An automated aircraft detection system to prevent illumination from the laser guide star beacons at the MMT and LBT

High powered guide star laser beams are a potential hazard for aircraft. Currently at the MMT telescope located on Mt. Hopkins in Southern Arizona, five Rayleigh guide stars create a total of 25 W of power at 532 nm wavelength. The ARGOS laser guide star for the Large Binocular Telescope (LBT) located on Mt. Graham in Southern Arizona will generate six Rayleigh guide stars with a total of 108 W at 532 nm. We present an automated system for use at the MMT and the LBT designed to detect aircraft and shutter the lasers when aircraft illumination is pending. The detection system at the MMT uses a single wide-angle CCD camera mounted to the optical support structure of the telescope. The LBT system employs two of the same CCD cameras, and an additional bore-sighted thermal infrared camera. The visible cameras integrate frames for 0.5 s to produce streaks from anti-collision beacons required for all aircraft. The IR camera serves as a backup and to protect unlighted aircraft. In each case, adjacent frames are compared using image processing software to detect streaks and movement in the field. If an aircraft is detected, the position and projected trajectory are calculated and compared to the position of the laser beams. If an aircraft illumination appears likely, the laser safety shutter is closed and a message is sent to the laser operator. As a safety precaution, a heartbeat signal from the control computer is required to keep the laser shutter open.