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  • Photonics Improves Sky Gazing

Photonics Spectra
Apr 2006
Advanced in photonics are improving telescope resolutions and enabling astronomers to discover planets around distant stars.

They may not physically go — boldly or otherwise — where no one has gone before, but astronomers often see what no one has seen before. Thanks to photonic innovations and techniques, the ability to uncover the secrets of the cosmos continues to advance.

Developments in adaptive optics, for example, promise to further overcome atmospheric jitter and to help ground-based instruments function almost as well as those in space. Astronomers are combining the power of multiple telescopes into the equivalent of a single, giant device by exploiting fiber optics. And new techniques spot Earth-size worlds circling other stars.

Making stars

The telescopes at the W.M. Keck Observatory sit on the summit of Hawaii’s dormant volcano Mauna Kea, 13,796 ft above sea level. Although the location offers some of the best conditions on Earth for observations, it is not perfect. Turbulence in the column of air that extends up from Mauna Kea distorts the light from distant objects.

A solution is to employ adaptive optics, with machinery minutely tilting and tipping the mirror or altering its surface to negate atmospheric distortions. This must be done constantly because the conditions are always changing.

David Le Mignant, an adaptive optics instrument scientist with Keck, noted that a figure of merit, the Strehl ratio, compares a perfect world with the actual one. Uncompensated, a terrestrial infrared telescope has a ratio of perhaps 0.5 percent. In space, the number is 100 percent. With adaptive optics, the ratio for a ground-based instrument approximates that of space.

However, there is a problem: a lack of stars — in particular, guide stars. By optimizing the image of these point sources using adaptive optics, everything in the telescope’s field of view also is sharpened. Unfortunately, natural guide stars are not common, and that limits how much of the sky can be observed to perhaps 1 percent, Le Mignant said.

Astronomers thus make their own stars by firing beams of 589-nm laser light 90 km into the sky, where they strike a layer of sodium atoms. The atoms glow, producing an artificial guide star of about magnitude nine or 10, or about 50 to 100 times as faint as the faintest star that can be seen with the naked eye.

Using a laser to excite a layer of sodium atoms 90 km in the atmosphere, astronomers create artificial guide stars that enable instruments to see as well on the ground as they would in space. Right now this can be accomplished only in the infrared, but with laser and detector advances, it may be possible to do so in the visible as well. Courtesy of W.M. Keck Observatory.

A laser guide star system has been in use at Keck for the past year or so. Besides the man-made star, the system requires the use of a natural star of magnitude 19 or brighter, which is about 1000 times as dim as the glowing sodium atoms. Despite this constraint, the artificial star technique offers some significant advantages.

“In laser star mode, we get to 50 percent of the sky, which is a tremendous breakthrough,” Le Mignant stated.

He noted that plans call for the use of multiple laser guide stars, with perhaps five bracketing the target. For reasons having to do with the power of the lasers and the stability of the atmosphere at various wavelengths, the best correction will be in the near-infrared, where the Strehl ratio could top 90 percent. In the visible, the correction will get the instrument to 30 percent or so. This next-generation adaptive optics project remains some years away, and pushing the technique into the visible will require both more powerful lasers and faster detectors.

Tied with fiber

There is a practical limit to the size of a telescope’s mirror, beyond which the pull of gravity and other problems distort the optical surface to the point that it is unusable. At least a partial way around this is to combine the light collected by many telescopes using interferometry. This makes them act as a single device, with a baseline set by the distance between the mirrors that determines the resolution of the combined instrument.

If their output is combined via interferometry, two telescopes have the resolution of a single instrument as large as the distance between them. At the W.M. Keck Observatory, astronomers are using optical fibers rather than free-space links in an interferometric setup. That switch could allow for longer baselines, if the right kind of fiber can be developed and other kinks can be worked out of the system. Courtesy of W.M. Keck Observatory.

Traditionally, this is done with mirrors, but that creates a problem for long baselines, said Julien Woillez, an interferometry scientist at Keck. Mirrors are not perfect, and if the light is transmitted through the air, it is subject to turbulence.

The solution may be to use a different transmission medium — specifically, optical fiber. “We believe that by using fibers, especially in the case where they have a very good transmission, you can match and maybe do better than what you would do with mirrors,” Woillez said.

Two telescopes have been successfully connected, with the outcome described in the Jan. 13 issue of Science. Plans call for linking as many as seven Mauna Kea telescopes, creating a near-infrared instrument 800 m in diameter that will be capable of resolving the equivalent of a paper edge at a distance of 100 miles. Eventually, such fiber connections could allow baselines that are miles long.

