Cooling off Hubble's camera troubles - problems with Hubble Space Telescope's light detectors in the wide-field and planetary camera - Brief Article

Science News, June 18, 1994 by Ron Cowen

In April, Science News disclosed problems with the light detectors on Hubble's new wide-field and planetary camera (SN: 4/23/94, p.260). Operating the camera at -88[degrees]C, which is 10[degrees]C cooler than before, has now dramatically reduced the problem, says Hubble researcher John J. Hester of Arizona State University in Tempe.

At the warmer temperature, some stars appeared about 10 percent dimmer when imaged at the top of any of the detectors, known as charged-coupled devices (CCDs), rather than at the bottom. Scientists find that at -88[degrees]C, the variation is at most 3 percent, Hester says.

Another concern at the higher operating temperature had focused on individual CCD picture elements, or pixels, that became damaged by incoming radiation and produced "dark current." This current, a spurious signal generated even when no light falls on a detector, is common and usually easy to subtract from a CCD image. But the dark current produced by some of the damaged pixels on the Hubble camera varied over time in an unpredictable way. At the lower temperature, the troublesome dark current has declined significantly Hester notes.

Nonetheless, operating at a lower temperature means that contaminating material can more easily deposit on the CCDs, reducing their sensitivity in the ultraviolet. To rid the detectors of such deposits, scientists will warm the CCDs for several hours once a month.