The Mirror Makers
The fight is on for the chance to build the world's most advanced space telescope.
- By Ben Iannotta
- Air & Space magazine, November 2001
(Page 3 of 6)
Unlike Danbury, Ball—the same company that made your grandmother’s canning jars—is not known as a maker of large optical systems like Hubble’s primary mirror, let alone one that would be 10 times bigger. Its niche has been the design and manufacture of small scientific instruments. It made most of the instruments and cameras that sit behind Hubble’s main mirror, converting raw light from the mirror into pictures of the heavens. Because of schedule delays, Ball got the nod to build the SIRTF telescope and instruments through subcontractors after NASA halted Danbury’s work on that telescope’s 4.9-meter mirror.
NASA officials want the NGST mirror to be six to seven meters wide. A mirror that size could not possibly be ground from a single piece of glass, the way Hubble’s 2.4-meter mirror was, and must be made from segments. In order to stay within the telescope’s 6,600-pound payload limit, these need to be seven to 10 times lighter per square meter of surface area than Hubble’s mirror. NASA wants the new mirror to weigh no more than 44 pounds per square meter.
The size of the mirror—more than double that of Hubble’s, larger than any conceived of for a spacecraft mirror—started as a bold challenge from NASA Administrator Dan Goldin. In January 1996, Goldin stood up in front of the American Astronomical Society in San Antonio, Texas, and flabbergasted the audience by calling for the Next Generation Space Telescope to be built with a mirror eight meters across.
Carnegie’s Alan Dressler remembers sitting in the audience. He had just finished leading a meeting of astronomers who hoped to whet NASA’s appetite for a successor to Hubble. The Dressler Committee report, “The Hubble Space Telescope & Beyond,” called for a telescope with a mirror at least four meters across, or two-thirds wider than Hubble’s—half the size of Goldin’s proposed mirror. Dressler figured that was about the right size to magnify infrared waves and capture images of the very early universe. A bigger telescope would improve the resolution of the images somewhat, but the main goal was to get a mirror to the Lagrange point, far from Earth’s heat, where the infrared waves would stand out against the cold background of space, he says.
To reach space, the mirror will have to be sent atop an unmanned rocket, folded up like the leaves of a table or the petals of a flower. It must unfold in space without jamming. Then it has to hold its shape for 10 years in temperatures close to –457 degrees Fahrenheit, the point at which matter no longer has any thermal energy.
Engineers aren’t entirely sure what the extreme cold will do to the segments, and flaws invisible to the naked eye would blur the telescope’s images. The mirror will be supported by numerous tiny, electrically controlled mechanical arms, called actuators, which will, in a technique called adaptive optics, nudge each segment into place after deployment. Then, perhaps once a month, NGST will look out to a reference star. If the image is blurry, ground controllers will send commands up to the telescope to nudge the mirror segments back into place.
The project is so daunting that the potential contractors are relieved that NASA has reined in the program slightly. Earlier this year, agency officials signaled their willingness to settle for a telescope of six to seven meters instead of eight.
On the other hand, some astronomers are convinced that bigger is better, and they completely support Goldin’s vision of an eight-meter NGST mirror.