4.6.3 Orbital Bakeout Procedures

The ACIS focal plane and OBFs will probably be subject to non-volatile residual contamination while on-orbit. In order to mediate the detrimental effects of the buildup of these contaminants on the instrument's performance, a bakeout mode has been implemented. This involves elevating the temperature of those optical surfaces, by use of heaters on the focal plane and detector housing, and holding them at the elevated temperature for a time adequate to boil off the contaminants. The elevated temperature of the focal plane will be +30 C, the entire bakeout cycle will take approximately two days, and it will have to be executed as frequently as deemed necessary to prevent buildup on the OBFs from degrading system performance beyond acceptable levels. Valid science data from the focal plane will not be available during bakeout and a calibration cycle will be needed at the end of the bakeout, but ACIS must be left at the focal plane location to allow a vent path for the evaporating contaminants. The composition of these contaminants has not been defined to date; the bakeout frequency assumes that the contaminants will be vented somehow during the bakeout cycle and will not immediately re-deposit themselves on the OBF surfaces, although no venting analysis has been done to trace the path of the contaminants during bakeout.