Such connections cannot be made simply by running fiber between the telescopes. For instance, the Mauna Kea project makes use of single-mode fiber that transmits only the wavelength of interest. Beyond that, the various arms must be matched to within an interference fringe, or less than a wavelength of light.

That, Woillez said, imposes restrictions on the instrument. Because fibers have dispersion, researchers must use fixed and matched arm lengths. As stars move across the sky, delay arms must adjust for that. This cannot be done solely by switching in and out of banks of today’s optical fibers because their dispersion is too high.

Some promising avenues could lead to the development of fibers with low enough dispersion. Telecommunications operators also would like fibers with low or zero dispersion. Woillez noted that astronomers, however, are interested in achieving no dispersion over much larger wavelength ranges. “No dispersion means that you can basically choose the amount of fiber that you’re going to introduce in each arm.”

Lenses in the sky

Other telescopes are working together, albeit unconnected, to spot exoplanets — planets around stars other than the sun — using gravitational microlensing, a phenomenon by which an object in the right position between an observer and a star can focus the star’s light so that it temporarily appears brighter than it normally would. For finding exoplanets, this approach offers some advantages — for example, it does not matter that the planet’s own signal is swamped by the star it orbits. A downside is that the effect does not last long, said astrophysicist Jean-Philippe Beaulieu with the Institut d’Astrophysique de Paris. The duration is on the order of a few hours to a few days, depending on the mass of the planet.

Nonetheless, the technique has been used to find the smallest extrasolar planet yet around a relatively normal star. At five times the mass of the Earth, the planet orbits an M-class red dwarf at three times the distance of the Earth to the sun. The star is some 22,000 light-years away, close to the central bulge of our galaxy.

Beaulieu explained that one to two telescopes are on the lookout for a microlensing event, watching millions of stars each night. When an event is detected, a swarm of telescopes starts the frequent and continuous monitoring of it so that the temporary brightening from a planet can be detected and the mass of the planet determined.

These observations are done with the Probing Lensing Anomalies Network team, a worldwide collaboration of telescopes. Because the systems in the collaboration are scattered around the globe, the likelihood that the weather will be clear and that it will be night when an event is detected is enhanced. The collaboration includes RoboNet-1.0, a prototype network of 2-m robotic telescopes.

Over the next few years, the technique should detect a number of small-mass exoplanets and should enable astronomers to calculate the probability that a given M-class dwarf star has a planet with one to 15 times the mass of Earth, Beaulieu said. That information could be used in theories of planetary formation, especially involving those with characteristics similar to Earth.

Using an interferometer, astronomers can detect distant planets by the small Doppler shifts they induce in stellar spectra. The light is captured by a telescope, sent through a Michelson interferometer and then sent through a spectrograph. The result is dispersed fringes. Because of its construction, the device utilizes a greater percentage of incoming starlight than other extrasolar planet detectors and can be used with smaller telescopes.

Spotting planets inexpensively

Another approach to detecting exoplanets is to use instrumentation to detect the slight Doppler shifts that an orbiting planet induces in the movement of its star. For this, astronomers typically employ a spectrograph, which extracts the Doppler shift information from only a small percentage of incoming photons.

A recently implemented technique instead uses an interferometer, which has the advantage of capturing 20 percent or so of incoming photons. A group from the University of Florida in Gainesville, Tennessee State University in Nashville, Instituto de Astrofìsica de Canarias in La Laguna on Spain’s island of Tenerife, Pennsylvania State University in University Park and the University of Texas at Austin used this method to detect a planet orbiting a 600-million-year-old star some 100 light-years away.

Jian Ge, a University of Florida professor of astronomy, said that the interferometer developed for this work not only captures more light, but also is less expensive than a spectrograph. The new device is a fixed-delay interferometer and is similar to ones used to precisely measure solar oscillations over a very narrow band of wavelengths.

The relatively inexpensive device is a few feet long and weighs about 150 lb. It can be used on telescopes with mirrors that are less than 1 m in diameter. Such smaller telescopes are more plentiful than their more powerful cousins, and it is easier to get observation time on them. That means that more stars can be observed than would be possible with other methods. To boost this even more, Ge plans changes to enable as many as 100 stars to be observed at a time. Plans call for a survey of stars, looking for planetary systems that might be similar to our solar system.

“We’ll look at about 20,000 stars in the next two years with this instrument,” he said.

He noted that this initially will be done only in the visible region of the spectrum but that an infrared version of the device also will be built. When the survey is completed, astronomers likely will have a good idea how many stars have planets circling them, the size of the planets, and the distance between them and their stellar masters.

When we do go where no one has been before, we’ll have an idea where we’re going.

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