Background:

Under normal operations, the science sequence will be opening and closing the shutter at a prescribed cadence. Any condition which would immediately remove power to the NEB creates a risk in which the camera shutter may be left open, possibly overexposing the CCD and destroying the chip. Examples would be any conditions resulting in safehold and NEB power being removed. There is no hardware or software logic built in to close the shutter before power is removed. The amount of damage that the CCD would incur depends on how long the shutter is left open, whether the or not the telescope is focused on an active (i.e. flare) region, which filters are in place, etc. An ACS simulation was performed on the ground which shows the spacecraft drifting off the sun by more than a degree within the first 30 seconds after safehold entry. This greatly reduces the risk to the CCD, however recovery steps must be taken immediately after a safehold or NEB shutoff has occurred to fully protect the CCD.

Key Telemetry:

None clearly evident without VC1 playback to examine the shutter encoder position (IKSHENC) and the shutter busy bit (IKMTBSH) at the time of NEB turn off. Since the housekeeping telemetry update is every 5 seconds an a sequence cadence may be much faster, it is difficult to determine. If no sequence was running at the time (IKDPSQST), the shutter would be closed and no action would be necessary.

Recovery Procedure:

Since any safehold condition warrants additional station passes, the FOT should use the first pass to determine the cause of safehold entry and sun angle (probably on the order of 5-7 degrees). As long as the spacecraft is power safe and all systems look nominal the procedure TI_shutter_close should be run. The procedure is a quick procedure the performs the following actions:

- disable the SPE safehold controller

- power on the NEB

- power on the DPU

- power on the mechanisms

- issue a shutter reset command

- turn off the above systems in reverse order

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