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Adaptive Optics Affords Better View of Space

Photonics.com
Jul 2006
KIRTLAND AIR FORCE BASE, N.M., July 17, 2006 -- Most astronomers would probably say that a good telescope and good weather are necessary to capture a high-quality image of an object in space. But a scientist at the Air Force Research Laboratory (AFRL) Directed Energy Directorate would say true quality is only achievable with adaptive optics.

afrl21c.jpg"You could have a 100-meter (328 feet) mirror on a telescope, but without adaptive optics, you will get the same image resolution as an off-the-shelf 4-inch telescope," said Lt. Col. Dennis Montera, chief of the analysis branch at Starfire Optical Range, a research facility of the lab's Optics Div. at Kirtland Air Force Base. "You just cannot get the full use of the aperture without adaptive optics."

The Starfire Optical Range, an optical research facility operated by the Air ForceResearch Laboratory’s Directed Energy Directorate. (Photo: USAF)
Adaptive optics (AO) refers to optical systems that are able to compensate for optical effects, such as the twinkling of stars caused by air turbulence between the ground and space. In astronomy, the resulting higher-quality, higher-resolution images enable fainter objects to be detected and studied.

The Starfire Optical Range, which is outfitted with an adaptive optical telescope capability, develops optical sensing, imaging and laser propagation technologies to support Air Force aerospace missions. The Optics Div.'s mission is to conduct optical and imaging systems and to develop technologies to accurately propagate laser energy through a turbulent atmosphere. The division conducts experiments at the Starfire Optical Range, North Oscura Peak on White Sands Missile Range and at Hawaii's Maui Space Surveillance Site.

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The 3.5-meter telescope used in conjunction with the sodium guidestar laser system at Starfire Optical Range. (Photo: USAF)
Adaptive optics using laser guidestar techniques was pioneered at Starfire. The laser guidestar method uses a laser fired into the upper atmosphere to create an artificial star. The light from the artificial star received back at the telescope allows measurement of the distortion. Adaptive optics is then used to deform a mirror, to compensate for the turbulence, and enables scientists to get a sharper view of objects.

Craig Denman, a physicist with the directorate's Laser Div., and his team led the development of an all-solid-state sodium guidestar laser technology that can potentially help ground-based imagery surpass even that of the Hubble Space Telescope. The sodium laser is projected into the night sky, exciting a layer of sodium atoms in the upper atmosphere -- creating a very bright yellow light source that is used with AO to view space objects.

"Beginning as far back as 20 years ago, there was a need for the sodium guidestar laser," Denman said. "In the early 1990s, the Massachusetts Institute of Technology successfully developed a yellow photon solid-state laser for the Air Force Research Laboratory, but it never made it to the telescope because it was not compact enough," he said.

The first guidestar lasers used dyes as the lasing gain media and were very expensive and hazardous, he said. Denman's division devised, in-house, the first computer-automated all-solid-state sodium guidestar laser system that would work with Starfire's 3.5 meter telescope and that achieved 50 W of continuous-wave power.

The success of the technology, coupled with the adaptive optics built into the range's 3.5-meter (about 11.5 feet) telescope, provide 10 times more efficient photon return from the mesospheric guidestar, with five times more power than other systems, Denman said.

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The 3.5-meter telescope. (Photo: USAF)
Adaptive optics and guidestar technology at the laboratory were initially developed for defense and military applications, but the research also benefits the astronomy community, since AFRL scientists share research information with those in the astronomy field.

"The Air Force can spend time studying the intricacies of how to make guidestars better," Denman said. "It costs so much to do astronomy on the big telescopes that astronomers can't afford to do the more time-consuming laser and guidestar scientific studies that we can. It's a symbiotic relationship."

Equipment at the Starfire Optical Range includes a 1.5-meter (about five feet) aperture telescope, a one-meter (about three feet) aperture beam director, the 3.5-meter telescope and smaller telescopes used for atmospheric measurements. The work force includes approximately 30 military employees, 30 government civilians and 60 on-site contractors.

The 3.5-meter telescope that uses the sodium laser is 35 feet high and weighs approximately 275,000 pounds. Through a series of mirrors, light travels from the telescope to a central optics room housing primary adaptive optics and a tracking system. From that point, the compensated or uncompensated beam can be sent to any one of four optical laboratories.

By the end of the year, Starfire should also have its own mirror coating capability at the site's Telescope & Atmospheric Compensation Laboratory, which opened in 2004.

Cooperative research agreements with other observatories, such as Gemini and WM Keck, have been conducted from time to time to further astronomy research and Air Force technologies.


GLOSSARY
adaptive optics
Optical components or assemblies whose performance is monitored and controlled so as to compensate for aberrations, static or dynamic perturbations such as thermal, mechanical and acoustical disturbances, or to adapt to changing conditions, needs or missions. The most familiar example is the "rubber mirror,'' whose surface shape, and thus reflective qualities, can be controlled by electromechanical means. See also active optics; phase conjugation.
telescope
An afocal optical device made up of lenses or mirrors, usually with a magnification greater than unity, that renders distant objects more distinct, by enlarging their images on the retina.
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