ALI Solar Calibration Aperture
The Aperture Selector is an opaque slide plate located on the Aperture Cover used to vary sunlight on the solar calibration diffuser plate located within the instrument. During the first 19½ months on-orbit, the ALI performed 40 solar calibrations flawlessly. On July 5, 2002 , during the 41 st solar calibration, the aperture selector plate failed to fully close. Due to the latency in data reception and processing, one more solar calibration was performed with the plate appearing in a slightly different but stationary position. The initial failure occurred with Slot #5 97.43% open. At this point, the solar calibration script was changed to eliminate the activation command to the aperture selector motor. Over a period of about six weeks, three more solar calibrations were taken with the new script. Plate motion was not evident during any calibrations after the failure, but a combination of induced forces evidently caused the plate to move in small, uncontrolled increments so as to increase exposure of slots over time until no further movement occurred. At this point Slot #6 was 49.27% open. Six solar calibrations performed after this point in time showed the system response to be stable thereby giving further verification that the selector plate had eventually reached a fixed, stationary position. Since August 15, 2002 solar calibrations have remained stable within ± 1%.
A subsequent failure mode analysis by MIT Lincoln Labs concluded that the probable failure mode was that epoxy staking of the aperture plate to the ball nut was overstressed during ground test thermal soak at -30 ° C and failed during subsequent ground, launch, and orbital thermal and vibration environments. This failure in turn allowed the ball-nut to become unthreaded from the brass spacer and move away from the plate until it jammed due to particle entrapment thereby allowing the plate to freely slide along the ball screw until reaching the jammed ball-nut.
After the aperture plate uncontrolled motion stabilized, operational solar calibrations resumed albeit at a constant illumination level rather than over the entire dynamic range as before. Also as a result, the ability to perform a multiple step linearity check was lost. Notwithstanding the failure, radiometric response stability could continue to be monitored if the plate position remained fixed. In addition, it was concluded that there is no impact on nominal DCE dark current and imaging, no impact on special dark collections, and no impact on lunar calibration dark current and imaging.
MIT Lincoln Labs gave a presentation on the ALI solar calibration aperture selector failure on November 4, 2002. Their three sets of presentation charts can be viewed on the following links.
Subsequent to this failure, the operations procedures were changed such that ALI solar calibrations were no longer performed. All other operations procedures remained unchanged. The reason for this change was to avoid the possibility that the moveable aperture plate would stick in an unfavorable position so as to interfere with normal operations.