The main camera on the Hubble Space Telescope that has revolutionized astronomy with stunning pictures of the universe has stopped working, an instrument specialist who works with the camera said Saturday.

The Advanced Camera for Surveys, which increases the visual sharpness of the Hubble by 10-fold, stopped working Monday, when it went off line. Engineers are still trying to figure out exactly what happened and how to repair it.

"It's still off line today," Max Mutchler, an instruments specialist at the Space Telescope Science Institute in Baltimore, said Saturday.

Engineers are hopeful the problem can be fixed, said Ed Campion, a NASA spokesman at Goddard Space Flight Center, in Greenbelt, Md., which is responsible for managing the Hubble.

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Campion said a bad transistor could be causing the trouble. If so, a backup could be used. Another suspicion is that some of the camera's memory was disturbed by a cosmic event. That could be fixed by reloading the memory.

"Both possibilities are things that can be resolved here on the ground," Campion said.

The camera sent messages on Monday, indicating power supply voltages were above their high limits and causing the camera to stop working.

"At this point, the ACS is in a safe configuration, and further analysis is ongoing," according to a statement by the Space Telescope Science Institute.

The institute reported that preliminary reviews of the telemetry and technical details about possibly affected components of ACS have been carried out. But the root cause of the problem is still unknown.

"Further analysis and testing revolves around low-voltage power supplies as well as analog to digital converters," the institute said in its statement. "Analysis of ACS images taken before the suspend event shows no anomalies of any kind."

For now, the institute said, no ACS science observations will be carried out. Measures are being taken to advance non-ACS observations to fill the available time.

The third-generation Hubble instrument consists of three electronic cameras and a complement of filters and dispersers that detect light from the ultraviolet to the near infrared.

The ACS was installed on the Hubble during a servicing mission to the orbiting space telescope in March 2002.

The development of the Advanced Camera for Surveys was a joint operation between Johns Hopkins University, Goddard Space Flight Center, Ball Aerospace and the Space Telescope Science Institute.

The Hubble was launched in April 1990. The orbiting telescope needs new batteries and gyroscopes to keep it working beyond next year.