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Hubble's main camera back in action

Reactivated computer system bearing up

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The Hubble space telescope's main camera is back in action following the reactivation last week of the flying eye's backup computer system.

On Saturday morning, the 'scope's science computer commanded the Wide Field Planetary Camera 2 out of the safe mode in which the instrument had been slumbering since a computer failure on 16 October. NASA adds: "Additional commanding allowed engineers on the ground to assess the instrument's state of health and verify the contents of the camera's microprocessor memory."

The first images from the camera will be "for data calibration purposes", and the agency hopes to release an image later this week.

Hubble was last month blinded by the failure of the Control Unit/Science Data Formatter (CU/SDF) in its operational Side A Science Instrument Command and Data Handling unit (SIC&DH), which packets data from the 'scope's five main instruments for transmission back to Earth.

NASA deployed the redundant Side B system, and hoped to have Hubble back in full science mode on 17 October. However, while instrument reconfiguration "proceeded nominally" and the Wide Field Planetary Camera 2, Near Infrared Camera and Multi Object Spectrometer were successfully commanded from safe to operate modes, things didn't go quite so smoothly with the Advanced Camera for Surveys (ACS).

The agency explained that on 16 October "an anomaly occurred during the last steps of the commanding to the ACS". Its report elaborated: "When the low voltage power supply to the ACS Solar Blind Channel was commanded on, software running in a microprocessor in ACS detected an incorrect voltage level in the Solar Blind Channel and suspended ACS.

Several hours later, the Hubble computer "sensed the loss of a 'keep alive' signal from the NASA Standard Spacecraft Computer in the SIC&DH and correctly responded by saving the NSSC-I and the science instruments."

A NASA team duly investigated the matter, and now concludes that the ACS Solar Blind Channel's voltage woes were not caused by a hardware problem with the camera. NASA's update explains: "The anomaly was because of a limit-checking algorithm that triggered before the data that it was checking was valid. A commanding change on the instrument will eliminate this condition and both teams expect a nominal low voltage power supply turn-on when it is commanded on next week."

Regarding the "sudden halt" of the computer, the team found "that three separate events occurring with near-simultaneity were responses to a single triggering event".

Specifically, this triggering event was "most likely caused by a self-clearing short-circuit, or a transient open-circuit, in the SIC&DH system". NASA concludes: "One or more such events would not be highly improbable in hardware inactive since 1990, and will not harm the telescope, although it could cause another interruption of science operations."

Hubble's computer system is a venerable 486 piece of kit, installed in 1990 to replace the original 1970s vintage DF-224. NASA has details of its Intel-based "advanced new computer" here (pdf). ®

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