Adaptive Optics Research Reaches the Stars

While driving down Interstate 10 on a hot summer day, motorists may notice heat waves shimmering about the pavement ahead.

The blurry image shows the distortion that can occur from heating the atmosphere, said Michael Giles and David Voelz, electrical engineering professors at New Mexico State University.

For the past few years, Giles and Voelz have been researching techniques to correct the distortion that affects telescope images.

“Atmospheric distortion is what causes the stars to twinkle or to blink on and off. It also blurs objects,” Giles said. “What we are trying to do is remove that so we could get better images.”

Voelz said distortion caused by the atmosphere occurs all the time and is difficult to see with the naked eye. But when astronomers use large telescopes to see something through the atmosphere, objects can appear blurry.

To clearup the image, researchers have been working with adaptive optics systems -- systems that use a deformable mirror (a mirror that bends) or other optics such as a liquid crystal device that will correct for the distortions in the wavefronts that come through the atmosphere.

About two years ago, NMSU researchers received a $240,000 grant from the federal Defense University Research Instrumentation Program to buy specialized instruments for their optics laboratory. The equipment included a high-resolution wave front sensor system, two high-speed, high-resolution liquid crystal phase modulators, a phase-measuring interferometer system and a high-speed deformable mirror system.

Since then, Giles and Voelz have been involved in several celestial research projects that apply adaptive optics systems. Recently, NMSU has been performing a research task for the Magdalena Ridge Observatory (MRO), which is under construction in the Magdalena Mountains west of Socorro.

“The task is to investigate the possibility of implementing inexpensive adaptive optics systems on the 10 telescopes that will eventually comprise the array of telescopes used in the interferometer,” he said.

The MRO will utilize an imaging stellar interferometer, which consists of an array of movable telescopes each about 1.5 meters in diameter, Giles said. The telescopes are spread across an area whose dimensions can be changed from tens of meters to hundreds of meters just by moving the outer telescopes or increasing the separation between telescopes.

“The method we are proposing uses a single metric – the amount of light from the incoming light wave from a distant star that passes through a small pinhole placed at the focal point of the telescope,” Giles said.

He added that when the wavefront is distorted by the atmosphere, the focused spot of light will be spread out because the distortion in the wavefront actually causes it to blur. As a deformable mirror corrects the wavefront distortions, the focused spot becomes smaller and smaller so that more light passes through the small pinhole at the focal point.

“We will use this increasing light intensity passing through the pinhole as a measure of how well the deformable mirror is correcting the distortions in the wavefront,” he said. “This metric will be used to control the deformable mirror, changing its shape until the best possible correction of wavefront is obtained.”

Doctoral student Chueh Ting is using the project as his dissertation. His work should be done this summer.

Another project that taps into the expertise of NMSU faculty involves the National Science Foundation in association with the Air Force Office of Scientific Research (NSF/AFOSR).

Last November, Voelz and Nancy Chanover of NMSU’s astronomy department received a two-year $180,000 grant for a project called the “Spectral and Polarimetric Imaging of Solar System Bodies” from the NSF/AFOSR.

The researchers are trying to develop an understanding of the haze and aerosol properties in the lower atmosphere of solar system bodies such as Titan, Saturn’s largest moon.

Voelz and students from NMSU’s Klipsch School of Electrical and Computer Engineering constructed an Acousto-Optical Tunable Filter (AOTF) camera.

“The device is a narrowband filter that is electrically tunable so the camera is able to collect images at various selectable wavelengths,” Voelz said.

In January, the camera was sent to Hawaii, where the Air Force keeps its Advanced Electro Optical System telescope. The telescope is equipped with an adaptive optics system that allows the Air Force to see clearly through the atmosphere for space tracking and imaging missions.

“This system allowed us to collect clearer images of Titan,” Voelz said. “Without the use of adaptive optics, images of Titan in visible light would be significantly smeared.”

Voelz said the recent increased interest in Titan is due to the Cassini-Huygens space probe, which will reach the Saturnian region this July. Its mission is to study Saturn and its moons, and a highlight will occur at the end of the year when Cassini ejects the Huygens probe to drop into Titan’s atmosphere.

“Scientists hope that by studying the chemistry of Titan’s atmosphere, which has methane and ethane in it, they will be able to learn more about ours,” he said. “Hopefully, our work will help them a little bit.”

Copyright © 2004, Brian Webb. All rights reserved.