Press Release

New Facility to Improve Airborne Telescope’s Clarity

By SpaceRef Editor
June 12, 2001
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A NASA airborne observatory’s images of space will be sharper and
more precise, thanks to a new mirror coating facility being installed
at the agency’s Ames Research Center, Moffett Field, CA.


Constructed of stainless steel, the mirror coating facility resembles
a huge pressure cooker.  It measures approximately 4.3 meters
(14 feet) in diameter, stands about 4.9 meters (16 feet) high, and
weighs 10 metric tons (22,000 pounds). Scientists will use it
periodically to recoat the 2.7-meter (106.3-inch) diameter
telescope’s primary mirror in NASA’s Stratospheric Observatory For
Infrared Astronomy (SOFIA).  Installation of the new coating
facility in the SOFIA Mission and Operations Center at Ames, located
in the heart of California’s Silicon Valley, will take several
weeks.  The coating facility supplier is Chart, Inc.,
Westborough, MA.


"We’re very pleased that this critical, unique element of
SOFIA’s ground support system has arrived here at Ames," said
SOFIA Project Manager Chris Wiltsee. "This facility will play a
major role in the future missions of SOFIA."


Scientists require SOFIA’s sophisticated telescope be kept
immaculately clean in order to ensure accurate astronomical
observations.  "Once a year, we will use the mirror coating
facility to replace the high-precision coating on the telescope’s
mirror," explained Eric Becklin, chief scientist with
Universities Space Research Association (USRA), NASA’s prime
contractor for the SOFIA project.  As needed, the top of the
stainless steel vacuum chamber will be lifted off and SOFIA’s mirror
will be lowered into the chamber, where it will receive a delicate
coating of aluminum that is about one 300th the thickness of a human
hair.  The total amount of aluminum that will coat the 60-square
foot mirror surface is roughly equivalent to the quantity of aluminum
that may be found in about one fourth of an average soda
can.

The coating process involves
vaporizing aluminum.  Inside the facility’s chamber is a
filament array system containing more than 60 tungsten filaments.
These are similar to the filaments inside many light bulbs, but much
larger, and are connected to a high-current, low-voltage power
supply.  Before beginning the coating process, the old coating
on the mirror is chemically stripped away and the bare glass is
thoroughly cleaned.  A worker enters the chamber and hangs
several strips of very pure aluminum on each filament.

The mirror assembly is then lowered into the chamber and everything
but the mirror’s front surface is shielded with special
materials.  After the top is reinstalled, powerful pumps remove
air from the chamber to create a near vacuum.  Next, the
filaments are electrified, and the resulting heat generated within
them vaporizes the aluminum.  The vaporized aluminum then
adheres to the unshielded mirror’s surface, thereby providing it with
a thin aluminum coating.  To minimize the build-up of dust on
the mirror surface that could damage the coating or otherwise degrade
scientific performance, scientists also will use pressurized carbon
dioxide gas to clean the mirror once a week.  A special
wand-shaped nozzle condenses the gas into "snow" as it
flows onto the mirror.


"Using the wand, the spray is directed across the surface of the
mirror at a glancing angle," explained Patrick Waddell, USRA’s
associate director of the SOFIA Mission and Support Group. 
"The carbon dioxide snowflakes carry the dust away." 
Although this dramatically slows down the need for coating, the
mirror will continue to degrade, according to Waddell.  To
further help keep it clean, workers periodically will also gently
wash the telescope’s mirror with a water-based liquid.


SOFIA’s astronomical observations will be conducted at an altitude of
about 41,000 feet aboard a modified Boeing 747SP aircraft operated
and maintained by United Airlines.  While using airborne
telescopes is not new, SOFIA will be the world’s largest and most
powerful, considerably larger and more sophisticated than its
predecessor, the Kuiper Airborne Observatory that was based at Ames
from 1971 to 1995.

NASA awarded a $484.2 million contract to Universities Space Research
Association,     Columbia, MD, in December 1996,
to acquire, develop and operate SOFIA.  Other team members
include Raytheon Aircraft Integration Systems, Waco, TX; United
Airlines, San Francisco; the University of California, Los Angeles,
Berkeley and Santa Cruz, CA; the Astronomical Society of the Pacific,
San Francisco; the SETI Institute, Mountain View, CA; and Sterling
Federal Systems, Redwood City, CA.  SOFIA’s complex telescope is
being developed by DLR, the German Aerospace Center, located in Bonn.
The specifications for the mirror coating facility were developed by
NASA Ames with assistance from USRA and the University of California
Observatories in Santa Cruz, CA.


Annual operating costs of SOFIA are anticipated to be about $40
million.  SOFIA’s first test flight is currently scheduled in
October 2003 at Raytheon’s Waco, TX, flight facility.  SOFIA is
scheduled to arrive at Ames in May 2004 for final testing preparatory
to full-scale operations starting in late 2004.  Further
information about SOFIA is available on the SOFIA web site, located
at: http://sofia.arc.nasa.gov

SpaceRef staff editor.