Space technology improves view of 3rd largest telescope
Posted: June 16, 2001

NASA engineers in Alabama have been climbing a Texas mountain for the past year to help astronomers reach deeper into space with the world's third-largest telescope.

The McDonald Observatory on Mount Fowlkes near Fort Davis, Texas, is home to the Hobby-Eberly Telescope. When astronomers there needed expertise in how to handle temperature extremes that affect the telescope's viewing capability, they hired the engineers of NASA's Marshall Space Flight Center in Huntsville, Ala.

Interior of the 91-segment Hobby-Eberly Telescope primary mirror. Photo: McDonald Observatory
With more than 30 years of experience developing sophisticated optical systems for space exploration, the Marshall Center is NASA's lead center for optics manufacturing and technology development.

The University of Texas at Austin, which owns and operates the McDonald Observatory, awarded the Marshall Center a $695,000 contract in November 1999 to design a Segment Alignment Maintenance System for the Hobby-Eberly Telescope.

John Rakoczy, lead engineer at Marshall working on the alignment system, said the project is a chance for Marshall's optics team to showcase its talents by working on ground-based telescopes in addition to those designed to operate in space. Other ground-based observatories could be potential customers of Marshall's optics facilities and team, Rakoczy said.

"By teaming our expertise with industries, not only do they benefit, but the space program benefits as well," Rakoczy said. "Working on the Hobby-Eberly Telescope gives the McDonald Observatory the benefit of our years of optics experience. At the same time, this project gives the Marshall team the opportunity to further our knowledge about working with segmented mirrors."

With a 36-foot (11-meter) primary mirror made up of 91 hexagonal segments, the telescope is the third largest in the world.

As telescopes have become bigger, both on the ground and in space, the reflecting mirrors that make them work are increasingly being made in segments - that is, with smaller mirrors fitted together to make one large mirror. Since even small temperature fluctuations can cause these mirror segments to move out of alignment, and thus limit a telescope's focusing capability, one remedy is to incorporate a system that will automatically keep the segments aligned and in focus.

"Temperature changes are the great enemy of telescopes," Rakoczy said. "Even a fraction of a degree can affect alignment of large, segmented mirrors. When you've got something as big as the Hobby-Eberly Telescope, you're trying to keep the mirror segments aligned within tens of nanometers."

A nanometer is equal to one-billionth of a meter -- a distance so small that it can't be seen with the human eye or even with a conventional optical microscope. For comparison, the head of a pin is about a million nanometers in diameter.

The alignment system uses electronic sensors to monitor the gaps between mirror segments. When the sensors detect any change in mirror alignment, the system compensates by sending computer-controlled directions to a series of small motors under each mirror segment. These directions are determined using highly sophisticated mathematical algorithms. Three motors, or actuators, are under each mirror segment and move the segment back into correct alignment.

The Marshall Center is developing the control system and software and is responsible for overall system integration. The center is teamed with Blue Line Engineering of Colorado Springs, Colo., which is providing the sensors and electronics.

"Blue Line is responsible for defining the overall system architecture and developing the sensor assemblies, local electronics, and distributed system control processing - everything in the dome," said Edward "Sandy" Montgomery, manager of the program for the Marshall Center. "MSFC is responsible for the part of the Segment Alignment Maintenance System residing in the control room."

A prototype system was successfully tested on a few of the mirrors between October 2000 and April 2001 with a design review completed in May. Final acceptance testing of the complete system will begin after fabrication and installation this fall.

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