Introduction

Although multi-element detectors are common in optical and IR astronomy, they are not yet commonly used in radio astronomy. This is partly because other techniques, such as synthesis interferometry, exist, and because of the technological difficulties in producing arrays that match the performance of highly optimized single-beam devices. Two notable differences exist between optical/IR and radio array detectors. The first is that the field of view of a typical radio telescope often does not allow a large number of detectors (Ruze 1965). The second is that, because of diffraction effects, it is extremely difficult to fill the focal plane and efficiently illuminate the aperture of a radio telescope simultaneously (Johansson 1995; Fisher 1997).

The Parkes 21 cm multibeam receiver (Staveley-Smith et al. 1996) is a conventional focal-plane array, with 13 beams arranged in a hexagonal grid. The spacing between adjacent beams is about two beamwidths, and therefore the array undersamples the focal plane. It is, however, optimized for efficient illumination and low system temperatures at 21 cm, and for the first time allows blind 21 cm surveys of very large areas of sky to be carried out.

Much of this volume is devoted to scientific issues relating to the new-generation surveys about to be made with the new Parkes 21 cm multibeam receiver. In this paper, I focus on the parameters of the two major extragalactic HI surveys proposed by the Multibeam Survey Working Group and give an overview of possible observing techniques which fill in the undersampled gaps either by dithering or scanning.

© Copyright Astronomical Society of Australia 1997