In the 1980s, PSL collaborated with the Haleakala Observatory on Maui to build a telescope ("Equatorial Mount") to detect gamma rays. In 1994 PSL built a similar telescope ("GASP" or Gamma Astronomy-South Pole) with the UW-Madison Physics Department which was installed at the South Pole. Both telescopes measure gamma rays by detecting the Cherenkov light created when cosmic rays (which are subatomic particles of enormous energy) pass through the air faster than light travels through the air. The light is collected by the multiple mirrors of the telescopes and focused onto clusters of photomultipliers which record the signal and send it to a computer for processing.