"The MM-Wave Upgrade"

Fig. 1: Five antennas of the ATCA. (copyright CSIRO, photo J. Masterson)

• The spectrum at millimetre wavelengths is incredibly rich in spectral lines, most of them coming from rotational transitions of various molecules and their isotopes (e.g., C_3H_2, SiO, 13CS, HCN, HCO+, CH_3CN, CS, SO, 13CO, CN, CO), and allows us, for example, to analyse the chemical composition of molecular clouds in our Galaxy.
• Objects such as the Galactic Centre, the southern Galactic Plane, our nearest neighbours the Magellanic Clouds including supernova SN1987A, and many southern stars, galaxies and quasars will be observed for the first time at high frequencies and high angular resolution (> 0.3").
• Water-vapour masers at 22 GHz have recently been used to infer a black hole in the centre of the galaxy NGC 4258. We will use the ATCA to measure the kinematics of masers in many southern galaxies which are similar to NGC 4258 (e.g., Circinus, NGC 4945) and search for masers in other galaxies.
• A large amount of cold molecular gas is a good indicator of star formation in galaxies as stars are made out of this gas. In the nuclei of strongly interacting or merging galaxies the star formation rates are particularly high (see Fig. 2) and, consequently, we find a high concentration of dense molecular gas there.
• Whereas most telescopes work at either centimetre- or millimetre-wave- lengths (because of the different requirements and observing techniques) we will have a huge telescope which can do both.

Fig. 2: The nuclei of two colliding galaxies, NGC 4038/9 also called ``The Antennae'', observed at a wavelength of 13 cm.

• design and construction of six cryogenically cooled 3.5-mm receivers covering the frequency range of at least 85-95 GHz; this includes one spare. Only the five antennae on the 3-km arm of the ATCA will be outfitted with these receivers;
• design and construction of seven cryogenically cooled 12-mm receivers covering the frequency range of about 18-25 GHz; this includes one spare. The receivers will be co-located with the 3.5-mm receivers in the same cryogenic dewar in the receiver turret. Options for a 7-mm future upgrade will be considered in the design of the dewar;
• extending the solid surface of five ATCA antennae from 15- to 22-m diameter. Holography will be used to adjust the full surface of each antenna to an accuracy of 0.15 mm;
• study of atmospheric phase correction strategies, e.g. by using water-vapour radiometers operating at either 225 GHz or 22 GHz;
• local oscillator upgrade to achieve the desired phase stability for operation at millimetre wavelengths using optical fibres;
• add several extra antenna stations on the existing east-west track;
• construction of a short north-south spur with several new antenna stations.

Fig. 3: Six telescopes make 15 two-element interferometers. Shown are the tracks of each interferometer observing a source at a declination of -50 degrees. Displayed on the left is the uv-coverage of a 12-h observation with the telescopes aligned on the current east-west track (Figure 1). The more dense and uniform the coverage the easier it is to reconstruct an image of the source from the observations. Seen on the right side is a combination of two 6-h observations, one with a standard east-west array and the other with a new configuration including the north-south spur. This observing mode will be used mainly at high frequencies to avoid taking data at low elevation where the atmospheric attenuation is large. --- Using a Java applet you can create your own Virtual Radio Interferometer (V.R.I.) at http://wwwnar.atnf.csiro.au/astronomy/vri.html.

Fig. 4: The 22-m Mopra antenna. (copyright CSIRO, photo J. Masterson)