ACS Suspends Operations

On Monday, 19 June 2006, at 1:15 pm EDT (17:15 UT), the Advanced Camera for Surveys (ACS) issued status buffer messages indicating that raw telemetry readings of the +15V and +5V power supply voltages monitored in the CCD Electronics Box (CEB) were outside normal operating limits. Altogether, 36 telemetry points in the Wide Field Camera (WFC) and High Resolution Camera (HRC) CEBs are monitored for real or apparent limit violations that will cause the ACS to suspend. A dump of the ACS's internal engineering data buffer revealed that all 158 CEB telemetry points began reading anomalously at the time of the event. This event occurred while the HST was outside the SAA, and during a period of no ACS commanding.

Currently, the ACS is in a safe configuration, and analysis continues. Preliminary interpretations of the telemetry and technical details about possibly affected components of ACS have been made. While the root cause of the ACS suspend remains undetermined, a leading, test-supported explanation of the telemetry is loss of a circuit that supplies +15 V power to the CEBs. ACS images taken before the suspend event appear to be entirely nominal. Up to the time of the anomaly, all engineering telemetry was also nominal.

The ACS electronics that are presently most suspect are redundant. The Side 1 electronics have been used since the installation of ACS. A switch to Side 2 electronics is being considered, and a decision will be taken in the near future.

For the time being, no ACS science observations will be carried out. HST observations are currently under way using NICMOS, WFPC2 and the Fine Guidance Sensors.