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Index of CfA Almanac stories.

The CfA Almanac

December 1998


THE THREE T-SHIRTS: The current, founding, and predecessor directors of the Harvard- Smithsonian Center for Astrophysics, respectively, Irwin Shapiro (left), George Field (right), and Fred Whipple (center), help celebrate the CfA's Silver Anniversary by hoisting the 1990s equivalents of institutional banners at the 25th Birthday Bash in the Perkin Courtyard October 7. (Photo by Jon Chase, Harvard News Office)


Everybody knows his bedtime is exactly 11 pm--on the dot, without exception--every night. So why was Jim Moran bugging Irwin Shapiro at 10:59 pm, Friday, October 23?

Moran, calling from the Submillimeter Array site in Westford, assured Irwin that he would be pleased by this late-night disruption of his usual routine. And, indeed, he was--for Moran was calling to report that he and about eight other members of the SMA Project had just succeeded in obtaining "fringes" on the planet Saturn with the two prototype antennas at Haystack.

Needless to say, Irwin was excited by that call. However, when sleep finally came, it was sound and serene: The SMA interferometer was going to work!

Indeed, at the start of the very first attempt to observe an astronomical source, the initial two elements of the planned eight-antenna Submillimeter Array (SMA) now under construction by SAO and Taiwan's Institute of Astronomy and Astrophysics had gotten strong and stable "fringes" from an astronomical object, with a very high signal-to-noise ratio, thus achieving the submillimeter-wave equivalent of "first light" for this unique instrument.

"Fringes" are the name of the game in interferometry and refer to the distinctive patterns produced by the simultaneous receipt and combination of radio signals from two separate telescopes, in this case, two 6-meter-diameter antennas now operating in a Massachusetts hay field but slated for later installation near the top of the Mauna Kea volcano in Hawaii.

* * *

first fringes card

Irwin Shapiro (left) and his late-night caller Jim Moran display the giant "first fringes card" displayed at the impromptu party held October 28 to celebrate that historic technical achievement. Notice that the celebratory cake seems to be decorated in a wavelength visible only to submillimeter detectors! (Photo by Jim Cornell) Two days later, in a more detailed report for the technical types who had been intently monitoring the "SMA Weekly Calendar" for positive news about this major SAO initiative, Moran described the operations at Haystack the night he kept Irwin Shapiro from early retirement:

"By late last Friday afternoon all the IF/LO equipment was in place and checked out for the initial phase stability tests on the monochromatic 230 GHz beacon.

At first there was some difficulty in locking the vector voltmeter to the beacon signal at the IF frequency of 1GHz. This problem was overcome by passing the received signals first through the holography IF system which had narrow band filters at 21 MHZ.

"With the vector voltmeter locked up, one of the secondary mirrors was moved through a range of several millimeters to check the phase scale. With the secondary in a fixed position, the phase drifted slowly, about 20 degrees over a period of 10 minutes. Changes of up to about 10 degrees were seen on time scales as short as 30 seconds. With the drift removed, the rms phase deviation over 10 minutes was about 4-5 degrees...The assembled crew were rather impressed with this level of phase stability on the very first test. (To place this result in perspective, the initial electronic stability specification of the VLA, circa 1980, was 1 degree rms of phase fluctuation per Gigahertz...)

"Emboldened by the phase stability results, the crew turned the interferometer towards Jupiter with the IF signals feeding the 100 MHZ bandwidth analog correlator. Alas, by the time all was ready, Jupiter was an hour past transit and well out of the delay window (10 nanoseconds). Undeterred, the crew decided to try for Saturn, which was 90 minutes from transit.

"Amazingly, obvious fringes appeared immediately on the chart recorder. The fringe rate was exactly as expected, 0.75 Hz, and the `sausage' pattern in the fringes caused by interaction of the two sidebands separated by 10 GHz was clearly evident. A quick check showed that the fringes were about the amplitude to be expected when the source transited, about 0.5 percent of the system noise.

"How could this be when Saturn was so far from transit? After some head scratching, it was realized that no special care had been taken to equalize the fiber-optic cable that run from the two antennas. Fringes 90 minutes before transit implied a relative delay of about 16 nanoseconds. (A check on Saturday confirmed that the relative cable lengths were different by 5 meters.) There must be some moral about the early bird getting lucky!

