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."