21-cm Imaging Spectrometer for the Green Bank Telescope

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National Radio Astronomy Observatory

Green Bank Telescope

BFA web site

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Project overview:
The National Radio Astronomy Observatory is developing a 21-cm imaging spectrometer or beam-forming array (BFA) for the Green Bank Telescope (GBT) that is capable of producing several simultaneous beams on the sky. The likely configuration of the beam-forming array would be a set of at least thirty-seven electromagnetic sampling elements located at the prime focus of the GBT, and through an appropriate complex weighting scheme, seven high-quality independent beams could be formed and spaced at arbitrary intervals on the sky. The beams may be formed in hardware or software with the latter option being more flexible and therefore preferred. The beam-forming array is the most natural, efficient, and logical way to increase the field of view of a radio telescope and would represent a breakthrough in technology. Such an instrument would open up many new survey possibilities in spectral line and continuum imagining, and in electromagnetic field polarization studies. It will also enable searches for new pulsars along the Galactic Plane. Consult the BFA web site for additional information about the science objectives.

The development of a very low noise radio frequency sampling element will be performed in the IPRA laboratory. The element, which consists of an antenna coupled to a low noise amplifier, will be cryogenically cooled to 15 K using a closed-cycle refrigeration system to increase the receiver's overall sensitivity. Students participating in this project will have an opportunity to learn about modern cryogenic systems, low noise electronics, and antenna design.

Another challenge in building the array is the large signal processing requirement. There are three general categories of signal processors: analog signal phasing and addition, digital delaying and addition, and digital cross-correlation followed by delaying and summation. The first method may be most suitable for a very wide bandwidth continuum receiver for which the signal processing speeds are not economically feasible with current digital technology. We will therefore focus only on the latter two methods. We are interested in exploring the application of parallel computing (Beowulf clusters) to this problem, and students with a background in signal processing and computer science are encouraged to become involved.

For additional Information please e-mail Rick Fisher: rfisher@nrao.edu.

References:

J.R. Fisher and R.F. Bradley, "Full Sampling Array Feeds for Radio Telescopes", Proceedings of the SPIE, Radio Telescopes, Vol. 4015, pp 308-318, 2000.

J.R. Fisher and R.F. Bradley, "Full Sampling Focal Plane Array", Proceedings of Imaging at Radio Through Millimeter Wavelength Workshop, Tucson, AZ, June 6-9, 1999, ASP Conference Series No. 217, eds. J. Mangum and S.J.E. Radford, pp 11-18, 1999.

Fisher, J.R., R.F. Bradley, and K.S. Saini, "Progress on the Evaluation of a Phased Array Feed," 1997 URSI North American Radio Science Meeting Digest, Montreal, Canada, p. 611, July 1997.

Saini, K.S., A Phased Array Feed for Reflector Antennas, Master of Science (Electrical Engineering) Thesis, University of Virginia, May, 1997.

J. Richard Fisher, "Phased Array Feeds for Low Noise Reflector Antennas," NRAO Electronics Division Internal Report No. 307, Sept. 24, 1996.

Saini, K.S. and R F. Bradley, " The Sinuous Antenna - A Dual Polarized Element for Wide- Band Phased Array Feed Application," NRAO Electronics Division Internal Report No. 301, Feb. 13, 1996.

Saini, K.S. and R F. Bradley, " The Sinuous Antenna - A Dual Polarized Element for Wide- Band Phased Array Feed Application," URSI National Radio Science Meeting Digest, Boulder, CO., p. 246, Jan.1996.


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