Fallbrook Radio Telescope (FLIRT)

There used to be a radio telescope in Fallbrook! It was called FLIRT, for the Fallbrook Low-Frequency Immediate Response Telescope, and was located at the northeast corner of the Fallbrook Naval Weapons Station.

This radio telescope was a great example of little science, where "little" refers to dollars, not in science results. "Big" science is best exemplified by the multi-billion dollar Hubble Space Telescope.

The array was built by Bill Coles, Rod Frehlich and Masayoshi Kojima at UCSD from 1982 to 1984, after an earlier antenna at Carlsbad was destroyed by a brush fire. The original purpose of the telescope was to do interplanetary scintillation studies, which refers to the twinkling of radio objects by the interplanetary medium, which is much like the twinkling of visible stars by the atmosphere of the Earth, which it did for three years.

Basically, Coles et al were studying the dynamics of the solar wind using the "twinkling" of compact radio sources as they passed through the solar wind as viewed from Earth. The signal from the Fallbrook array was correlated with signals from two other custom arrays in southern CA.

The Fallbrook array was a special purpose instrument, so it was not much use as a telescope for most other projects. It has a beam width measured in degrees, which is some 30 times worse than most radio telescopes. Also, it had poor gain stability. But it did have a large collecting area, which made it as sensitive as some pretty famous radio telescopes. Also, it was steered electronically (via phase delays, no mechanical parts).

FLIRT was built with full phase shifters on each element, since Coles et al wanted to test beam-forming algorithms. Although they never got the time to work on the topic, they came up with a new and clever calibration system which worked very well. It is described in an IEEE paper that grew out of an undergrad project - see IEEE Proc. 82, pp. 697-704, 1994.

Radio telescopes often don't look like much compared to the beautiful dome and telescope of optical telescopes like the Palomar 200". In this case, the telescope looked like a complicated mess of clothesline hung up in a grid! Technically, the telescope was a bunch of full-wave dipole antennas strung up in a useful configuration.

The telescope proved useful for two other later projects. Yashwant Gupta used FLIRT for his Ph.D. thesis to observe interstellar scintillation of pulsars. After this project, the telescope was idle for two years and was on the verge of being dismantled when Dr. Steve Thorsett thought of a very interesting use for it.

The idea was to look for radio counterparts to Gamma Ray Bursters. Gamma Ray Bursters (GRB) are strange beasts that suddenly produce copious amounts of gamma rays (shorter wavelength than x-rays). In the 1990s, what was interesting about GRBs is that no one knew what they were - and hence they were "mysterious". They produce a burst of radiation lasting only seconds, and then they are seen no more. Obviously, this makes them pretty difficult to follow-up!

The usual method astronomers use to decipher the origin of such mysterious objects is to "hit them with everything we have". We train large optical telescopes on their positions, we train large radio telescopes on their positions, and we train telescopes in space on their positions. Note the key words: "on their positions". In the case of GRBs, that's the one thing we didn't have.

The reason is that until 1999 the satellites which observe GRBs couldn't tell very well where the signal comes from. Gamma Ray telescopes are basically a black box that measures the amount of energy left by gamma rays as they hit the black box, but cannot tell the direction of those gamma rays. The basic problem is that gamma rays are so darn energetic that they can't easily be focussed, like light rays can, with lenses. They just blast through most materials! Hence the black box used to detect gamma rays is basically just a proverbial impenetrable object that stops the gamma rays in their tracks and measures how hard they hit.

Steve Thorsett figured out that a radio telescope would be sensitive enough to observe the radio waves that might be expected to accompany a gamma ray burst. If a radio telescope could be devoted toward quickly looking at the relatively large area of the sky where a burst was sighted, then its much better position-finding ability could nail down the source of the GRB. FLIRT proved perfect for the task of doing studies of large areas of the sky looking for interesting events. Steve, with Rick Balsano from Princeton University, put together an experiment to test this idea, beginning in 1996.

Rick set the telescope up so it could instantly respond to reports of an observed GRB. He had collaborators working with NASA's Compton Gamma-Ray Observatory Satellite (GRO), who immediately told Rick when they observed a burst. Rick then steered the telescope, usually by computer modem from Princeton, by setting the phase offset for each dipole antenna, to look at the general direction of the sky indicated by GRO as the location from which the burst came.

At this time there were two schools of theoretical thought on GRBs. Either they were at the very edge of the universe or they were very close to the Earth. If they were very close to the Earth FLIRT could have detected them and also estimated how far away they were. If they were extremely distant FLIRT would not be able to detect them at all. The general opinion was that GRBs were most likely to be distant, so the FLIRT experiment was a "long shot", but would have been extremely exciting if a detection were made.

However, the need for such radio follow-up was essentially eliminated when a satellite was launched (BeppoSAX in 1996) that had both gamma-ray and X-ray detectors, since X-ray detectors can give very good position locations. In 1997, Kulkarni et al were able to measure radio emission from a GRB and show that it was distant. This eliminated the need for FLIRT.

Rick's research at FLIRT ended in 1998, and I'll try to get a report from him of his results from FLIRT.

The UCSD lease came up in 1999, and Bill Coles decided that they were unlikely to use FLIRT again. Hence all traces of FLIRT were removed from October 1998 through March 1999, including digging up all the buried wires, to restore the site to its use as grazing land.

I thank Bill Coles and Rick Balsano for providing much of the information about FLIRT and its projects.

Go to Fallbrook Information: History

Copyright © 1997-2000 by Tom Chester.
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Last update: 14 September 2000.