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Telescope Assembly
CDR, FORCAST CDR, CTT Systems 
This spring, a milestone took place
in Germany with the Critical Design Review (CDR) for the SOFIA
telescope assembly. Taking place at the MAN
Technologie offices in Mainz-Gustavsberg, the CDR's successful
completion served as a green light for final release of telescope
system design data and drawings, along with approval of manufacturing
plans.
CDRs are one-time-only events in the
lives of space agency programs around the world, serving as a bridge
between design and manufacturing stages. A successful CDR means
that the design is validated and will meet its requirements, is
backed up with solid documentation and has been proven to be safe.
In effect, it grants permission to begin manufacturing of hardware.
The SOFIA telescope assembly CDR was
unique because so many of the reviewers were based in the U.S.
and had to travel to Germany for the event. In order to maximize
reviewers' time onsite, a large number of documents were forwarded
to them in advance. The resulting pre-approvals of documentation
left time for unusually thorough discussion of what are known as "Review
Item Discrepancies", or RIDS. Typical of every CDR, RIDS are open
questions that need to be resolved before final approvals can be
granted.
The consortium of German companies
building the telescope for the German
Aerospace Center (DLR), including MAN Technologie and Kayser
Threde, showed that they have met the many challenges of building
a large, lightweight telescope. This is an ambitious project by
every measure, requiring a composite material (graphite epoxy)
for the structure and lightweighted optics on a complex suspension
assembly that isolates the entire 40,000-lb. telescope from the
aircraft, all while maintaining the structural integrity needed
to meet FAA requirements.
The telescope's primary mirror had
a separate CDR in 1997 due to the long time that it takes to fabricate
large, lightweighted mirrors. The mirror coating facility, which
will be housed in the aircraft's hangar at the Ames Research Center,
had its CDR in mid-June, and each of SOFIA's four facility instruments
are scheduled to complete their individual CDRs by this fall.
One of the first of the instruments
to have run the CDR gauntlet is FORCAST,
which successfully completed its CDR at the end of May. FORCAST,
short for Faint Object infraRed CAmera for the SOFIA Telescope,
is a mid- to far-infrared two-channel camera with selectable filters
for imaging in the 4-25 and/or 25-40 micron regions. Under the
guidance of Principal Investigator Dr.
Terry Herter of Cornell
University in Ithaca, New York, FORCAST is a facility-class
instrument, meaning it will be kept in the SOFIA hangar at the
NASA Ames Research Center on a permanent basis and will be run
and maintained by the observatory staff for SOFIA users.
Fact sheets, flyers and links about
all 10 initial instruments are available at FirstLight
Instruments
 The
qualification test unit of the SOFIA desiccant dryer
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As just one example, in Sweden, CTT
Systems AB, a specialty air conditioning company, is adapting
its Zonal Drying System to the SOFIA aircraft to combat condensation
on the telescope during the descent phase after each completed
mission flight. The dryer will provide the telescope cavity with
extremely dry air; this dry air distribution will continue after
landing, until the temperature of the telescope structure is
equal to ground temperature.
The 20-kilogram (44-lb.) dryer works
by taking air from the passenger cabin, drying it and then blowing
it into a targeted area of the aircraft (in this case the telescope
cavity), lowering the relative humidity of the air. Using the desiccant
wheel principle, the dryer is a cylinder-formed dehumidifier that
continuously returns processed humidity back into the cabin.
June
20, 2000
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