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News Release
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February 16, 1995
FOR IMMEDIATE RELEASE
CONTACT: Emil Venere
esv@resource.ca.jhu.edu
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Astro-2: A Shuttle-based Ultraviolet Astronomical
Observatory
By William P. Blair
The Johns Hopkins University
NASA's shuttle Endeavour is ready for the launch of mission
STS-67 on March 2, 1995. As is often the case, the crew is a
mixture of seasoned astronauts and eager newcomers. But cradled
in its cargo bay will be a cluster of unique ultraviolet
telescopes called the Astro Observatory, a set of mechanized eyes
to scan the heavens in ways unavailable to astronomers on the
ground.
In essence, the Astro telescopes turn the Shuttle into an
ultraviolet "observatory". The telescopes are interfaced to the
orbiter through various pieces of Spacelab hardware and are
totally dependent on the Shuttle for power, telemetry, and
pointing. To make the analogy complete, Astro will even be
operated in orbit by astronomers specially trained for this
purpose. The first flight, designated Astro-1, occurred in
December 1990 and lasted 9 days. During its maiden voyage,
Astro-1 observed everything from planets to quasars to the
tenuous gas of interstellar space. The flight of Astro-2 will
build on this experience in a number of important ways.
A TRIO OF INSTRUMENTS
At the heart of the payload are three telescopes, each designed
to expand the horizon of a particular aspect of ultraviolet
astronomy. They are called the Hopkins Ultraviolet Telescope
(HUT), the Wisconsin Ultraviolet Photo-Polarimeter Experiment
(WUPPE), and the Ultraviolet Imaging Telescope (UIT). These
telescopes all grew out of NASA's sounding-rocket program and
were under development for years before the first mission. As a
matter of fact, the three telescopes were next in line awaiting a
March 1986 launch to observe Halley's comet at the time of the
Challenger disaster in January 1986.
As its name suggests, HUT was designed and built at The Johns
Hopkins University in Baltimore, Maryland. It consists of a
0.9-meter (36-inch) f/2.0 primary mirror with a spectrograph at
the prime focus. The spectrograph's detector is an array of
electronic diodes fed by a microchannel plate intensifier. In
normal operation, HUT samples most of the far ultraviolet
spectrum (830 to 1860 angstroms) with about 3 angstrom
resolution. (One "angstrom" is equal to one hundred-millionth of
a centimeter, and is the unit of preference for measuring the
wavelengths of ultraviolet and optical light.)
This instrument is at its best from 900 to 1200 angstroms, a
region that is inaccessible to the Hubble Space Telescope. Here
we can observe a wide variety of objects (including quasars,
white dwarf stars, cataclysmic variable stars, and nebulae) for
the first time. Although HUT also can observe the extreme
ultraviolet band from 420 to 930 angstroms, this will only be
useful for very nearby objects such as those in our solar system.
Because of the opacity of interstellar hydrogen gas below 912
angstroms, it is not expected
that many more distant objects will be visible in the EUV band at
the sensitivity level provided by HUT.
WUPPE is the product of a team of scientists and engineers at the
University of Wisconsin, Madison. Its 0.5-meter (20-inch)
primary mirror feeds a spectrophotometer at the f/10 Cassegrain
focus that is sensitive from 1400 to 3200 angstroms. The
instrument has two intensified diode-array detectors and rotating
filters that determine precisely the polarization of ultraviolet
light as a function of wavelength.
Very little work has been done previously on the polarization of
light from stars, nebulae, and galaxies in the ultraviolet.
WUPPE opens this window, especially for stars that are too bright
for Hubble's Faint Object Spectrograph and for extended nebulae
too large for Hubble's small field of view to be effective. When
light is scattered by dust particles it acquires some degree of
polarization, so WUPPE measurements help elucidate the
composition and structure of dust in interstellar space and
circumstellar clouds. WUPPE was designed to have large dynamic
range, allowing the high precision necessary to detect even small
amounts of polarization.
In contrast to the spectroscopic capabilities of the other
ultraviolet instruments, UIT provides the observatory with
imaging capability. Developed at NASA's Goddard Space Flight
Center in Greenbelt, Maryland, UIT is a powerful combination of a
0.4-meter (15-inch) telescope, image intensifiers, and Kodak
IIa-O film. It is also equipped with a number of broad and
narrow band filters covering the region from 1200 to 3200
angstroms. Each film frame records a circular field with a
diameter of 40 arc-minutes and 2 arc-second resolution. This is
more than 250 times the size of the field of view provided by the
Wide Field/Planetary Camera 2 on HST. Carried in two
1,000-frame cassettes, the exposed film will be returned to Earth
for processing, digitization, and analysis.
