After five years of designing, enhancing, building, cabling and testing the time had finally come for the engineering groups in Sydney and Narrabri to put the prototype 3mm systems on two antennas of the ATCA. The floods that had inundated half the town of Narrabri incl. major access roads to the telescope just receded in time to let equipment and people through. Several sleepless nights and lots of coffee needed to meet the deadline were then followed by a few busy days of installation and system testing. Then, just before midnight on the last day of November, first light was achieved and witnessed by a large number of people in the Narrabri ATCA control room. Congratulations to everyone who made this possible !!
The 3mm system which currently provides two antennas (CA03 and CA04) with 84-91 GHz prototype receivers will progressively be upgraded to the full array of six antennas with receivers covering 84-115 GHz. In the meantime millimetre testing is under way whenever there is time available in the regular centimetre observing schedule, weather permitting.
At the heart of the new millimetre receivers are indium phosphide "monolithic microwave integrated circuits" (InP MMICs), cooled to 20 K, the product of a joint effort between the Australia Telescope National Facility (ATNF) and the CSIRO Division of Telecommunications and Industrial Physics (CTIP). The MMICs are one of several components being jointly developed under a special program established by former CSIRO Chief Executive, Malcolm McIntosh, to develop millimetre-wave integrated circuits for radio astronomy and telecommunications.
My own ATCA millimetre experience took place a few days after first light, on the 10th of December 2000 and in January 2001. SiO masers are instantly rewarding as the strongest are booming in during the first 10 sec integration cycle. So, I observed a few of them (Orion SiO, VX Sgr, R Dor, R Aqr, S Pav, X Pav, etc.) at velocity resolutions between 4 MHz and 31.25 kHz just for fun. Figure 2 shows a "long" integration (21 min.) spectrum of Orion SiO taken on the 11th of January with the 153m ATCA baseline. Any line centre or bandwidth adjustments can be done immediately as no tuning is required after the initial setup of the 3mm system. Planets are equally important and great calibrators as their millimetre continuum emission is so strong.
The highlight of my first millimetre observing session on December 10th, 2000, was the detection of HCO+ absorption against the nucleus of the nearby radio galaxy Centaurus A (see Figures 3) using the 31m ATCA baseline. The rest frequency of HCO+ is 89.188518 GHz. We used a bandwidth of 32 MHz divided into 128 channels giving a channel width of about 0.84 km/s. The spectrum is centred on the systemic velocity of Centaurus A (v_sys = 552 km/s, where the strongest HCO+ absorption line occurs. This line is also the broadest with a width of about 6 km/s, whereas the other two lines near 540 km/s are much narrower (about 1.5 km/s). The molecular absorption lines in Centaurus A are well-known (Gardner & Whiteoak 1976, MNRAS 175, L9; Eckart et al. 1990, ApJ 365, 522; Israel 1992, A&A 265, 487) and have recently been studied in detail by Wiklind & Combes (1997, A&A 324, 51) using the Swedish ESO Submillimetre Telescope (SEST). They also detect much fainter HCO+ absorption complexes at 560 - 640 km/s and potentially 500 - 540 km/s, although the latter range is uncertain due to potentail confusion by HCO+ emission and uncertain baseline removal. Future, more sensitive, ATCA absorption line measurements against the core of Centaurus A, which is smaller than 2mas according to 3cm VLBI observations by Jones et al. (1996, ApJ 466, L63), can provide several improvements over single dish observations: a flat baseline, emission filtering and higher sensitivity. Phase instabilities can be overcome by self-calibration. In July 2001 we expect a third antenna of the ATCA to be operational at 3mm, so imaging can begin in earnest ! With additional antennas getting ready every few months, a detailed study of the distribution of molecular gas in the dust lane of Centaurus A (Wild & Eckart 2000, A&A 359, 483) as well as the outer shells (Charmandaris, Combes and van der Hulst 2000, A&A 356, L1) will then be possible.
The ATCA 3mm system is easy to use: no tuning is required to change frequency, a benefit of the new-technology cooled InP MMICs which were jointly developed by ATNF and CTIP. The delay calibration is stable over the whole band and long time intervals. Since all millimetre testing has so far happened in the Australian summer, we are looking forward to the first winter observations. The new North-South spur (with a length of 214m) which was recently added to the East-West array will facilitate millimetre observations as shadowing of antennas at low elevation can be avoided.
For more details see http://www.atnf.csiro.au/mnrf/3mm_details.html.