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Southern Hemisphere's Largest Telescope Hamstrung by Optical Problems

As South Africa angles for a massive telescope project, its own SALT telescope has been offline for a year
South African Large Telescope



BRUCE DORMINEY

SUTHERLAND, South Africa—High on an arid plateau, an international group of visiting astronomers stands squinting into a simmering orange sunset. Giddy just to be here, they scramble atop a cluster of ancient volcanic boulders to peer over the edge of a vast camouflage green overlook. They have been attending an international conference in Cape Town on communicating astronomy with the public and are only too excited to finally be on the grounds of Africa's premier astronomical observatory.

"Here" is a grueling four-hour drive from Cape Town, where the nearly five-year-old Southern African Large Telescope (SALT) should be preparing for a night's observing. Instead, it stands silent. Its giant 10- by 11-meter, segmented mirror remains fixed; its control room's computer screens empty of celestial targets. SALT has been in repair mode since April 2009, and it probably will not be able to continue its ambitious scientific mission until June at the earliest. Although SALT's technical team of South African astronomers and optical engineers says that the telescope's international partners have been patient during the long delay, everyone is clearly anxious to restore the telescope to full health.

Billed as the single largest optical and near-infrared telescope in the Southern hemisphere—a new and improved version of the Hobby-Eberly Telescope (HET) in Texas—SALT has been hobbled by image quality issues from the get-go.

"We started to do first science towards the end of 2005 and detected this imaging problem in the first year," says David Buckley, director of SALT astronomy operations, from his corner SALT office at the Cape Town headquarters of the South African Astronomical Observatory (SAAO). "But it took four years to stop the science because we didn't know its cause; the only way to diagnose it was to do observations. We were allowing the telescope to limp along doing what science it could."

The imaging problems arose in what is known as the spherical aberration corrector, a grouping of four separate mirrors high above the telescope's primary mirror.

"To someone who's just looking at pretty pictures, it's fine," Buckley says. "To a professional astronomer, objects on the outside of these images look like cashew nuts with little tails on them. These images were compromised by a mirror alignment problem. But in April of last year, we knew how to solve it, and had the tools and the personnel to do it."

Inexpensive, but at a price
As large telescopes go, SALT was a bargain, costing just $20 million to build. But the same design features that kept its price down have led to its long-running complications. SALT's spherical-mirror telescope was just one-fifth the cost of a conventional fully steerable parabolic-mirror telescope, like those at either of the Keck telescopes in Hawaii. SALT's primary mirror can rotate a full 360 degrees, but its angle remains fixed at 37 degrees in the vertical, restricting the telescope's view at any given time to a relatively small ring of the sky.

With a parabolic mirror, the incident light rays all come to a common focus. But SALT's spherical mirror creates a curved focal plane that must be corrected for if the telescope's incoming photons are to be successfully focused for imaging or spectroscopy.

The spherical aberration corrector, or SAC, should have compensated for the fact that the light from different parts of the primary mirror ends up in different focal positions, says Encarni Romero Colmenero, a SALT staff astronomer. Light from the celestial target first hits the spherical primary mirror, bounces up to SAC, hits all four of its mirrors and finally ends up at one focal point.

But that is not what happened. Instead, SALT's images were distorted by the relative motions of the mirrors inside the corrector, caused in large part by how SAC's mechanical interface was bolted onto the bottom of the telescope's tracker. The tracker, located above the primary mirror, follows the telescope's celestial targets as Earth rotates. In contrast, a conventional telescope's whole structure moves to track its targets.

"The telescope has been down so long because we had to dismantle SAC and check that the mirrors had not been damaged; then put them back and realign them," says astrophysicist Phil Charles, director of the SAAO. "We've completely redesigned the mechanical mounting that holds the mirrors and how it connects with the tracker."

The observatory's future
Once SALT is back online, it will still be hobbled by the country's lagging telecom system, which already caused the telescope some data communication problems. While Cape Town has become a trendy, cosmopolitan city with an infrastructure more than ready to host World Cup soccer in June, South Africa overall remains a developing nation.

Charles says such optical and data-transmission issues will not permanently damage the SAAO's reputation for world-class astronomy. After all, as Buckley points out, large, state-of-the-art telescopes are often besieged by engineering glitches—to wit, Arizona's new Large Binocular Telescope is at least a year and a half behind schedule. The troubles appear not to have dampened the expectations of SALT's future users, since it has seemingly never been a question of whether the problems can be fixed, but when. Charles notes that even after the problem with the image corrector had been discovered, SALT brought in two additional partners: the American Museum of Natural History in New York City and India's Inter-University Center for Astronomy and Astrophysics.

SALT's 15 institutional partners in seven countries plan to use the observatory to study everything from asteroids to dark energy. And although partner astronomers are welcome to apply for observing time, Buckley candidly advises them not to show up expecting to oversee their own observational campaigns.

"This is a queue-schedule telescope," Buckley says. "Any one night it may be doing a dozen observational programs. So you don't want some novice asking which button to press." Astronomers using SALT will instead run remotely submitted observing programs, which advocates of this approach claim is a much more efficient way of using telescope time.

Queue scheduling means SALT can investigate objects on timescales from days to years. Its specialty will be time-domain spectroscopy—useful for observing accretion by galactic black holes, surveys of distant supernovae and even searches for extrasolar planets. As Larry Ramsey, a Penn State University astrophysicist on SALT's board of directors, says, the telescope won't try to compete with Hawaii's Keck Observatory or Chile's Very Large Telescope array but will play to its own unique strengths.

Sister scopes
If SALT succeeds, it's good news for more than the astronomers who get observing time there. Buckley argues that the SAAO's ability to completely diagnose and repair SALT without outside help is testament to its capacity to build and run a large telescope. Meanwhile, South Africa is in competition with Australia to land the $1.5-billion SKA (Square Kilometer Array), a planned one-square-kilometer telescope comprising numerous radio dishes working in concert.

South Africa's prototype for the project is the Karoo Array Telescope (MeerKAT), an array of 80 radio antennas, each 12 meters across, that will be located 200 kilometers north of SALT. A dedicated 600-kilometer data line for MeerKAT, now under construction, will bring the telescope array's data stream to Cape Town.

It is too early to know whether South Africa will win its SKA bid, but despite a few internal squabbles, the country is taking steps to tackle its infrastructure shortcomings and its scientific need for new human capital. As for SALT, it now supercedes a 1940s-era 1.9-meter optical telescope, one of several smaller telescopes the SAAO operates at its Sutherland facility, as the largest in South Africa.

Back at Sutherland later that same evening, the visiting astronomers gather at the base of the SAAO's observing plateau, transfixed by the full swath of the Milky Way, the Southern Cross and the Magellanic Clouds before them. It's the kind of clear, moonless night that was so crucial for 15th-century European sea explorers steering their way south to the bottom of Africa. With any luck, the giant telescope will soon get up to speed, and such celestial treasure will take on wholly new scientific significance.

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