Introduction

The Molonglo Radio Observatory is situated in a flat valley, 700m above sea level in the Great Dividing Range near Bungendore, about 30km east of Canberra. The site was one of several considered in the mid-1950s for the 64m radio telescope eventually built by the CSIRO at Parkes. In 1961 Bernard Mills chose the site as the most suitable for his new (408MHz) Cross-type radio telescope (Mills et al. 1963). The land was acquired by the University of Sydney and the telescope, which became known as the One Mile Cross was opened by Prime Minister Sir Robert Menzies in November 1965. During the next 12 years the instrument was used for a number of major astronomical investigations (Mills 1991), including: production of the Molonglo Reference Catalogue of over 12000 radio sources; discovery of supernova remnants in the Galactic Plane and the Large Magellanic Cloud; determination of accurate positions and flux densities of galaxies and quasars; discovery of a large number of pulsars including the pulsar associated with the Vela supernova remnant.

In the late 1970s it became apparent that the Cross would soon complete its planned programme of surveying the radio sky south of and a decision was made to convert the east and west arms of the Molonglo telescope to an aperture synthesis instrument capable of higher sensitivity and angular resolution. This was achieved by increasing the frequency of operation by approximately a factor of two. Some constraint was imposed by the resonances of the existing line feed structure, but the choice of 843MHz was made in consultation with Telecom to avoid likely sources of interference from radio telephony services (Mills 1981). While 843 MHz is not in an internationally protected band for radio astronomy, the use of this frequency by the Molonglo Observatory was entered as a footnote in the Spectrum Band Plan.

The Molonglo telescope in its new incarnation started observations in 1980 and has been in continuous operation, with several technical improvements, since then. It is now known as the Molonglo Observatory Synthesis Telescope (MOST). Until the early 1990s, the site proved to be free of interference in the 843MHz band, and the MOST enjoyed quiet observing conditions. With the introduction of analogue mobile telephone services, the telescope began to experience occasional bursts of interference. At the present time (1997) such interference bursts have become much more frequent, inevitably degrading the quality of radio images and necessitating additional time consuming image processing to minimise the loss of information.

In 1995 we were dismayed to learn that the Australian Government were planning to sell the 825-845MHz band of the electromagnetic spectrum. The use of part of this band by the MOST has been acknowlegded for over 17 years, during which time the telescope has made many internationally recognised contributions to astronomy. Observations would be seriously threatened by radio frequency interference if the band were to be released for unrestricted use. The University entered into discussions with the Spectrum Management Agency (SMA) and it was agreed that a series of tests should be carried out to determine the sensitivity of the MOST to interfering signals from local low power transmitters. The results, together with theoretical predictions, are intended to provide the technical basis for protection of MOST observations when the 825-845MHz band is sold for commercial use.

© Copyright Astronomical Society of Australia 1997