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RADIO ASTRONOMY

Introduction

The DSES 18-meter dishes represent a unique opportunity for our members to do radio astronomy studies. Recently we have become equipped to observe neutral hydrogen (HI) in the Milky Way galaxy. Neutral hydrogen is known to change its spin axis under certain stimuli thus causing it to produce radio emissions at 21 cm (1420.406 MHz). This phenomenon provides a means by which hydrogen clouds in the Milky Way can be observed. The history of the discovery of the HI line can be found in How Ewen and Purcell discovered the 21-cm interstellar hydrogen line by Stephan, K.D. in IEEE Xplore® (Subscription reqd).

It is also possible to determine the relative velocity between the Earth and various hydrogen clouds by observing the doppler shift in the received signals. One possible application is to use the doppler data to study of the motions of the radial arms of the Milky Way Galaxy by observing the HI emissions from various nebulae.

The equipment for this project consists of a feedhorn placed at the focal point of the dish, two low noise amplifiers (LNA) attached to orthogonal elements in the feedhorn, a coupler, a low loss cable to our control room, and a SpectraCyber Astronomical Gas/Molecule Spectrometer - a combination 1420 MHz receiver and spectrometer.

The system can be used either to track a given target or to perform drift scans where the dish receives whatever passes overhead while the dish is in its parked position pointing at the zenith. An example of the type of data that can be recorded during a drift scan is shown in this movie of emissions from the Crab Nebula (105k) that were received using SpectraCyber receiver.

The procedure for calibrating our system is to observe various flux calibration sources.

A critical component in the receiving system is the feedhorn. Its design and positioning must be chosen so as to achieve the maximum signal to noise ratio (S/N). Illustrated here are some of the feedhorn configurations we have been testing along with records of their performance.

 

Another option we are considering is to observe emissions from hydroxyl radicals (OH) which are also associated with hydrogen clouds. HO radicals have spectral lines at 1665.402 MHz and other frequencies. One approach for receiving equipment would be to use LNAs with down converters to 1420 MHz to match our SpectraCyber spectrometer.

An alternative receiver system that we have acquired for observing hydroxyl emissions, and one that is continuously tunable to over a wide band of frequencies, consists of a combination of an ICom 8500 receiver and a SDR-14 software defined receiver. The Icom serves as a front end and its IF output is connected to the SDR-14. We have modified the Icom by disabling the AGC to allow it to be used to for radio astronomy. The SDR-14 is made by RFSPACE and is a very versatile receiver.

 

DSES Research Proposals

Activity Reports

  • 01/04/05 Report on Soft Gamma-Ray burst detection; Hydrogen receiver calibrations using 3C033
  • 12/26/04 Report on radio astronomy and other work projects at T-22
  • 11/13/04 Progress Report on radio astronomy activities at DSES T-22
  • 10/11/04 First full-up test of tracking an extra galactic radio source under computer control
  • 07/12/05 Deep Impact Tracking – Final Report

Other Radio Astronomy Websites