically sample the radiation monitor flag. When the flag is asserted, the ACIS software shall abort the current science run, flag the bias maps as invalid, and issue a command to the DEAs, disabling power to the CCDs. The most recent science run start and/or stop command which arrives while the flag is asserted shall be saved, along with its parameter block, as the desired state of the instrument once the radiation subsides. DEA Housekeeping will continue, however, but report that the DEA boards are off.
When the radiation monitor flag is de-asserted, ACIS shall issue a command to the DEAs to re-enable power to the CCDs, re-load the DEA board settings, and re-calibrate its A/D converters. If a science run was in-progress when the monitor was originally asserted and a stop command has not yet been received, or if a subsequent start command was received while the monitor is active, the instrument shall start the desired science run, forcing a re-computation of the pixel-by-pixel bias maps.
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