Scanning the entire sky in gamma-rays, photons with over 50 million times the energy of visible light, the Fermi mission's Large Area Telescope (LAT) explores the high-energy universe.

The heliotropic, great-circle and Hipparcos telescope reference frames are explicitly defined in the light of the scanning law of Hipparcos satellite.

Then a cross scanning of beam is presented to adjust the laser beam until it falls in the center of the telescope field of view.