A Correlating Receiver System for a Compound Interferometer

Abstract

The first daily maps showing the two-dimensional radio brightness distribution over the Sun were produced at Fleurs ten years ago when the 64-antenna grating cross was completed.The maps had a resolution of 3' arc at A = 21 cm.When the Fleurs field station was given to the University by CSIRO in 1963 it was decided to use the antennas of the grating cross and add to them four or more larger antennas (45 ft diameter) to produce a pair of high resolving-power compound interferometers.The feature of the new system is that the telescope behaves as a large number of two-element interferometers operated simultaneously so that each compound interferometer gives a series of Fourier harmonics of the onedimensional brightness distribution over a region of the sky approximately one square degree in subtended solid angle.Observations of a source over about twelve hours, during which time the direction of the one-dimensional scanning changes through 180° with respect to the source, will allow a two-dimensional brightness radio distribution over the source to be derived. As a result of the long integrating time, the sensitivity of the system will be very high although the antennas are small.Today I shall describe how the new radiotelescope is being used now and will be used in the future for solar radio observations.(1) Two-dimensional solar maps at X = 21 cm are made usually two or three times a week with the old grating cross. (2) The east-west compound interferometer is used to scan the Sun each day at a wavelength of 21 cm to determine the one-dimensional brightness distribution over the Sun at a resolution of 35". In addition lower resolution fan-beam scans are made at a wavelength of 43 cm. (3) In future, use will be made of the independent outputs from each antenna of the east-west compound interferometer to derive almost instantaneously the onedimensional brightness distribution over the Sun. This type of observation will be used when the Sun is showing some burst activity. The resolution will be about 35" at X = 21 cm and 1'.2 at X = 43 cm. (4) At a lower resolution (2'.5 at X = 21 cm) we intend to replace the present television-type scanning of the grating cross, which takes about one hour to complete a scan of the Sun, by the following procedure. We shall take the combined output of the antennas of the eastwest arm of the cross and correlate this voltage separately with the output of each antenna of the northsouth arm. At any instant the one-dimensional brightness distribution along a narrow north-south strip of the Sun is determined. During two minutes the Sun will pass through the fan beam of the east-west array and the two-dimensional brightness distribution will be obtained. A further development of this will involve a more rapid scan in the east-west direction than is provided by the rotation of the Earth. It appears possible to reduce the time of a complete scan to a few seconds by slewing the beam of the east-west array. (5) During the summer when the Sun is at its most southerly declination we shall follow ...