The assembled crew (Todd Hunger, Charles Katz, Jim Moran, Nimesh Patel, Masao Saito, William Snow, Sridharan Tirupati, and Ken Young), along with Paul Ouellette, celebrated with pizza and a stash of Rabbit Run Sauvignon Blanc left over from the SMA Advisory Committee meeting."

two-element interferometer

The two-element interferometer. (Photo by Jim Moran)


Astronomers from the CfA and their colleagues at five other institutions are literally looking at the universe in a new light following the successful launch of NASA's Submillimeter Wave Astronomy Satellite (SWAS) on December 5 after several days (and years!) of delay.

The first space borne instrument to observe the heavens in submillimeter radiation--a narrow band of cosmic emission lying between infrared and radio waves on the electromagnetic spectrum--SWAS is especially well suited for studying the birth of stars, a process now hidden deep within obscuring clouds of dust and gas.

The mission, launched at 7:58 pm EST (4:58 pm PST) on Saturday, December 5, is enabling astronomers to probe deep into the heart of these stellar nurseries to gather data on the structure, motions, and nature of the matter that creates new stars.

"Although stars are the building blocks of the universe, little is known of how stars are conceived, born, and grow to maturity," says Gary Melnick of the CfA, and Principal Investigator for the SWAS instrument. "Until now, these vital processes were among the least understood steps in cosmic evolution."

The detectors carried by the SWAS observatory are tuned to the characteristic emission frequencies of five different atoms and molecules, including water (H2O) and molecular oxygen (O2). These atoms and molecules are important for several reasons. First, they are predicted to be important coolants of the interstellar clouds collapsing to form stars and planets. Second, the detection of these atoms and molecules can provide important, but hitherto missing, information about the composition and chemistry of those clouds. And, finally, knowledge about the abundance of such life-sustaining molecules as water and molecular oxygen in the parent clouds may shed light on how these species are deposited in the rocks and atmospheres of newly forming planets.

"During its mission, SWAS will observe hundreds of regions of ongoing star formation within our galaxy. The answers SWAS will provide are important not only to the understanding of the formation of future stellar systems, but also to the understanding of the processes that led to the formation of the Sun, the Earth, and the other planets and moons in our own solar system," says Melnick.

Much cosmic submillimeter emission cannot be easily studied from the ground--even at high mountain-top or airborne astronomical observatories--because of interference from the large quantities of water and molecular oxygen within the Earth's own atmosphere. At an orbital altitude of some 370 miles above Earth, SWAS will have an unobstructed view of the heavens.

SWAS got a jump-start into space by being dropped from the belly of a specially modified L-1011 aircraft flown out of Vandenberg Air Force Base, CA. The Pegasus-XL launch vehicle, built by Orbital Sciences Corporation, is a three-stage, solid-propellant booster system designed to be carried aloft by this aircraft. The Pegasus-XL vehicle was released when the aircraft reached an altitude of about 40,000 feet (12,200 meters) and had an airspeed of Mach 0.8.

The SWAS observatory is orbiting the Earth every 97 minutes and typically observes three to five astronomical objects per orbit. The resultant data are stored in the spacecraft memory and relayed to a ground station twice per day. Within 24 hours of receipt at the ground station, these data are received at the SWAS Science Operations Center located at the CfA. There, the science content of the data is analyzed and new astronomical targets are selected for observation. The mission is designed for a two-year duration.

In addition to Principal Investigator Melnick, the SWAS co-investigators are Alexander Dalgarno, Giovanni Fazio, John Stauffer, and Patrick Thaddeus, also of the CfA; Neil Erickson and Ronald Shell, University of Massachusetts; Paul Goldsmith and Martin Harwit, Cornell University; David Koch and David Hollenbach, NASA Ames Research Center; David Neufeld, Johns Hopkins University; and Rudolf Schieder and Gisbert Winnewisser, University of Cologne, Germany.

The SWAS observatory, including both instrument and spacecraft portion, weighs only 625 pounds. The instrument is comprised of seven major subsystems: 1) the signal detectors built by the Millitech Corporation; 2) an acoustical-optical spectrometer provided by the University of Cologne; and 3) the telescope assembly, 4) star tracker assembly, 5) instrument control electronics, 6) instrument structure, and 7) thermal controls provided by Ball Aerospace & Technologies Corporation, which also integrated all the components and prepared the instrument for launch. The SWAS spacecraft was designed and built by NASA's Goddard Space Flight Center, Greenbelt, MD. SWAS is one of NASA's Small Explorer (SMEX) satellites, designed and built to be small and economical, yet scientifically powerful.

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