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Office of News and Information
212 Whitehead Hall / 3400 N. Charles Street
Baltimore, Maryland 21218-2692
Phone: (410) 516-7160 / Fax (410) 516-5251

February 16, 1995
CONTACT: Emil Venere

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.


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 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.


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 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.


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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