Telescope ASPH 611 Term Project
A University of Calgary Department of Physics and Astronomy
Graduate Course in Radio Astronomy

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Introduction

Dave Gibson, December 12, 2005

The 21 cm line of Hydrogen is one of the most studied spectral lines in radio astronomy; used to, among other things, trace galactic structures and the interstellar medium. Detection proved a difficult task in the 1940's when its presence in space was first predicted by H.C. Van de Hulst, under the instruction of Jan Oort. Van de Hulst hypothesized that because the parallel magnetic moment was slightly more energetic than the antiparallel moment (due to spin interactions), when a transition from F=1 to F=0 to the ground state 12S1/2 occurred the resulting energy emission would be a radio wave1. He was able to calculate this proposed radio wave's wavelength using physical constants, presenting a wavelength of 21 cm as the expected value, though he was skeptical of detection due to the rarity of the particular transition.2

Although the knowledge of its potential existence was there, it would not be until 1951 that another student, Doc Ewen, over the course of a year would build an instrument capable of the detecting the line at the Van de Hulst's predicted wavelength of 21 cm. Though others at the time, including the Dutch and Australians, had made attempts it would be American Ewen's frequency switching techniques combined with the horn antenna and other hardware that would grant him success3. Verification of his detection would come shortly there after as the other efforts adopted Ewen's techniques yielding the same results.

hornantenna

The detection of hydrogen would generate new discoveries and ideas, both in astronomy and other applications. In astronomy, combined with Einstein's theory of relativity, galactic structures were beginning to be traced using Doppler shifted hydrogen lines. Observations of the interstellar medium showed a more complex structure than had been previously theorized, with less uniformity. In 1959, Philip Morrison and Guiseppe Cocconi would suggest a search for artificial radio signals as a means for identifying extraterrestrial life4, an idea Frank Blake would use creating of Project Ozma. In a more conventional application, hydrogen masers would be used as frequency standards used to improve the accuracy of chronometry5. Today hydrogen continues to be a well-observed molecule, with projects such as the Canadian Galactic Plane Survey (CGPS) producing full, high-resolution map of the galactic plane. Continuous observation and more refined techniques have increased the accuracy of the known frequency of hydrogen, defined today to be 1.420405751786(30) x 109 Hz6.

Our radio astronomy group was given the task of reliving the failures and successes of the first detections of hydrogen, building a system capable of such a task. Already equipped with a radio telescope built in a previous project, we were given a data acquisition card and a computer with the task of capturing data, converting it into a useful format, and then verifying the detection of an HI source. This would involve several steps, including characterization, software production, calibration and finally the acquisition of useful data. Though the technology was more advanced than those that performed this task in the past, more advanced hardware can lead to more advanced problems required the same use of ingenuity and problem solving skills used over 50 years ago.

  1. K. Rohlfs, "Tools of Radio Astronomy", (Springer, Verlag, Berlin, 1990)
  2. NRAO, "History of Radio Astronomy", http://www.nrao.edu/whatisra/hist_oortvandehulst.shtml
  3. NRAO, "History of Radio Astronomy", http://www.nrao.edu/whatisra/hist_ewenpurcell.shtml
  4. Cocconi, G., and Morrison, P. "Searching for Interstellar Communication", Nature, 1959).
  5. J.W.V Storey, Ashely, Naray and Lloyd, "The 21cm Line of Atomic Hydrogen", (UNSW, 1993).
  6. K. Rohlfs, "Tools of Radio Astronomy", (Springer, Verlag, Berlin, 1990)

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