Observations of the Sun with the Molonglo Cross

Extract

Transit observations of the Sun can be made with the Molonglo radio telescope which is now in full operation at 408 MHz. Thus the Sun can be scanned once per day by eleven separate beams evenly spaced in declination. Because of foreshortening of the north-south arm with increasing zenith angle, the total coverage varies and in winter about 28′ arc can be covered by the eleven beams, whereas only 15′ is covered in mid-summer. This also means that the beamwidth in declination changes from 2′.9 at the zenith to about 5′.8 for the mid-winter sun. The beam is 2′.8 wide in right ascension. A further complication is the fact that the positions of the eleven beams cannot be chosen arbitrarily but can only be selected from a predetermined set of beam positions. These latter have been fixed by the phasing connections built into the telescope and cannot be changed easily. Hence for all the above reasons the strip of Sun which can be scanned will vary from day to day.