Surprisingly, very little ultraviolet imagery of astronomical
objects has actually been obtained, and much of what exists has
come from short rocket flights or from high altitude
balloon-borne telescopes. During the night portion of a shuttle
orbit, UIT is able to detect blue stars to about apparent visual
magnitude 25, permitting studies of the spatial distributions of
hot stars in globular clusters and galaxies. By recording a
variety of target fields, UIT gives us our first generalized view
of the ultraviolet sky.
THE LEGACY OF ASTRO-1
The premiere flight of the observatory took place in December
1990 and lasted 9 days. In addition to the ultraviolet
telescopes, an X-ray instrument called the Broad Band X-ray
Telescope (BBXRT) was flown on a separate pointing system from
the ultraviolet telescopes. The mission had its share of
difficulties to overcome, including problems with the pointing
systems, waste water management on the shuttle, and electronic
problems on the aft flight deck. Ultimately, however, these
problems were solved or worked around and the mission must be
considered one of the most successful scientific shuttle
missions. As of January 1995, over 125 scientific articles,
including 75 in refereed journals, have been published by members
of the four instrument teams. In addition, another 11 papers have
been submitted for publication or are in press, and many others
are still in progress. Even more impressive is the wide range of
science that has been accomplished, covering everything from
solar system objects and the local interstellar medium to distant
quasars, from star clusters to galaxies to individual nebulae and
stars.
From its inception, the Astro payload was expected to have
multiple flights, but a long series of delays and schedule
pressures in the post-Challenger era had forced NASA to declare
that the Astro-1 opportunity would be the payload's only flight.
However, in light of the success of the first flight, NASA
reconsidered and in May 1991 NASA announced that Astro-2 was in
their plans. However, because of the similarity of the BBXRT to
the Japanese/NASA satellite ASCA, only the ultraviolet telescopes
will participate in Astro-2.
IMPROVEMENTS FOR ASTRO-2
The time since the first flight has been used to implement a
number of changes to hardware and procedures that should ensure
even more impressive results from Astro-2. Much effort has gone
into developing an understanding of the problems incurred by the
Instrument Pointing System during Astro-1. This will result in a
significant improvement in the quality and quantity of data
returned on Astro-2. In addition, the work and time scale
involved in preparing the pre-mission timeline has streamlined,
thanks to a task team that includes members of both the
instrument teams and NASA planners. Procedures and software for
improved real-time replanning have also been revamped, based on
the crucial role played by real-time replanning in the success of
Astro-1. Also, performing certain real-time procedures by way of
ground commanding should improve observatory operations and
efficiency.
HUT is the only of the telescopes to have undergone a significant
upgrade since the first mission, made possible by improvements to
the special coating materials used to reflect far ultraviolet
light. In particular, a material called silicon carbide has been
developed that has nearly a factor of two better reflectivity in
HUT's primary wavelength range. As the optical coating facility
at NASA Goddard Space Flight Center developed the capability to
coat larger and larger optics with silicon carbide, they coated
first the grating for the HUT spectrograph and then the HUT
back-up primary mirror. When subsequent tests showed excellent
coatings, both of these optical elements were incorporated into
the rebuilt HUT, with the result that a factor of three or more
better performance is expected from this component of the
observatory.
In addition to these improvements, another significant change for
Astro-2 is "community involvement." Although each of the
instruments was developed by a team of scientists and engineers
at a particular university or government facility, the
observatory has a wider appeal. In 1993 NASA solicited proposals
from the general astronomical community for participation in the
observatory's second flight. After scientific and technical peer
review, NASA selected 10 proposals for inclusion into the
scientific program. This has produced an even broader
perspective to the range of observations that will be attempted
and the scientific investigations that will be carried out. The
success of this limited Guest Investigator program can already be
evidenced in the rapport amongst the science teams and the
inclusion of Guest Investigators in mission planning and
real-time operations decisions.
