Suppose you had to focus a pair of binoculars blindfolded--and if you failed, a $700 million investment would be lost in space. That was essentially the task facing University of Minnesota astronomy professor Bob Gehrz, a member of the team that tested the optical performance of the mirrors on the Space Infrared Telescope Facility (SIRTF), the last of NASA's Great Observatories. When a Delta rocket lifts SIRTF into space Friday, April 18, Gehrz will be at Cape Canaveral to watch the launch of a mission whose success will be riding on its mirror optics.
In his position as a facility scientist with the SIRTF Science Working Group, Gehrz helped oversee the fabrication and testing of the telescope's optics. His mission was to make sure there were no more incidents like the one that befell the Hubble Space Telescope (HST), whose focus was initially blurry.
"The trouble with Hubble was that they tested its mirrors separately and didn't make sure they worked well together," said Gehrz, a former president of the American Astronomical Society. "Our motto on this project was, 'We test as we fly, and fly as we test.' We wanted to make sure there were no surprises."
This time, getting the optics right the first time was even more important. Because HST orbits Earth, astronauts were able to fix it, but SIRTF will not orbit Earth. Instead, it will orbit the sun in the same path as Earth but millions of miles behind. Therefore, once it's in orbit, no manual fixes will be possible.
SIRTF is designed to pick up tiny heat signals from some of the coolest and some of the most distant objects in the universe. To block out thermal noise, it must be shielded from Earth's and the sun's heat and be kept in a liquid helium-insulated chamber at a chilly five degrees K. -- that is, five degrees above absolute zero. That meant Gehrz and his colleagues had to test the optics at five degrees.
The telecope has two curved mirrors, a large primary mirror and a smaller secondary mirror, both made from ultralight--but strong--beryllium. During testing, a flat mirror, nicknamed OSCAR, was brought in to direct light through the telescope and into an infrared camera. To focus the telescope, the scientists adjusted the distance between the two beryllium mirrors by moving the secondary mirror. The mirrors are about 1.5 meters apart, and if the telescope is to work properly, the distance must be correct to within one three-hundredth of a millimeter. Also, for the testing to be as accurate as possible, OSCAR had to be perfectly flat.
"OSCAR was perfectly flat at room temperature, but when cooled to five degrees, it curved," said Gehrz. "It was very slight--the curve would have fit a sphere with a radius of three kilometers. But that, plus the fact that we couldn't eliminate the effects of gravity, meant there will be a small uncertainty in the focus when the telescope is put in orbit."
To deal with the uncertainty, Gehrz and Ed Romana of the Jet Propulsion Laboratory (Pasadena, Calif.) are leading a team of more than 20 scientists who will start snapping pictures when SIRTF reaches orbit and will watch how the image clarity changes as the telescope cools. The crunch will come after about 30 days, when they will decide whether to refocus the telescope. Because the reliability of the motor that moves the secondary mirror is uncertain, the team has agreed that they will make no more than three moves to get within one three-hundredth of a millimeter of perfect focus distance. But they can't see the results of a move until afterward, when they take a picture of a celestial object.
"If we find we' re in focus to begin with, we'll leave the focus alone," said Gehrz. "If not, we'll move it, and we have a strategy to figure out if we're moving in the right direction and how far it should go."
University of Minnesota colleagues Charles "Chick" Woodward, associate professor of astronomy, and Elisha Polomski, a postdoctoral fellow, will collaborate with Gehrz, who is a member of the SIRTF Guaranteed Time Observations Program, which allots time to members of the Science Working Group to do research on SIRTF. The University of Minnesota team will get 100 hours of time during the first year of operation. SIRTF's useful lifetime will be at least 2.5 years, the researchers said.
With SIRTF, the University of Minnesota astronomers plan research in four areas, said Woodward.
"In part, our SIRTF observations may dramatically change our ideas about the formation of Earthlike planets," said Woodward. "SIRTF may establish that the formation of terrestrial planets is commonplace in our galaxy."
"We hope to see objects from the beginning of the universe--objects too distant to be detected by telescopes operating on visible light," said Gehrz. "It should help us understand the way all kinds of celestial objects form and evolve."
OSCAR, made at the REOSC facility in France, consisted of fused silica glass made by Corning Glassworks in Canton, N.Y. The two working mirrors were made by Tinsley Optics (Richmond, Calif.) under contract to Ball Aerospace (Boulder, Colo.), where optical testing was carried out.