Notes from the Australia Telescope Compact Array millimetre upgrade

Work has been continuing on the MNRF upgrade since our initial 3-mm single-baseline observations last November (see ATNF News No. 43, February 2001). In June/July, we installed improved versions of the 3-mm and 12-mm indium phosphide receivers in antennas 3 and 4 with dual-polarisation capability. At the end of September, the 3-mm receivers on antenna 2 became fully functional, and the new 3-element system appears to be performing well. A high-frequency (13.6-GHz) local oscillator (LO) signal is now distributed to the antennas via optical fibre, and an absorber mounted on a paddle can be swung in front of each 3-mm horn by computer control in order to measure the system temperature. Cyclic pointing errors have been eliminated by adjusting the encoder electronics.

The 3-mm science team, led by Baerbel Koribalski, has been generously allocated time for test observations, following the June and September shutdowns. Spectra of several interesting sources were obtained with the single-baseline system in July. Also during that time, we successfully implemented a "fast-switching" technique for phase calibration using the Compact Array's mosaicing mode, and made estimates of antenna sensitivity by observing Uranus across the entire 3-mm band (planets are among the only sources with well-known millimetre fluxes). More recently in September, we've observed several sources with the three-element system in the 750D configuration (baselines of 180 - 580 m) and are planning additional observations in the new ultra-compact EW352 configuration (baselines of 45 - 120 m) in October, which will hopefully provide sufficient (u,v) coverage to generate our first astronomical images.

Shown here are two of the spectra taken in July: (1) SiS (5-4) emission from the envelope of the evolved star IRC+10216; and (2) HCO+ (1-0) emission from the N113 molecular cloud of the LMC. The third plot shows the closure phase vs. time for a nine-minute observation on the bright quasar PKS B1921-293, taken with the three-element system on 28 September. While phase variations on individual baselines were as large as 360 degrees, the closure phase is consistently close to zero, as expected.

SiS (5 - 4) emission from IRC+10216.

HCO⁺ (1 - 0) emission from N113.

3-mm closure phases for PKS B1921-293.

Tony Wong
(Tony.Wong@csiro.au)