THE MISSION PLAN
The Astro Observatory is operated directly by the astronauts from
the AFT flight deck of the shuttle or in combination with
controllers on the ground in the Payload Operations Control
Center at NASA Marshall Space Flight Center. The observatory
operates as an attached payload, with the shuttle and Spacelab
systems providing power, pointing, and telemetry. The Astro-2
mission is scheduled to be 16 days in duration, which will make
this the longest shuttle mission to date. The nominal plan calls
for a roughly 24 hour checkout period (although it took longer
than this on Astro-1), followed immediately by science operations
for the duration of the mission. Although various shuttle tests
and other activities (such as waste water dumps) are required,
everything possible is being done to maximize the time spent
observing a wide variety of astronomical objects. The
ultraviolet instruments are mounted on the Instrument
Pointing System (IPS), a Spacelab component developed for NASA by
the European Space Agency and used both on Astro-1 in 1990 and on
the Spacelab-2 mission in 1985. The IPS provides a stable
platform, keeps the telecopes aligned, and provides various
pointing and tracking capabilities to the telescopes. During
Astro-1 the IPS had some difficulties locking onto guide stars
properly, although an alternate technique allowed the astronauts
to manually point the IPS and track targets on the HUT TV camera
using a hand paddle (much in the same way this is done with
ground-based telescopes that do not have auto-guiders). In
general, the astronauts were able to provide pointing stability
of about 2 - 3 arcsec or better. However, in "optical hold", the
IPS should be able to achieve sub-arcsecond stability. Much work
has been done by a special task team put together by the mission
management team at NASA Marshall Space Flight Center to ensure
that the IPS works properly for Astro-2.
The ultraviolet telescope assembly will rest on two Spacelab
pallets in Endeavour's cargo bay, where it will ride into an
orbit some 350 kilometers (190 nautical miles) high and inclined
28.5 degrees to Earth's equator. A night launch is anticipated,
which will orient the orbit so that orbital passes through the
high particle background part of the orbit (the so-called South
Atlantic Anomaly, or SAA) will occur mainly on the daylit side.
High energy particles can affect instrument operation and
increase the background levels in electronic detectors. Since
the "natural" background (that is, scattered light and
ultraviolet atmospheric airglow emissions) is also higher on the
daylit side, this preserves the orbital night passes for
observations of the faintest (and often highest priority)
astronomical targets.
Once in space, the aligned UV telescopes are pointed by using a
combination of shuttle maneuvers and slews of the IPS. In
addition to tracking guide stars, the system will utilize a
complex image motion compensation system to try to eliminate
jitter during observations caused by crew motions and thruster
firings. This is particularly important for UIT to maintain the
quality of its imagery (since the images are recorded on
film).
The Astro-2 crew of seven includes four professional scientists.
Payload specialists Dr. Samuel Durrance from Johns Hopkins
University and Dr. Ronald Parise from Computer Sciences
Corporation and NASA Goddard Space Flight Center will control the
telescopes from Endeavour's aft flight deck. Both flew
previously on Astro-1 and provide a wealth of experience and
continuity to the project. Mission specialists Dr. Tammy
Jernigan and Dr. John Grunsfeld are career astronauts who will be
responsible mainly for the IPS and other Spacelab systems that
power the payload. Jernigan has flown twice previously and has
been designated the Payload Commander for Astro-2. Grunsfeld was
selected in the astronaut class of 1992 and Astro-2 will be his
first flight. Commander Stephan S. Oswald, pilot William G.
Gregory, and mission specialist Wendy B. Lawrence will control
the shuttle's ascent, descent, and general operations, as well as
performing the many maneuvers of the orbiter to point the
telescopes. Commander Oswald has piloted two previous shuttle
missions, while Astro-2 will be the first flight for Gregory and
Lawrence. Split into two teams, the crew will work 12-hour
shifts to keep the observatory operating
constantly while in orbit.
MISSION PLANNING AND GROUND OPERATIONS
Operating from low earth orbit poses some interesting challenges
to planning astronomical observations because of the constantly
changing visibilities of the objects to be observed. Using
special software that calculates detailed target visibilities and
checks constraints, the instrument teams develop a nominal
timeline of desired observations and turn this over to mission
planners at NASA Marshall Space Flight Center. NASA personnel
then implement this timeline into a pre-mission plan, checking
additional constraints and generating the supporting documents
and computer files necessary to support the mission. In
real-time operations, the scientists work closely with the NASA
personnel to implement desired changes to the pre-mission
timeline, adjusting as needed to events as the mission unfolds.
Astro-1 marked the first use of the new Payload Operations and
Control Center (POCC) at NASA's Marshall Space Flight Center in
Huntsville, Alabama. Since that time, numerous other Spacelab
and related payloads have been operated from this facility.
(Shuttle operations, however, are still accomplished from Johnson
Space Center in Houston.) Scientists and engineers from the
ultraviolet instrument teams are able to inspect the incoming
science and/or engineering data and troubleshoot any problems
that may occur. They will then uplink any necessary changes to
the observing plan or instrument operating configurations.
