The main camera on the Hubble Space Telescope, which has revolutionized astronomy with its stunning pictures of the universe, has stopped working, engineers who work on the camera said Saturday. The Advanced Camera for Surveys, a third-generation instrument installed by a space shuttle crew in 2002, went off line Monday, and engineers are still trying to figure out what happened and how to repair it. The camera sent messages Monday indicating power supply voltages were above their upper limits and causing it to stop working. A bad transistor could be causing the trouble, Campion said. 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.