Not to shoot down aircraft in the name of science

Astronomers and air travel maintained a peaceful coexistence for over a century. That is, until astronomers started firing powerful lasers into the atmosphere -sky dwellers, human or otherwise, are no big fan of those. Bodies like the Federal Aviation Administration and US Space Command have imposed restrictions on the use of the high-powered lasers astronomers now routinely fire into the atmosphere to assist with their observations, to prevent anything or anyone flying across the sky from potential harm from the lasers. While astronomical lasers aren’t powerful enough to physically damage an airplane, they could easily injure pilots or passengers, and damage sensitive imaging equipment on Earth-observing satellites.

Last week scientists from the University of California at San Diego reported on their work to develop an integrated aircraft warning system for astronomical telescopes, that gives accurate warnings about approaching aircraft and helps minimise the observatory downtime due to passing planes.

The advent of adaptive optics systems on astronomical telescopes was the golden ticket for astronomers: no more blur and speckling from the turbulent gas in the atmosphere. These systems take light from a reference source and analyse in real-time the distortions in the image introduced by the atmosphere, and apply the necessary correction to the telescope optics to whip the image back into shape in an instant, without the need for lengthy post-processing. To perform this cycle several hundred times each second the target is observed requires a bright guide star that emits enough photons on timescales of milliseconds to perform the image analysis. Furthermore, as the size of turbulent cells in the atmosphere is finite, the reference star must be located close to the science target – typically within a few arcseconds’ range. This meant that adaptive optics could in practice only be used in very few cases. The golden ticket: not so shiny up close.

In the 1980s, inspired by atmospheric scientists using laser beacons to probe the upper atmosphere, French astronomers Foy & Labeyrie published a paper arguing for the use laser beacons to create artificial stars in the sky, near the science target, and use these for sensing the distortions from atmospheric turbulence. Their paper shows remarkable insight, and essentially tells us most of what we know today about laser guide stars: they correctly identified several mechanisms that could produce a bright spot in the sky – Rayleigh scattering around 10 km, or sodium fluorescence around 90 km, predicted the error in the turbulence measurement from the finite height of the backscattered spot, known as the cone effect, and even propose the use of multiple beacons to improve the accuracy of the measurement.

Single laser guide stars are in fairly routine operation on several of the world’s largest telescopes today, and they have enabled some fantastic new science, such as the study in amazing detail of the crowded region round our Galaxy’s central black hole.

The presence of humans, and of sensitive satellite equipment, poses a limit on the use of laser guide stars. At Lick Observatory, for example, lasers can only be fired between 11 pm and 5 am, when air traffic is light. Plane spotters outside the dome warn for approaching aircraft. That’s right, to use this piece of cutting edge technology requires an actual human being on the lookout. Dickensian or what?

The system developed in San Diego is simple but neat. Aircraft are in continuous communication with air traffic control radars and Traffic Collision Avoidance System. They are fitted with transponders that send out pulsed responses with information on the aircraft encoded. At Apache Point Observatory, the UCSD team simply fitted the sky-facing side of the telescope structure, i.e. the back of the secondary mirror, as pictured, with antennae tuned to the frequency of these signals and pointing along the same direction as the laser. Using a combination of narrow and broad beam antennae, the received signals can be decoded to work out the angular distance to the approaching aircraft. If one comes too close, the laser is shuttered.

The system was in operation for 7 months earlier this year at the Apache Point 3.5-m telescope and the authors report that all closure events were due to actual aircraft – so no false detections. Only when pointed at an actual airport did the antennae have some trouble because of the many overlapping signals.

Passing airplanes are not the main source of laser guide star downtime – at busy observing sites they are often shuttered because they get in the way of other telescopes’ observations – but they can certainly be a nuisance, not least for the grad students or engineers braving the cold to spot planes all night. With our future optical/infrared ground-based facilities all increasingly relying on adaptive optics and laser guide stars, a nice cheap and automated system like this one can make a big difference in the efficiency of modern day observatory operations.