2 The telescope

The telescope was an unfilled aperture in the form of a T with dimensions 96$\lambda$ $\times$ 2.5$\lambda$ east-west (EW) and 32.5$\lambda$ $\times$ 4$\lambda$ north-south (NS). This array formed a nearly Gaussian beam of extent 1.1 $\times$ 1.7 degrees (EW $\times$ NS) at the zenith (declination 48.8$^\circ$). The telescope was operated as a meridian transit instrument, steerable in declination between -30$^\circ$ and the North Celestial Pole by the adjustment of the phases of rows of dipole elements in the north-south direction. Away from the zenith, foreshortening of the array broadened the beam in this direction to 1.7$^\circ$ secant(Z), where Z is the zenith angle. The gain of the telescope in directions away from the zenith was further reduced by the response of the basic array element, a full-wave dipole ${\lambda}\over{8}$ above a reflecting screen. Aperture grading by means of attenuators was used to keep sidelobes to a level of a few percent.

The pencil-beam response was formed by multiplying the signals from the EW and NS arms. A problem in T- and cross-format telescopes arises from the region in which the two arms intersect. If this region is removed, a broad negative response is produced around the narrow pencil beam, and the telescope filters out the lowest spatial frequencies, leading to a poor representation of the broadest angular components of the emission. To overcome this problem, the signals from the elements in the overlap region were split and fed to both the EW and NS arrays.

The gain of the antenna (in effect the ratio between the flux density of a radio source and the antenna temperature it produces in the main beam) was established using an assumed flux density of 29100 $\pm$ 1500 Jy for Cygnus A. Details of the original flux-density scale and the subsequent revision can be found in Roger et al. (1973) and in Roger et al. (1986).