The Fermi Gamma-ray Space Telescope was launched to study gamma rays, not sunshine. Yet that's what it has done, most recently last week, when one of its instruments registered signals from a solar eclipse.
Shawne Workman writes in today's edition of SLAC Today :
During Wednesday's total solar eclipse, the moon blacked out the sun from vantage points in India, Asia and the Pacific Ocean for as long as 6 minutes, 39 seconds. Of course, it also shadowed the intervening space, between the moon and Earth—including, by chance, a swath of the Fermi Gamma-ray Space Telescope's orbit.
The telescope passed through the eclipse at roughly 3:30 Universal Time (3:30 a.m. at zero longitude, or 8:30 p.m. PDT). The main power voltage to the Large Area Telescope took a dip as the sun's power-charging rays hid behind the moon. The eclipse created a downward spike in the LAT's regular cycle of increasing voltage as the battery charges in the sun, followed by a drop as the battery discharges during the telescope's brief night.
It turns out that the sun itself is a source of gamma rays, although very faint ones. They're created when high-energy cosmic rays hit the sun's atmosphere. So the LAT can watch the sun moving across its field of view in a matter of hours against a background of stars, and is monitoring those emissions around the clock and in high quality for the first time, according to principal investigator Peter Michelson. Read more here from the Feb. 19, 2009 issue of SLAC Today. Here's a scientific poster about that work, as well as coverage of Michelson's talk in science journalist Ivan Semeniuk's Embedded Universe blog.