The Molonglo Galactic Plane Surveys

The Molonglo Observatory Synthesis Telescope (MOST) is a radio interferometer operating in the continuum at 843 MHz with 43 " x 43 " cosec/delta/ resolution. It is a highly redundant east-west synthesis array which is very sensitive to weak, extended emission. The MOST operates in a frequency regime where nonthermal (synchrotron) emission dominates, while at the same time, thermal sources are still generally optically thin and readily detectable. Hence, it is an excellent instrument for morphological studies of Galactic supernova remnants (SNRs) and HII regions and their relationship with the interstellar medium (ISM). The first epoch Molonglo Galactic Plane Survey (MGPS1) covers the area 245 degrees less than or equal to l less than or equal to 355 degrees, /b/ less than or equal to 1.5 degrees and is now available to the astronomical community. Complete coverage of the 330 square degrees was achieved with more than 450 overlapping fields, each representing a 12 hr synthesis observation of an area 70' x 70' cosec/delta/ with a typical rms noise (1 sigma) of similar to 1 mJy beam(-1). Recently, the MOST has been upgraded to widen the field of view to 160' x 160' cosec/delta/. Replacement of the pre-amplifiers and an innovative method of multiplexing and phase-switching has allowed a fivefold increase in area of the field of view, without degradation of the sensitivity or resolution. A second epoch Galactic Plane survey (MGPS2) has commenced; it is planned to cover the region 240 degrees less than or equal to l less than or equal to 365 degrees, /b/ less than or equal to 10 degrees, which totals 2500 square degrees. This survey is the Galactic extension of the Sydney University Molonglo Sky Survey (SUMSS). Expected outcomes from the two Galactic surveys include an inventory of the discrete extended sources (SNRs and HII regions), a catalogue of the unresolved and variable sources, perhaps including some hitherto unknown Galactic population, and detailed tracking of the large-scale filamentary structures out of the Plane. The distribution of external galaxies located behind the Plane is also being studied.