The Telescope

The MOST is a multi-element interferometer operating at 843MHz with a 3MHz bandwidth (FWHM). The reflectors of the MOST are two co-linear cylindrical paraboloids aligned east-west, each 11.6m wide and 778m long. These two arms are separated by a 15m gap and have a total aperture area of more than 18000m (Robertson 1991). The MOST incorporates 352 low noise preamplifiers, one for each 12.5 (4.4m)section. The line feed of each section is a resonant waveguide excited by a linear array of circularly polarised ring antennas (spaced at 0.540). The amplified signals from the sections are combined in groups of four, via computer controlled phase-shifters, to form the 88 basic interferometer elements (bays) of the MOST. The intermediate frequency signals from each bay are processed to form a set of real-time fan beams, which are sampled every 24s. The natural coordinate system for the MOST is analogous to an alt-alt mounting: tilt is the angle of rotation of the entire structure about its east-west axis, measured from the zenith with north being positive; meridian distance (MD) is the angle between a beam and the plane of the meridian, positive to the west (Robertson 1991). In its usual mode of operation the MOST observes for 12 hours to form a high resolution image by the process of back projection (Perley 1979; Crawford 1984). During an observation, the bays track the chosen field centre. A mechanical drive system tilts the entire structure to the appropriate elevation. At the same time linear phase and delay gradients are applied to the line feed, thus guiding the beams in MD. The RF phase shifters are used to make small rapid offsets in the MD pointing of the bays. This facility, installed in 1995, enables the field of MOST to be widened by time sharing (Large et al. 1994). The MOST forms radio continuum images with a maximum field size of 2.7cosec, a resolution of 43 cosec() and an rms noise level of 1 mJy per beam.

© Copyright Astronomical Society of Australia 1997