The Hobby-Eberly Telescope is getting a facelift.
Engineers lopped off the package of mirrors, instruments, and electronics sitting 60 feet above its primary mirror are placing it with one that is heavier and more sophisticated. The replacement will give the giant telescope a wider yet sharper view of the sky and support a new set of powerful instruments that will allow it to begin probing the mystery of dark energy.
The technological rhinoplasty is part of an extensive telescope upgrade for the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), which is scheduled to begin full science operations in 2014.
HETDEX will measure the positions and motions of more than one million galaxies, which will help determine how the rate at which the universe expands has changed over time. HETDEX also will look for patterns in the way the galaxies are distributed. These observations should help theorists determine the nature of dark energy, a force that is causing the universe to expand faster as it ages.
To accomplish its goals, HETDEX will use a new set of spectrographs, known as VIRUS, which will measure how fast each galaxy is moving away from us and make other observations.
To accomplish its work, HETDEX will need a wide view of the sky. HET currently sees a patch of sky just four arcminutes across, or about one-seventh the diameter of the full Moon. Such a view is too constricted for HETDEX, which will need to see thousands of galaxies each night.
"To understand what dark energy is we need a really wide field," says Karl Gebhardt, an astronomy professor at The University of Texas at Austin and a HETDEX project scientist. "With the current field of view, the telescope would take tens to 50 years to do what we want."
The solution is to increase the field of view to 22 arcminutes, which is slightly smaller than the size of the full Moon, allowing HET to see 25 times more sky with each observation than it does now. "That will permit us to do much more efficient surveys of the sky, and much faster surveys," says John Good, an engineering scientist at McDonald Observatory and design and engineering manager for the upgrades.
Achieving the wider field of view requires a new set of mirrors, known as a corrector, mounted high above HET's primary mirror. The mirrors collect light from the huge primary mirror and correct the images over the wide field of view, bringing them into sharp focus.
The corrector will be twice as big and about four times as heavy as the current one, which is too massive for the current telescope to support.
The solution? A facelift.
Most large research telescopes can move in any direction to track astronomical objects as Earth turns on its axis. To reduce cost, though, HET was designed differently. It can rotate from side to side, but it doesn't move up and down. Instead, the corrector package moves across the primary mirror's field of view on a mechanical system known as a tracker. The combination of this motion and the telescope's rotation allows HET to track targets for about an hour at a time.
The new, heavier corrector package, along with new electronics and a nest of optical fibers that feed light to the VIRUS spectrographs, will require a new tracker assembly. The total package will weigh more than 40,000 pounds (19,000 kg), or about five times more than the current tracker assembly.
"Basically, the tracker is a big robotics system," says Joe Beno, associate director of the University of Texas Center for Electromechanics, which is assembling the new tracker. "It's got about 13 different major actuators. Each one of those has a series of control and subcontrol loops and safety stops. So it has about 150 things in the mechanical system to position the optical package where we want it."
All of those pieces must work together to provide extremely precise control of the corrector. "The optical package has to be positioned within a cylinder that's 10 microns in diameter and about 10 microns high. A human hair is about 90 microns, so [the precision is] about a tenth of a human hair," Beno says.
"We have to track a spot that's less than a thousandth of an inch in diameter," adds Good. "When you're looking at small-scale dimensions like this, to that precision, the world becomes very springy and shaky. We like to think of it in terms of being just kind of a big, shaking Jell-O mold. You can actually walk up to the present telescope, which weighs 100 tons, while it's focused on a star and you can push it with your hand and you can move that telescope off of its track. So it gives you a respect and appreciation for how difficult it is to make a big, heavy machine track something that's hundreds of millions of light-years out in space."
Despite the increased size and weight, though, the new tracker should provide more accurate pointing than the current system, Gebhardt says. "The current tracker needs constant realignment and constant tweaking to get a good image quality. The new tracker is designed to take this into account, and to accommodate the motion of the telescope and the motion of the star as it goes across the sky much more accurately than the old tracker."
The new system will monitor the position of a bright star, called a guide star. A computer-guided system will move the tracker to keep the guide star precisely aligned, sharpening the telescope's view.
"A crucial part of the telescope upgrade will be the new optical feedback systems to keep the optical payload within the tight alignment that is required," says Gary Hill, principal investigator of HETDEX. The new system will monitor the positions and image quality of bright stars, and a computer system will use that information to guide the telescope and keep it aligned, sharpening the telescope's view.
The new tracker also will increase the amount of light HET can gather. Its primary mirror is 11 meters in diameter, but the correct mirrors gather light from only 9.2 meters of the mirror surface at any given time. The upgrades, however, will make it possible to use 10 meters of the mirror surface, increasing its light-gathering power by about 18 percent and maintaining HET's rank as the third-largest telescope in the world.
Engineers assembled the new tracker at the Center for Electromechanics, an engineering research group at UT-Austin's J.J. Pickle campus. Some of the components were off-the-shelf units, while others were custom manufactured.
In 2014, the entire package was disassembled and shipped to McDonald Observatory. HET has been shut down while the old tracker is removed and the new one is installed and tested.
"If we've given ourselves the green light in the laboratory here in Austin, we're anticipating that we will have worked out most if not all of the significant bugs in the operation of the system," says Good. "It'll be an interesting and busy time."
If everything goes well, the upgraded telescope should return to service later this year, with HETDEX beginning full science operations soon after.