Solar Observatory Operating

Press, Volume CIII, Issue 30593, 9 November 1964, Page 10

Solar Observatory Operating

On the roof of C block at the Christchurch Technical College are two large aerial arrays which together form a radio telescope. The telescope, built to observe radio emission from the sun, came into operation last week after two years’ preparation.

The telescope is a leisuretime interest of one of the science masters at the college, Mr T. F. Mackrell. It was made possible through extensive facilities provided by the college board of managers. The telescope will be used, in particular, to chart the level of radio emission from the sun during the present International Year of the Quiet Sun, and during later periods when solar activity is expected to be greater. To correlate the radio activity of the sun on any particular day with visible disturbances on the sun, regular observations are made around noon on clear days through a 3in optical telescope provided by the board. (Radio observations can be made irrespective of the weather.) The level of solar radio

emission will also be compared continuously with fluctuations in the earth’s magnetic field, data for which will be supplied by the Magnetic Survey of the Department of Scientific and Industrial Research from its Amberley observatory.

From this part of the study, Mr Mackrell hopes in particular to investigate a possible correlation between certain quite small fluctuations in the level of solar radiation and minor disturbances in the magnetic field. These fluctuations have apparently no direct connexion with sunspots or the sunspot cycle. An event in June next year which Mr Mackrell hopes to observe with his radio telescope is the occultation by the sun of the Crab nebula, an intense radio source in the constellation Orion.

The telescope is a phaseswitching interferometer, the large distance (more than 200 ft) between the two arrays forming the interferometer base-line. Each array consists of 24 half-wave dipoles (similar in function to the rods of television aerials) mounted in front of a wirenetting reflector 250 sq. ft in area. The reflectors and arrays are adjustable by a vertical movement to point directly at the sun at its upper transit (highest point in the sky) each day, but do

not follow its daily path; instead, the sun passes through the interference pattern of the two arrays so that the strength of its radio signal rises and falls as it crosses. The phase-switching arrangement helps to eliminate noise.

Mr Mackrell tried two other varieties of radio telescope before choosing this type, but in the others the level of noise from very-high-frequency radio, television, satellite, transmissions, spark plugs, and other sources of radiation was unacceptably high. The phase-switching interferometer shuts out most of the noise, and that which gets through is readily identifiable. The chosen frequency of 210 megacycles is relatively quiet.