The Measurement Astrophysics Research Group works on enhancing ground-based astronomy measurements.

by Steve Carr

In a unique collaboration that utilizes ground-based astronomy measurements, researchers from the University of New Mexico Measurement Astrophysics (MAP) Research Group and Georgia Tech are developing techniques using an atmospheric diagnostic tool that could revolutionize the precision of ground-based astronomical measurements.

ALE, or Atmospheric LiDAR for Extinction, an instrument funded by the National Science Foundation, is enabling astronomers to measure quantitatively the fraction of light from astronomical objects lost as it travels through Earth’s atmosphere.

“Light from distant galaxies travels towards Earth-based telescopes for millions of years, and in the last millisecond of its trip about 20 percent of that valuable light, which carries information about the very structure of the universe, is lost as it traverses our atmosphere,” says principal investigator and UNM Professor of Physics and Astronomy John McGraw.

“For centuries, astronomers have looked through the atmosphere with their telescopes, but have seldom looked at the atmosphere in an effort to precisely correct for this lost light,” he says.

ALE works by transmitting rapid pulses of eye-safe laser light into the atmosphere, which are scattered back to two detectors providing a distance-resolved profile of gas, particles and clouds, allowing astronomers to calculate precisely the amount of light lost in traversing the atmosphere. 

At its “first light” celebration at the UNM Campus Observatory last February, NSF Astronomy Division Director G.W. Van Citters activated ALE. A green laser light produced a beam that illuminated the night against a backdrop of cloudy skies. Within a minute, ALE was providing data on the features of the atmosphere at that particular time that might affect astronomical observations.

“The main goal of ALE is to determine extinction through the atmosphere,” says Julie Smith, one of the many graduate students working on research leading to a Ph.D. or M.S. in physics and astronomy or optics. “Extinction is caused by clouds and molecules and particles of dust and things like that which scatter and absorb light.

“With the telescope we shoot the laser up and collect all the backscattered light and from that information we can back out extinction coefficients. By doing so, we can apply that information to the photometric data we get from the main telescope, and we can correct our photometric images for the light loss in the atmosphere,” she says.

The result is a precision profile of the structure of Earth’s atmosphere. While Lidar technology is not new, ALE is the first instrument to be applied to precision astronomical measurements.

ALE is now in routine operation and has already provided valuable insight about the “astronomical atmosphere,” the atmosphere through which ground-based telescopes must observe. Even the clearest of nights show layers of light absorbing and light scattering materials for which astronomers need to make corrections if they are to make the most precise possible observations.

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