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 An Interview with
Dr. Harvey Tananbaum, Director
Chandra X-Ray Center
August 26, 1996
Background:
The Advanced X-ray Astrophysics Facility (AXAF) is scheduled for
launch aboard the Space Shuttle in August 1998. We present here the
second of a series of interviews with Dr. Harvey Tananbaum, Director
of the AXAF Science Center, on the status of the project.
A major accomplishment during the period from August 95 to August 96
was the coating of the mirrors at Optical Coating Laboratories, Inc.
in Santa Rosa, California. The mirrors were painstakingly cleaned and
then precisely coated with layers of chromium and iridium, less than
a millionth of an inch thick. The successful grinding, polishing and
coating of the AXAF mirrors were historic technical accomplishments.
They are the smoothest and cleanest mirrors ever made. After the
coating process was completed, the mirrors were transported by
air-ride van to Eastman Kodak Company in Rochester, New York, where
they are being assembled into a support structure called the high
resolution mirror assembly and aligned with one another.
Other important tasks underway during this time were the development
of the scientific instruments which will record the direction of
arrival and the distribution with energy of the X-rays focused by the
mirrors, as well as the spacecraft module that will house the
instruments and electronics that enable the data to be transmitted
back to Earth.
Q: When we talked to you about a year ago, the
X-ray mirrors were scheduled to be coated. We understand that it went
well.
HT:The coating was very successful. All the mirrors met the
requirements, and the job was completed essentially on schedule.
X-ray measurements on witness samples in our labs at SAO
(Smithsonian Astrophysical Observatory) by Dr. Suzanne Romaine and
her team confirmed the excellent quality of the coatings. Everyone
was very pleased with OCLI (Optical Coating Laboratories, Inc., in
Santa Rosa, CA.). We will use them again if the opportunity
arises.
Q: How are other aspects of the program
going?
HT: This has been a challenging year in four areas: the
construction of the mirror assembly, development of the CCD
detectors, building the science instrument module; and keeping the
weight on the observatory down. Ball (Ball Aerospace and Technology
Corporation) had to deal with a number of issues involving the
structural integrity and thermal design of the science instrument
module. The weight of the ACIS (AXAF CCD Imaging Spectrometer) went
up by approximately 50 pounds, further complicating the
situation.
Q: Is this increase in weight a
problem?
HT: It has been a challenge to the team because the overall
increase has put us close to the limit allowed for the focal plane
end of the observatory. The limit involves both the maximum weight
that can be carried and the vibration loads to be experienced in
the Shuttle. We had to update the predicted vibration loads taking
into account the increased weight and other updates to the
Observatory design. All of this resulted in a schedule impact as
well.
Q: Will the delays on the science instrument
module impact the testing schedule?
HT: No. We were able to rearrange the sequence of events and Ball
built a special carrier to support and move the science instruments
at the X-ray Calibration Facility at Marshall Space Flight Center.
After calibration at MSFC, the science instruments will be shipped
to Ball around July 1997 for integration into the science
instrument module, so the overall program schedule or the August
1998 launch date will not be impacted.
Q: How about ACIS?
HT: They (the Penn State/MIT Consortium) fell behind schedule for
several reasons. They had difficulty in getting good CCD
(charged-coupled devices, electronic semiconductors used in digital
imaging devices on telescopes and in digital cameras and
camcorders) chips, then they encountered a series of technical and
personnel issues--the launch of the Rossi XTE (X-ray Timing
Explorer) satellite was delayed; that prevented a few key people
from joining the ACIS team at the anticipated time--and finally,
the flexprints, or connectors that link the CCDs to the data
processors, failed during testing. They seem to have all that
straightened out and to be coming along for a delivery of one unit
in early March and the full set of flight CCDs in early
April.
Q: How did the mirror assembly at Kodak
go?
HT: The assembly of the mirrors was one of the most technically
demanding tasks of the entire program. At least as challenging as
the grinding and polishing of the mirrors. That's why we practiced
with the VETA (Verification Engineering Test Article). Four pairs
of heavy, fragile mirrors had to be aligned to an accuracy of a
tenth of an arc second. That is equivalent to aligning them to an
accuracy of about a fiftieth of the width of a human hair. Yet they
had to be secure enough to survive launch--an incredibly difficult
engineering feat.
Q: What sort of difficulties did they
encounter?
HT: One example will give you an idea of what they were up against.
To minimize the distortion due to gravity, they assembled and
aligned the mirrors in a vertical orientation in a 60-foot tall
tower. They used lasers and mirrors to test the alignment. They
found that the body heat of the technicians changed the air
temperature by a fraction of a degree and threw off the
measurements. Opening and closing the door to the tower would set
up air currents that would cause errors. Or turning the light in
the tower on and off. So, they left a bank of eight fluorescent
bulbs on all the time. The problem was that these light, which were
ten feet away, caused temperature differences of a few hundredths
of a degree, enough to change the local density of the air, its
index of refraction, or ability to bend light, and produced a
mirage effect which changed the focus of the test beam ever so
slightly. So they had to set up a procedure wherein they kept the
lights on only for three or four minutes at a time. That's the kind
of problem they had to deal with, day in and day out.
Q: But they met the challenge.
HT: It appears that they have. But we won't know for sure until we
finish the testing at the X-ray calibration facility at Marshall.
As I said in the beginning, it has been a challenging year, but I
like to think that the team is meeting the challenges. A program
like this doesn't run without difficulties. We have overcome these
difficulties. I hope that we are as effective in dealing with
whatever crops up over the next year.
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