24 Sept 1992
On day 261 the FOC f/48 camera was being configured to start an observation sequence. The intensifier high voltage came up as commanded to the nominal value of 36 KV, stayed up for approximately 30 seconds, and then switched off and remained off for the entire observation set. A decision was taken, after a quick analysis of the telemetry data, to bring the FOC to a SAFE mode, in order to avoid a successive switch-on scheduled for the following night. The FOC will remain in SAFE mode throughout this week (SMS 256) and a recovery is planned for day 272. There is no reason to believe that the f/48 high voltage problem will affect the f/96 camera, since the power supplies of the two cameras are independent. Therefore, normal f/96 camera operations will be resumed after recovery, while every f/48 activity is presently suspended until a detailed analysis is completed.
Consultations with ESA and British Aerospace engineers have started, to establish the nature of the problem. A similar behaviour had been observed during ground testing of prototype units. After these events the units returned to normal operations by using slower power-on sequences. The design of a recovery procedure using a slower high voltage turn-on for the f/48 camera is now in progress. Besides a power supply anomaly, other possibilities involve corona breakdown in or around the intensifier tube, or burning of dust particles (whiskers) inside the intensifier. With the available data, it is not possible, at the moment, to discriminate between these possibilities. Analysis of the f/48 images obtained during the last few months is in progress. Evidence has been found of an increasingly higher dark count background, which may or may not be related to this event.
We expect to be able to switch on the f/48 and test its performance in late October, in time to advise the Cycle 3 TAC meeting scheduled for mid-November.
We will also advise the Cycle 2 users on our findings and indicate whether and how they can achieve the original scientific objectives of their programs.