Marshall Center space mirror technology helps Texas telescope improve its view
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.
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.
The following Web link contains more information on the Hobby-Eberly
Telescope:
