ALI Focal Plane
Contamination of the focal plane by an unknown substance was recognized in October 1998 during the characterization and calibration of the ALI at Lincoln Laboratory. This contaminant may be completely eliminated by raising the temperature of the focal plane above 260 K. Prior to launch, several bake outs occurred in an attempt to eliminate the source of the contaminant. In January 1999, the entire instrument was baked out at 303K while under vacuum for one week and then later for an additional two days. The focal plane was also baked out for three hours at 273 K in October 1999 and for one day at 273K in July 2000 during spacecraft thermal vacuum testing at GSFC.
In the event that on-orbit bake outs would become necessary, an additional heater was added to the focal plane radiator in February 1999. This heater, along with others on the instrument, can raise the temperature of the focal plane to 270 K on orbit.
The location of ALI contamination has been identified as the top surfaces of the spectral filters overlaying the focal plane detectors. A Charge Coupled Device (CCD) camera was placed at the focus of a collimator and images of the filter surfaces were obtained. The resulting pictures clearly show that a residue had formed on the surface of the filters during contamination build-up. Additionally, all evidence of the residue has been eliminated as a result of the bake out. This conclusion is supported by the levels of post-bake out data returning to baseline levels once the focal plane had been cooled to 220 K.
Focal plane contamination appears in three forms: pixel-to-pixel variation, mean level shifts, and bowing. Further details of these three forms of contamination can be found in the ALI Validation Report (Section 220.127.116.11).
Much information pertaining to the contamination of the ALI focal plane was obtained during ground testing between October 1998 and November 2001. The contaminant appeared to condense on the surfaces of the spectral filters lying above the detectors when the focal plane was operated at 220K. However, once the focal plane is warmed above 260 K the contaminant “boils off” and detector responses return to baseline levels. This implies that mirror surfaces, when maintained above 273 K at all times, will not collect contaminants during ground testing or during orbital operations.
Although the source of the contamination is not known, the leading suspect is the black paint (Z306) coating the inside of the telescope to reduce stray light. Bake out of the telescope surround structure waslimited to 70 hours and it is possible that residual outgassing of the paint may be sticking onto the filter surfaces when the focal plane is cold.
As a result of ALI focal plane contamination, the ALI operations procedure incorporated an imaging procedure whereby, every 15 days and for a 15 hour period, the cover was opened and a bake out was performed. That procedure resulted in the elimination of focal plane residue such that no loss of imaging quality was experienced.