The Measurement Astrophysics (MAP) Research Team in the University of New Mexico's Department of Physics and Astronomy, with the help of a three-year, $980,000 grant from the National Science Foundation, has devised instruments and techniques for acquiring more accurate measurements of the brightness of stars and galaxies using ground-based telescopes observing through the Earth's weather-dominated atmosphere.

The techniques developed at UNM will ultimately allow ground-based telescopes world-wide to measure stars and galaxies with space-based precision. The MAP Team will use these techniques to establish about 100 bright stars as brightness calibration standards for use by all astronomers, by spacecraft in orbit (such as weather satellites), and even by industry.

UNM Professor John McGraw and Research Assistant Professor Peter Zimmer are leading the team that includes UNM professors, research staff, graduate, undergraduate students and collaborators from other universities and national laboratories.

"Our current understanding of the universe is that about 25 percent of its mass is "dark matter" principally associated with galaxies, and that another 70 percent of its mass is represented by a more recently discovered "dark energy" that apparently causes the rate at which the universe expands to increase – the accelerating universe," explained McGraw. "The inference of dark matter and dark energy is based upon precise measurements of several types, including measurements made with ground-based telescopes."

However, no one can describe the nature of either dark energy or dark matter says McGraw.

"It is virtually certain that learning about the "dark universe" will require more accurate observations of the universe, including by the extremely large 10m – 30m (and even 100m) diameter ground-based telescopes now under design," said McGraw. "Understanding the nature of the universe requires more accurate astronomical observations, specifically the brightness of galaxies and the exploding stars that allow researchers to gauge their distances."

"All the really large new telescopes will be built on the ground, because it is far too expensive to launch more than a select few telescopes," said Zimmer, "so astronomers working at the visible and infrared wavelengths that penetrate Earth's atmosphere will have to contend with the variable amount of light lost as it passes through clouds, dust, haze, pollution and through the clear air itself."

The key breakthrough was the idea that if astronomers wanted to look ‘through' Earth's turbid atmosphere, they must also look ‘at' the atmosphere to determine accurately how much light is lost from distant objects as it passes through Earth's atmosphere. The UNM MAP team instruments will provide a way to correct for this light loss allowing ground-based telescopes to achieve the level of measurement precision required to investigate the ‘dark universe' and other research areas.

Built upon previous research by McGraw, the MAP team devised a "clear air" lidar that can routinely measure the absolute transmission of light through the atmosphere at one or a few specific wavelengths, or colors. They also implemented a simple telescope, an "objective grating spectrometer," with a pedigree dating to the late 1800's, to measure the amount of light lost at all visible wavelengths. Finally, they devised a sky camera that observes stars to check and confirm the accuracy of the standard star measurements.

"This suite of instruments, currently in operation at UNM's Campus Observatory, will be valuable for deriving accurate corrections for light lost in transiting Earth's atmosphere at observatories worldwide, wherever brightness measurements of stars and galaxies are being made," said McGraw.

The final result of this research, when completed, will be a catalog of constant stars calibrated to fundamental brightness standards maintained by the National Institute of Standards and Technology.

"This set of stars," said Zimmer, "referenced to international laboratory brightness standards, will be observed worldwide and used not only to calibrate astronomical telescopes, but any light sensing device that can see the sky. Thus calibrated standard stars have applications in many remote sensing fields, including climate change research, and defense applications, especially when the sensors requiring calibration are aboard spacecraft in orbit."

"The required repeated measurements of the standard stars will provide ‘a thousand points of well calibrated light'" said McGraw, "that illuminate Earth's atmosphere every clear night, allowing scientists to monitor the state of Earth's atmosphere, including the abundance of greenhouse gases such as water vapor, and in the future, carbon dioxide and methane."

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