Getting a clearer read on stars

On a broad mesa 12 miles southeast of Flagstaff, Ariz., astronomers are testing the notion that when measuring some of the brightest stars in the sky, six eyes are better than one.

The mesa is home to a unique array of telescopic "eyes" - light-gathering mirrors that allow astronomers to study the surfaces of stars, search for extrasolar planets, and map positions of stars with high precision.

The array, dubbed the Navy Prototype Optical Interferometer (NPOI), is already yielding insights into nearby stars. And the lessons the development team learns here are expected to benefit a similar but larger array planned for New Mexico's Magdalena Ridge, near Socorro. Earlier this month, Magdalena Ridge project officials signed a contract for the first of 10 telescopes that will dot the site. The team expects to generate its first images in 2007.

Although the NPOI is still a work in progress, astronomers using it have discovered that familiar stars thought to be lone suns in the night sky actually are double-star systems. They've mapped brightness variations across the surface of stars, leading to deeper insights into the stars' properties. And they've given detailed shape to shells of hot gas that old stars are ejecting before they go through their final death throes.

To accomplish this, the facility uses a technique called interferometry, in which light from the six widely spaced mirrors is gathered and combined to yield images of astronomical objects.

The array is decidedly modest compared with behemoth "light buckets" such as the twin Keck telescopes in Hawaii. The light-capturing mirrors at Keck are 10 meters across, compared with 50 centimeters for the NPOI. But when it comes to ground-based telescopes distinguishing two tightly-spaced objects, the NPOI can't be beat - at least not yet. The spacing of the array gives the facility the resolving power of a mirror 437 meters across, far surpassing the resolution of any single ground-based telescope.

"If you could turn the array on its side and look at the San Francisco peaks" some 20 miles away, "you could measure the diameter of a mouse's eyelash" on the summit of the tallest peak, enthuses Nathaniel White, a senior astronomer at the Lowell Observatory in Flagstaff who heads the NPOI development effort.

Much of the demand for this precision comes from the US Navy, which is charged with accurately measuring the positions of stars for navigation. In addition to the six movable mirrors, the facility hosts four more dedicated to astrometry or star-mapping. The Naval Research Laboratory is also using the interferometer to study the distorting effects of Earth's atmosphere on light passing through it.

In these days of global-positioning satellites, it may seem strange to think in terms of celestial navigation, Dr. White acknowledges. But even GPS satellites need a frame of reference from which they determine their own positions and develop their vaunted accuracy. Stars supply that reference frame, he says. The more accurate the stars' positions can be measured, the more accurate the GPS system will be.

Beyond earthbound navigation lies a need to navigate spacecraft as they hurtle to planets and asteroids. The first interstellar missions, which probably won't launch before the end of the century, would push celestial navigation to new limits, notes Kenneth Johnston, scientific director of the US Naval Observatory in Washington. And while NPOI won't achieve the accuracy such long-range missions would require, some of the techniques pioneered here will be useful in designing space-based observatories for astrometry that will achieve those accuracies.

The interferometer here takes a page from its radio-astronomy counterpart, the Very Large Array in Socorro, N.M. The six mirrors, or siderostats, are mounted at regular intervals along a Y-shaped set of tubes. The mirrors aim light down into optical devices, which send a tightly focused beam through the tubes to a central lab. There, astronomers combine the light and produce the images.

Unlike radio and infrared interferometers, NPOI is working at shorter wavelengths of the electromagnetic spectrum. At wavelengths that short, optics must be aligned to within millionths of a meter. That requires stable lasers for measuring that alignment of optics and lightning-fast computers to adjust them. These help ensure that light beams from mirrors at varying distances arrive at the detector simultaneously and with little dispersion. Without that on-time arrival and tight focus, the sharply defined images would turn into visual mush.

The challenge of maintaining beam quality over distances of up to a kilometer or more is preventing the group from moving the siderostats out to their fullest distance - which translates into the facility's highest resolution. Moreover, Dr. Johnston says, the group is trying to develop optics that allow astronomers to take full advantage of all the light the mirrors collect. Up to now, they've been able to use only the light hitting the central 20 centimeters of the mirror, substantially limiting the faintness of the objects they can study. Johnston anticipates that it may take another year or two to clear these hurdles.

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