1.1.5 UV Imaging

WFPC2 had a design goal of 1% photometric stability at 1470Å over a month. This requires a contamination collection rate of less than 47 ng cm^-2 month^-1 on the cold CCD window. Hence, the following features were designed into WFPC2 in an effort to reduce contaminants:

1. Venting and baffling, particularly of the electronics, were redesigned to isolate the optical cavity.
2. There was an extensive component selection and bake-out program, and specialized cleaning procedures.
3. Molecular absorbers (Zeolite) were incorporated.

The CCDs were initially operated at -77°C after launch, which was a compromise between being as warm as possible for contamination reasons, while being sufficiently cold for an adequate dark rate. However, at this temperature significant photometric errors were introduced by low-level traps in the CCDs. This problem with the charge transfer efficiency of the CCDs has been reduced since 23 April 1994 by operating the CCDs at -88°C, but this leads to significantly higher contamination rates than hoped for. On-orbit measurements indicate that there is now a decrease in throughput at a repeatable rate of ~30% per month at 1700Å. Monthly decontamination procedures are able to remove the contaminants completely and recover this loss. As of Cycle 12, the interval between decontaminations has been increased from 30 days to approximately 49 days.