Laura Lewis

NASA Ames Research Center, Moffett Field, CA

(Phone: 650/604-2162, 650/604-9000)

Edna DeVore

USRA/SOFIA Public Affairs

(Phone: 650/604-2130)

RELEASE: 99-65


NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA) project
team has achieved a major milestone toward completing the world’s largest
airborne observatory.

French engineers reconfigured the 2.7-meter primary mirror of the telescope
to be light enough to meet the stringent weight requirements of an airborne
telescope, yet strong enough to withstand the open-cavity environment it
will experience mounted in a highly modified Boeing 747SP flying at nearly
41,000 feet.

“This is the largest telescope mirror ever to be lightweighted using a
mechanical cutting process,” said SOFIA Chief Scientist Eric Becklin of the
Universities Space Research Association. “There were definitely some
technical challenges and some risk of damaging the mirror. However, the
mirror came through in excellent condition, and we can now move on to the
more conventional figuring process.”

The painstaking lightweighting process required removing over 3,600
kilograms (approximately 7,900 pounds) from the back of the SOFIA primary
mirror. During the nearly 18-month process, the mirror’s weight was
reduced by over 80% from 4,500 kilograms (9,922 pounds) to 880 kg (1,940
pounds). The lightweighting procedure, that used a huge, high-precision
milling machine, resulted in a strong, lighter-weight mirror with a
honeycomb back.

Schott Glasswerke of Mainz, Germany cast the mirror blank. Engineers at
the French company, REOSC, are now beginning to grind and polish the front
surface of the mirror. This phase is expected to take approximately a year.
REOSC has produced primary mirrors for the European Southern Observatory’s
Very Large Telescope (VLT) and the international Gemini project.

The SOFIA mirror will be figured to a focal ratio of f:1.28. The
complete telescope system, a Cassegrain with Nasmyth focus, will have a
focal ratio of f:19.6. It will operate primarily in the infrared and
submillimeter region of the spectrum, from .8 to 1,600 microns, but will
also be used in visible wavelengths. The entire telescope assembly is
expected to weigh about 20,000 kilograms (approximately 44,000 pounds).

Under an international agreement between the United States and the German
government, the German Aerospace Center (DLR) is responsible for the design
and construction of the SOFIA telescope. In return for supplying the
telescope and part of the ongoing funding of the observatory operation, the
Germans will receive 20% of SOFIA’s observing time to allocate to its
science community. A consortium of MAN Technologie and Kayser-Threde
companies in Germany is designing and building the telescope for the DLR.

SOFIA, with its 2.7-meter (106-inch) telescope, is NASA’s next generation
airborne observatory, replacing the now-retired Kuiper Airborne Observatory
that had a .9-meter (36-inch) telescope. SOFIA will be based at NASA’s
Ames Research Center at Moffett Field, CA, and is expected to become
operational in 2002. From its 41,000-foot vantage-point, astronomers will
be above more than 99% of the infrared-absorbing atmospheric water vapor
that limits what they can study using ground-based observatories.

SOFIA will provide an excellent platform for the study of black holes,
galactic evolution, the chemical composition of interstellar gas clouds,
complex organic molecules in space, and how stars and solar systems form.
SOFIA is being developed, and will be operated for NASA by a consortium led
by the Universities Space Research Association.

For more information about SOFIA, visit the SOFIA home page located at

Color images with captions to accompany this story are available at the
SOFIA web site:

Additional background about SOFIA is located at