Australia Telescope Compact Array report

Staff

After one year at the Observatory, Cliff Harvey has resigned to take other work in the Narrabri district. Cliff worked for most of his term in the Electronics Group as an assistant to Alan Day in the cryogenics area.

Although David Rayner was not formally a member of Observatory staff, we have seen so much of him over recent years that he feels like one of the family! He has now left us and his departure was marked by a riotous evening at Chan's Restaurant in Narrabri. David was a joint ATNF-University of Tasmania postgraduate student. His thesis work involved measuring circular polarisation with the Compact Array and he succeeded in reducing the measurement errors to levels unprecedented in radioastronomy. His work has rekindled theoretical interest in astrophysical circular polarisation. Since completing his PhD David has worked in several temporary roles with the ATNF, including a three-month sojourn in Narrabri, on various aspects of system performance related to operation at mm wavelengths. In particular his contributions to pointing calibration have been very valuable. His final contribution was lecturing at the Synthesis Imaging Workshop. Bob Sault gives a summary of the Synthesis Imaging Workshop elsewhere in this issue.

Operations

New Configuration

Part of the current ATCA upgrade provides new antenna stations to support improved compact antenna configurations, ultimately using the new North Spur to provide N-S baselines. The first new configuration to become available is the EW352 array, scheduled from 3 - 24 October 2001, with antennas on stations W102, W104, W109, W112, W125 (and antenna 6 on W392 as usual). This configuration, with its complimentary partner EW367, replaces the old 375-m array which was used for the last time on 12 July 2001. RIP 375-m.

MNRF Compact Array upgrade

Since the last edition of the ATNF News there have been two substantial interruptions to scheduled astronomy for further MNRF installation work. Substantial advances have been made in four important areas of the upgrade.

Local oscillator and signal distribution

In the final upgraded system the original "daisy-chained" coaxial cable based LO distribution to antennas will be replaced by dedicated optical fibres from the Control Building to each antenna station. Also, the digitised astronomical signals (the "IFs") will be transmitted from each antenna on single-mode rather than the original multi-mode fibres, and dedicated single-mode fibres will also provide time signal and ethernet services to each antenna station. In earlier editions of the ATNF News we have reported the laying of more than 40 km of optical fibre cables along the Array. Work has continued with the connecting (splicing) of these fibres at each end. Since the recent installation periods the 160-MHz LO reference is distributed to five of the six antennas on fibre, and a new high frequency reference (13-GHz) is distributed to the three antennas with new mm-wave receivers. Ethernet and time signals are now available on five antennas. The most recent progress is the transfer of data transmission to single-mode fibres in antennas 2 and 5. This allows the operation of those antennas on the new antenna stations W104 and W125.

Receivers

Three millimetre receiver packages are now installed on the Array in antennas 2, 3 and 4. Each operates at 12-mm (16089 - 18888 and 20089 - 22488 MHz) and 3-mm (84906 - 87305 and 88506 - 91305 MHz). Some 3-mm observations have already been made with three baselines. However a delay in provision of a module in the 12-mm conversion system for antenna 2 has postponed the availability of a three-baseline system at that band.

Water Vapour Radiometers

It is hoped that the upgraded Compact Array will be capable of correcting phase fluctuations of atmospheric origin through measurement of the water vapour column density for each antenna using a Water Vapour Radiometer. The radiometer design is a four-channel 22-GHz system which is distinct to the main astronomy receiver packages. Each radiometer uses a separate horn that is mounted close to the astronomy horn (the offset between radiometer and astronomy horns is approximately five arcminutes). Testing of two prototypes over recent months has provided some encouraging results:

· Temperature stability is excellent. Additionally, as designed, the multi-channel nature of the system gives it extra immunity to fluctutations caused by temperature changes in the components;

· The system temperature of the units is higher than expected. However their raw sensitivity is still quite adequate;

· Using simple calibration of the radiometer output, we can reduce the phase error to the equivalent of about 350mm of pathlength. The aim had been to correct to about 100mm. The dominant residual error is from 1/f noise originating in the RF amplifiers of the radiometer. Errors with timescales greater than about three to five minutes dominate the error power. We do not believe we can eliminate this by using better amplifiers or by cooling them; it would be possible by using fast astronomical switching. Alternatively, we may also be able to eliminate this by regularly switching in a simple load.

Antenna Control Computers

Three prototype Antenna Control Computers (ACCs) have been installed in antennas 2, 3 and 4 in parallel with the original PDP11/73 machines. The new units are connected via the new fibre based ethernet to the control computer network and are providing monitoring services for the new mm receiver packages.

David McConnell
Officer-in-Charge
(David.McConnell@csiro.au)