Science and engineering data from the observatory will be sent to
the ground in real time whenever the shuttle is in direct contact
with a TDRS communications satellite. At other times, this
information will be recorded on-board for downlink later. UIT
images are recorded on film that will be developed after the
shuttle lands, but HUT and WUPPE scientists will have access to
"quick-look" data in real time, offering the possibility of some
exciting discoveries during the mission. Eventually, all of the
data wind up at NASA Marshall Space Flight Center where they are
archived, transferred onto computer tape, processed, and supplied
to investigators within 60 days. Only then can detailed
calibration, reduction, and analysis of the scientific data take
place.
The astronomical observations planned for Astro-2 have been
specified by the teams that have dedicated years to building and
preparing the instruments. It is no small task to choreograph
some 415 pointings of the Shuttle at over 300 objects of interest
to one or more of the instrument teams. Hundreds of hours have
been spent by the instrument teams huddled in meetings selecting
the highest priority observations to be scheduled. After
generating a "science observations" timeline for the mission, the
planners at Marshall Space Flight Center have the daunting task
of determining the Shuttle's "maneuver" timeline, calculating
such things as the TDRS coverage and the mission "thermal
profile" (to prevent the Shuttle from getting too warm or too
cold), and finding guide stars and celestial roll angles for each
planned observation. Much of this work must be done in
conjunction with NASA's Johnson Space Center in Houston, which is
responsible for the operation of the Shuttle while in orbit.
The observatory is managed by NASA MSFC, which supervised its
development and integrated the telescopes with Spacelab's
electronics, the IPS, and other subsystems. Astro is a
complicated and expensive collection of instruments that
demonstrates some of the technical and logistical problems of
doing astronomy from space. While the "observatory" analogy
holds true in some respects, this observatory has to be able to
locate and track objects as faint as 16th magnitude while the
entire observatory hurtles through space at 5 miles per second!
In addition, each instrument team has had to overcome many
technological hurdles in creating these specialized machines.
The reason for all of this effort is simple: the potential payoff
in scientific knowledge is enormous. Each of Astro's instruments
should return about 700,000 seconds of data from Astro-2. A wide
range of astronomical objects will be observed, some at
previously unexplored wavelengths, others with techniques and
technologies that are unique. Compare this potential to the
expectations from a typical sounding rocket flight, where a
single instrument might gather 300 seconds of data on only one or
two objects!
The Astro Observatory is about to train its unique ultraviolet
eyes on the universe for the second time. With both the
scientific and operational experience from Astro-1 on which to
build, we look forward to a wealth of new discoveries!
Dr. William P. Blair is a Research Professor at Johns Hopkins
University and is a Deputy Project Scientist with the HUT
project. With scientists from all of the instrument teams, he
will await the mission's results at NASA-Marshall Space Flight
Center in Huntsville, AL, during the mission.
Hopkins Ultraviolet Telescope (HUT)
Principal Investigator: Arthur F. Davidsen
Project Scientist: Gerard A. Kriss
Institution: The Johns Hopkins University
Primary mirror: 90 cm (36 inches), f/2
Operational mode: Spectroscopy
TV Field of view: 9 by 12 arc minutes
Apertures: 12 arc seconds to 3.3 arc minutes
Wavelength range: 830 - 1860 angstroms
Spectral resolution: 3 angstroms
Size, mass: 1.1 m (dia.) by 3.7 m, 789 kg.
Ultraviolet Imaging Telescope (UIT)
Principal Investigator: Theodore Stecher
Institution: NASA Goddard Space Flight
Center
Primary mirror: 38 cm (15 inches), f/2
(effective)
Operational mode: Imagery (on 70mm IIa-O
film)
Field of view: 40 arc minutes
Wavelength range: 1200 to 3200 angstroms
Angular resolution: 2 arc seconds
Size, mass: 0.8 m (dia.) by 3.7 m, 474 kg.
Wisconsin Ultraviolet Photo-Polarimeter Experiment
(WUPPE)
Principal Investigator: Arthur D. Code
Co-Principal Investigator: Christopher M.
Anderson
Institution: University of Wisconsin,
Madison
Primary mirror: 50 cm (20 inches), f/10
Operational mode: Spectro-polarimetry
TV Field of view: 3.3 by 4.4 arc minutes
Apertures: 1.5 to 50 arc seconds
Wavelength range: 1400 to 3200 angstroms
Spectral resolution: 4 angstroms
Size, mass: 0.7 m (dia.) by 3.7 m, 446 kg.
William P. Blair, wpb@pha.jhu.edu
Date of last change: 1/11/95
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