The Ohio Sky Surveys

Sunrise at Ohio State University Radio Observatory. Photo by Herb Johnson. Click on the picture to see a larger version.

After construction in 1962 and following observations of various smaller regions, such as M31 (Andromeda Galaxy), a thorough survey was conducted from 1965 to 1971. The results were published in The Astronomical Journal in a series of articles, charts and source catalogs known collectively as "the Ohio Survey".

The Ohio Survey Click on picture for enlargement

The survey was primarily at a radio frequency of 1415 MHz, but data was also collected and evaluated at 2650 MHz and 612 MHz. Only one "channel" or band of frequencies was sampled for each frequency. The antenna was oriented to one declination at a time, and as the sky drifted past the meridian field of view, radio energy from that area was received and detected. Signal power was plotted on an analog chart recorder and also digitized and recorded on magnetic tape for later processing. (Note: Punched paper tape was used in the early days of the surveys.) A given declination was observed for a number of days before the telescope was moved to another declination in a systematic fashion.

The area surveyed was from declinations 63 degrees north to 36 degrees south, with a resolution at 1415 MHz of roughly 40 arc minutes in declination by 10 arc minutes in right ascension (RA). Over the course of the Survey, 19,620 sources at 1415 MHz were identified, of which 60% were previously uncataloged.

Some of the objects first identified by the Ohio Survey included quasars, objects of intense radiation and power at the edge of the then-known universe. The archived data subsequently permitted these and other sources to be reviewed over several years of observations. Later, the LOBES survey used most of the same apparatus as the Ohio Survey, and was able to automatically determine and verify the sources first charted by the Ohio Survey.

1415 MHz

The 1415 MHz receiver used a parametric amplifier, cooled with liquid nitrogen. System noise temperature was 120 Kelvin, the bandwidth was 8 MHz, and the integration time was typically 12 seconds, with an RMS noise of .025 Kelvin. Antenna efficiency was 40% to 50%. Sources were identified down to .25 Janskys (a measure of radio power). Beam resolution at 1415 MHz was 8 to 10 arc minutes in right ascension by 40 arc minutes in declination.

612 MHz

The 612 MHz receiver had a noise temperature of 650 Kelvin, and RMS noise of 0.15 Kelvin. Antenna efficiency was about 60% The beam resolution at 612 MHz was 23 arc minutes in right ascension by 85 arc minutes in declination.

Processing

In the early days of the surveys, punched paper tape was read off-line and archived onto magnetic tape on a day-to-day basis. Later, the data was recorded directly to magnetic tape. A number of days' data at each declination was processed to remove or average out noise. Radio sources were identified and cataloged largely by human observation, and their locations and values cataloged. Additional software took the processed data and produced stellar contour charts which were published with the list of identified sources. Magnetic tapes and punched cards of processed data were archived, many of them to 1996.

Archives

Paper chart recorders preserved the "raw" data for all three frequencies and these charts were manually reviewed. Additional reports and some survey results were directly derived manually from these charts. Almost all of these original paper charts are still archived as of 1996.

Construction Reference

"The Large Radio Telescope of Ohio State University" by Kraus; Sky and Telescope, July 1963. A detailed description of the construction and operation of the telescope.

Receivers Reference

"Maps of the Perseus Region at 600 and 1415 Mc/s", Dixon, Meng, Kraus; Nature, vol 206 no. 4973; Feb 20 1965. A description of the receivers.

Processing Reference

"A new High-Sensitivity Study of the M31 Region at 1415 MC/S" by Kraus, Dixon, and Fisher; Astrophysics Journal Vol 144, No 2; 1966. This contains a detailed and complete description of processing methods for the subsequent Ohio Survey.

Survey Reference

"Ohio Survey 2 supplement" by Rinsland, Dixon, Kraus; Astrophysics Journal volume 80, number 10, October 1975. A list of all the Ohio Surveys.

Additional Reference

Ohio Sky Survey - Wikipedia

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