In an agreement worth $500,000, NASA will purchase hundreds of custom silicon photodiodes from Centronic Inc. of Newbury Park, Calif., for use on the Space Station Freedom, which is set for launch in 1997. The photodiodes will be used to keep the telescopes and gyroscopes focused on a single point in space. Centronic has also supplied diodes for Stanford University's Gravity Probe B, an instrument used to test predictions of Einstein's general theory of relativity.