The Molonglo Observatory Synthesis Telescope (MOST)

Photo Copyright 2002 by George `Nyima' Warr
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The Molonglo Observatory Synthesis Telescope (MOST) is operated by the School of Physics of the University of Sydney. The telescope is located near Canberra, and was constructed by modification of the East-West arm of the former One-Mile Mills Cross telescope. Construction of the original telescope was begun in 1960 by Emeritus Professor Bernard Y. Mills; in recognition of this pioneering work and other innovative contributions to radio astronomy Bernie Mills was awarded the 2006 Grote Reber medal.

The MOST consists of two cylindrical paraboloids, 778m x 12m, separated by 15m and aligned East-West. A line feed system of 7744 circular dipoles collects the signal and feeds 176 preamplifiers and 88 IF amplifiers. The telescope is steered by mechanical rotation of the cylindrical paraboloids about their long axis, and by phasing the feed elements along the arms. The resulting `alt-alt' system can follow a field for +/- 6 hours (necessary for a complete synthesis with an East-West array) only if the field is south of declination -30 degrees. For fields near this limit the signal-to-noise ratio is lower for the first and last hour or so due to the lower gain of the system at large `meridian distance' angles.

The main specifications of the telescope are given in the Table below. The quoted noise level is larger than the thermal noise (which is about 0.2 mJy for a 23' image) because weak sidelobes from nearby strong sources are frequently important in setting the minimum reliable flux density (i.e. limiting the dynamic range). The precise effect varies from field to field depending on the sources present.

Although the telescope uses the well-known principle of rotational synthesis by an East-West array, the method by which this is realised is a novel one. Sixty-four fan beams spanning 23' are formed in real time by a hardware beam-forming device, and the responses added progressively into the image. At the end of a 12h observation the process produces a complete aperture synthesis image. The continuous uv coverage from 15m to 1.6 km results in good response to complex structure, and low sidelobe levels.

The 23' field covered by the beams may be enlarged by time-sharing the beams within the sampling interval. The resulting increase in noise level is often less than expected, because the sensitivity is often limited by side lobe confusion. The wide field project was completed in 1997, extending the available field of view to 161' x 161' cosec |dec| (see 'Increasing the Field Size of the Molonglo Observatory Synthesis Telescope', Publ. Astron. Soc. Australia v11, p44, 1994). Most observations are now made in this mode, although greatest sensitivity is still obtained with 23' fields.

The telescope was recently awarded funding by the Australian Government under the MNRF scheme to prototype technologies relevant to the next generation radio telescope, the Square Kilometre Array (SKA). Trials of the Molonglo Protoype (SKAMP) are being undertaken concurrent to the existing operation of the MOST.

Some further details concerning the MOST are given by Mills in Proc. Astron. Soc. Aust., 4, 156, 1981 and 6, 72, 1985, and Robertson in Aust. J. Phys., 44, 729, 1991. The MOST has a complementary role to the Australia Telescope Compact Array, in that the MOST can survey large and complex fields to a low flux density limit in just a single 12 hour period, but at a fixed frequency and with modest angular resolution (somewhat under 1').

For more information on the MOST, contact the Director, Prof. Dick Hunstead.

PLEASE NOTE: Since mid-2009, MOST has been off the air as development work on an FPGA-based digital backend came into full swing. The reconfigured MOST, known as SKAMP (SKA Molonglo Prototype), will have full spectral capability, initially over an expanded band at 843 MHz and later, with redesigned feeds, over a much wider band from 600-1200 MHz. SKAMP is an approved SKA demonstrator. Commissioning of SKAMP2 at 843 MHz with a 30 MHz bandwidth is expected to get underway towards the end of 2012.

Specifications of the MOST

Centre frequency 843 MHz
Bandwidth 3 MHz
Polarization Right Hand Circular (IEEE)
Declination range for full HA -90 deg to -30 deg
Synthesised beam 43" x 43"cosec |dec|
Field size (unmultiplexed) 23' x 23'cosec |dec|
Field size (multiplexed) 161' x 161'cosec |dec|
Effective noise after 12h 0.5 - 1 mJy/beam (1 sigma)
Surface brightness sensitivity 0.4 - 0.9 K (1 sigma)
Dynamic range (typical) 100:1