[Big Ear Masthead]

About the Big Ear Radio Telescope

Click on each photo below to view a larger version.

Basic Explanation

The Big Ear radio telescope, dismantled in 1998, was larger than three football fields in size and equivalent in sensitivity to a circular dish 52.5 meters (175 feet) in diameter. The telescope consisted of a flat tiltable reflector measuring 340 feet long by 100 feet high (less when tilted), a fixed standing paraboloidal (curved) reflector which measured 360 feet long by 70 feet high, an aluminum-covered ground plane measuring 360 feet wide by 500 feet long, and two feed horns mounted on a movable assembly. The reflectors were not solid, but were actually covered in a mesh pattern with small gauge wires. The spacing of the wires was such that the reflectors appeared to be solid to incoming radio waves.

[Flat Reflector]
Flat
Reflector
[Parabolic Reflector]
Parabolic
Reflector
[Ground Plane]
Aluminum
Ground Plane
[Mesh Covering]
Mesh
Covering

The flat tiltable reflector was pointed skyward to pick up radio waves coming in from space. The signals were then bounced over to the paraboloidal reflector where they were focused into a beam. This beam was reflected back across the ground plane to the feed horns. The aluminum layer of the ground plane kept the very weak signals from being absorbed into the ground and the signals generated by the ground from interfering with the desired signals.

[Feed Horns]
Feed Horns

[Feed Horns]
Another view
of feed horns
[Horns on Movable Assembly]
Feed horns on
movable assembly

The telescope surveyed the sky by remaining stationary and allowing the rotation of Earth to sweep its beam in a narrow circular path through the sky once each day. After a few days of observation, the beam was moved slightly up or down and the pattern was repeated. It took several years to thoroughly search the sky.

The beam of the telescope was elliptical, being forty minutes of arc in the declination (vertical) direction and eight minutes of arc in the right ascension (horizontal) direction, at 1400 MHz. This may be visualized by comparing it with the size of the Moon, which is a thirty minutes of arc diameter circle in Earth's sky.

Advanced Explanation - Not for the Astronomically-Challenged
By Herb Johnson

The telescope was a "drift field" design: that is, the rotation of the Earth scanned it in "right ascension" or RA across the sky from west to east (so the sky looked like it was moving east to west). The flat tiltable reflector was adjusted to point to a position in the south along the "meridian" line from south to north: that position was the "declination" of the telescope.

The part of the sky at the declination where the flat was "pointed" reflected that area into the paraboloid 500 feet away; it focused that image back across 420 feet to the horn cart, where the horns "saw" two areas 8 arc minutes wide by forty arc minutes tall. Over a ten-second period, small areas were scanned in frequency as they "drifted" by. One area was subtracted from the other to get rid of background noise including terrestrial noise and the sky background. This subtraction occurred by switching between the two beams at 79 Hz.

The horn images were separated by about 40 arc minutes (about 150 seconds of time for a source on the celestial equator), so a distant point source would only be in one main beam at a time. The LOBES (LO-Budget Extraterrestrial_intelligence Search) program looked for "strikes" to appear first in the east, then the west horn. When a strike was found, the narrow 10 kHz channel receiver was tuned to it. For a "strong" SETI candidate, the 100 kHz scan was stopped, a 10 kHz scan was performed while the source was in the beam, and the horn cart moved to follow the source across the sky. Also, an ICOM communuications receiver was tuned to the frequency of interest and audio recordings were made using AM, FM, and SSB detectors.

The Big Ear radio telescope was extremely sensitive. LOBES could detect in the continuum channel sources down to about 200 milliJanskys. For reference, the strongest stellar sources are ~2000 Janskys. Pulsars are a few Janskys, but pulse too quickly for Big Ear to "count" the pulses. The Sun is about a million Janskys. A Jansky is 10 to the minus 26 watts per square meter per Hertz.

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North American AstroPhysical Observatory.

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Last modified: August 12, 2005