During the operation of astronomical telescopes, laser beams frequently are emitted into the atmosphere for satellite or lunar ranging. They also can be used to create artificial guide stars for correcting image aberrations. The Federal Aviation Administration (FAA) requires that, when a laser beam is used, one or more individuals must be present outside the telescope to spot airplanes that might intersect the beam’s path and to tell scientists to shutter the beam. To make it easier to locate the aircraft, and to eliminate the need for human surveyors altogether, researchers from the University of California, San Diego (UCSD), created a detection system that can identify the presence of an airplane by its onboard transponder and shutter the laser beam when the aircraft is within 15° of it. The system is important for a pilot’s protection from laser illumination that could interfere with his or her vision. Besides temporary loss of night vision, the pilot is subject to what Thomas W. Murphy Jr. calls the “startle factor” – the tendency to become distracted or alarmed when the cockpit lights up unexpectedly. Murphy, one of the device’s inventors, is an associate professor of physics at the university. Never the twain shall meet Transponders are FAA-mandated traffic control instruments found on all commercial and on most private aircraft. They indicate the plane’s location and flight direction. To detect a transponder, the new system is equipped with two antennas – a broader one with an angle of 90° and a narrower one with a 30° angle – that are aligned with the optical axis of the initial laser beam. The narrow and broad beams receive a portion of the transponder’s power – which ranges between 70 and 500 W – to measure the angular distance from the aircraft to the main laser beam. When the airplane is determined to be too close to the main beam, it is shuttered. The antenna array is mounted on the sky-facing side of the secondary mirror support structure on the Apache Point Observatory 3.5-m telescope. “This [mechanism] can simply be a wand that is lowered into the beam to prevent its exit from the laser enclosure,” Murphy said. It takes less than 30 µs for the system to react to a signal. If a plane comes within range of the detection system, and factors such as saturation of a signal or a fast-acting aircraft cause inactivity from the antennas, a backup responder, or signal processing unit, will shutter the laser beam, while also logging the aircraft’s identity and altitude. The researchers tested the method at the Apache Point Observatory in Sunspot, N.M., between December 2008 and August 2009. The device was coupled with a 3.5-m telescope used in a series of lunar ranging experiments named APOLLO (Apache Point Observatory Lunar Laser-ranging Operation). There were glitches during testing. At the university, which is near two major airports, false triggers from beam reflections caused the system to shutter the laser beam 80 percent of the time during peak hours of the day. “The high density of traffic around UCSD, together with an expansive clutter of buildings on the ground, led to overlapping signals and multipath interference, respectively,” Murphy said. However, because observatories are situated in more remote locations, the system should not experience any problems under normal conditions, according to William A. Coles, co-developer of the system and professor of electrical and computer engineering at the university. To ensure the effectiveness of the device and to alleviate some of the effects from obstructing signals, the researchers are refining their technique. “We are preparing a minor upgrade that will be insensitive to orientation and have somewhat improved rejection of multipath interference,” Coles said.