A Radio Survey of the North Polar Region with a 4.5 minute of arc Pencil-beam System

Abstract

A series of observations has been made in order to test a new and more powerful method of aerial synthesis. An East–West interferometer of variable spacing is used, and observations over a period of 12 hours at each spacing are combined to provide the same resolution as a circular aperture having a diameter equal to the maximum spacing used. In the present tests, small sections of the 178 Mc/s interferometric system were used in pairs to provide different East–West spacings. Since the individual sections could not be steered in hour angle, the observations were restricted to an area of diameter 8° centred on the North Pole. The observations when combined provided a pencil-beam response 4′.5 arc in width, a resolution equivalent to that of a uniformly-excited circular aerial 3 400 ft in diameter. Although the observations were intended primarily as a test of the new method, they have also provided information of astronomical interest, by allowing the detection of individual sources having flux densities $$S \geqslant 0.25 \times 10^{-26}\enspace \text {w.m}^{-2} \enspace(\text {c/s})^{-1}$$ , about 1/8th of that of the weakest sources observed individually in the main 178 Mc/s synthesis programme. The positions of sources having $$S \geqslant 0.5 \times 10^{-26}\enspace \text {w.m}^{-2} \enspace(\text {c/s})^{-1}$$ could be determined with an accuracy of ±15″ in each coordinate. The new results have been useful in extending the number/flux density relationship.