Design and Operation of the ATCA

The Australia Telescope Compact Array is an earth-rotation aperture synthesis radio interferometer. Earth-rotation aperture synthesis was first used in the 1950s for radio observations of the sun. The technique was demonstrated by Christiansen and Warburton (1955) and Ryle (1962) and is well explained in Thompson, Moran & Swenson (1986, 2001) and Chapter 1 of Perley et al. (1989). Chapter 1 of Thompson, Moran & Swenson includes a historical review. Essentially, the antennas are used in pairs to form two-element interferometers. The visibility (i.e., the fraction of the signal common to both antennas of a pair) is derived by multiplying the signals together. By combining the correlated signals obtained over a long period of time and with a large range of spacings between antennas, the Compact Array measures the spatial coherence function:

$$V = \int{{\mathbf I}_{\nu (s)} e^{2 \pi i {\mathbf s} . ({\mathbf r_1} - {\mathbf r_2}) } d {\mathbf \Omega}}$$ (1)

where ${\mathbf I}_{\nu (s)}$ is the two dimensional intensity distribution on the sky, ${\mathbf s}$ is the unit vector in the direction of the celestial source, $({\mathbf r_1} - {\mathbf r_2})$ is the separation vector between antennas 1 and 2 and $d {\mathbf \Omega}$ indicates integration over the sphere.

By Fourier transformation of the results obtained, an image of a radio source is produced. The image is formed with the same angular resolution as for observations with a single antenna of diameter equal to the largest spacing.

The ATCA consists of six 22-m radio antennas. Five of the six antennas are positioned at station posts along a three kilometre railway track oriented east-west. Since September 2001, a 214m north-south track has been in operation, allowing the five movable antennas to be configured in more compact configurations than previous arrays, allowing full-synthesis observations to be made in less than 12 hours. The sixth antenna is fixed on a station three kilometres to the west of the western end of the 3km railway track, thus allowing a maximum ATCA baseline of six kilometres. Five of the six ATCA antennas are shown in the figure below. These five antennas can be positioned at any of 44 fixed stations. The stations contain ports that allow the antennas' data to be transferred to the correlator in the control building. Owing to the location of stations and other physical constraints, the smallest baseline increment available is approximately 15m, and the shortest physical baseline is approximately 30m.

Figure 1.1: Five of the antennas in the 122m array

Each year is divided into 2 observing semesters. During each semester array maintenance, upgrades and antenna reconfigurations (moving the antennas to different stations) as well as astronomical observations are scheduled. The location of six antennas at six stations is called an array, an array configuration, an antenna configuration, or often, simply a configuration. In order to obtain full ($u,v$) coverage with the ATCA, you need to observe, using many different configurations, for twelve hours with each configuration. Almost any program can be successfully carried out with less than complete uv coverage, however, and configurations are chosen so as to optimise single configuration imaging characteristics. Sophisticated off-line image processing techniques minimise the effect of missing uv coverage and allow reasonable images to be made with one configuration. For most programs one to four configurations provide the best compromise between dynamic range, uv coverage and time.

The configurations offered at present (except for the 122m option) have been designed to give optimum, minimum-redundancy coverage after a 12 hour observing period. Sets of four configurations for the principal arrays (0.75, 1.5 and 6.0km) are offered and form recommended observing sets. See the ATCA Configurations page for a list of currently offered configurations and their baseline lengths. Your choice of configuration depends on the extent, brightness and complexity of your source (see below).

The antennas have a Cassegrain design, i.e., the receivers are located in a turret that protrudes through a hole in the main reflector surface (see the figure above). The antennas have an altitude-azimuth mount with wrap limits as shown in this figure. The shaped (i.e., non-parabolic) dish and subreflector surfaces are designed to maximise the gain to antenna noise ratio. The reflecting surface of antennas 1-5 is solid panels that allow observations up to 116GHz. This is also true of the inner 15.3m of antenna 6. The outer reflecting surface of antenna 6 is perforated panels which are accurate enough to permit observations at frequencies up to 50GHz.

The subreflector mounted near the prime focus has a specially shaped pseudo-hyperbolic design, unlike the classical optical Cassegrains, which have parabolic secondary mirrors. The feedhorns are mounted along the main optic axis - this allows polarisation measurements to be made, with very low ($\sim 1\%$) instrumental polarisation. Subreflector focus is presently chosen to best suit 3mm observations: This means that the subreflectors positions may not be ideal for longer wavelengths. If you need better focus settings, discuss your requirements with local staff.

A major feature of the ATCA is its wide bandwidth operation. The feedhorns and front-end electronics operate over a very large range of frequencies, thus allowing uv coverage to be increased by multi-frequency synthesis and dual frequency observations. The feedhorns are compact, with a corrugated interior surface and are designed for maximum frequency coverage, low noise, low spillover, low reflection and low cross-polarisation sidelobe levels. The feedhorns allow two simultaneous orthogonal linear polarisations to be measured (at frequencies almost an octave apart), and have a main lobe with near-constant, symmetrical beamwidth (James 1984, Thomas et al. 1986).

The frequencies at which the Australia Telescope operates are listed in this table.

For information about the current status of the ATNF facilities see:
http://www.atnf.csiro.au/observers/apply/avail.html

For the most recent information on the status and capabilities of the millimetre wavelength receivers, see the following webpages:
User's Guide to 3mm Observing with ATCA:
http://www.atnf.csiro.au/observers/docs/3mm/index.html

Each range of frequencies is referred to as a band and it is an unfortunate but common practice for these bands to be referred to by letters. The relationship between letters and frequencies is also shown in the same table. For anyone not familiar with radio jargon, there will not appear to be any sense to these band designations. Band designations were inherited from Allied forces' secret codes used to refer to the bands during World War II. The letters were carefully chosen so as not to have any sensible or logical relation to frequencies in order to confound Axis engineers. This lingering legacy of military obfuscation can be a source of irritation and confusion for astronomers. In this manual, bands will be referred to by the wavelength of the approximate centre of the band or (especially at higher frequencies) by the frequency. There are presently four feedhorns mounted on each antenna (three on antenna 6): a large (2m high) feed-horn that operates at both the 20-cm/13-cm bands, and a somewhat smaller (50cm high) 6-cm/3-cm band feedhorn and two (a few cm high) feedhorns mounted on the same dewar for the 12mm and 3mm wavelengths.

The feedhorns and the receivers are mounted on a rotating turret. The rotating turret design ensures that the feedhorns are aligned with the optic axis of the antenna, allowing a wide field of view and dual-polarisation observations. The turrets are rotated automatically in accordance with the frequencies selected by the observing file. Rotating the turret orients the desired feed-horn and also ensures that the subsequent electronics suit the frequency at which you are observing. The 6-cm/3-cm band feed-horn is fitted with separate 6-cm and 3-cm receivers, while the 20-cm/13-cm band feedhorns have 20-cm and 13-cm receivers. All receivers run continuously and use cooled FET and HEMT amplifiers that provide wide bandwidths and total system temperatures between 30K to 65K, depending on frequency.

The signals collected by the feedhorns are fed to the receiver systems for amplification and conversion to lower, more manageable frequencies. During observations each antenna provides four independent intermediate frequency (IF) outputs (two frequency bands, two polarisations). These channels allow simultaneous observations of both polarisations at the two available frequencies in either dual-receiver system. The frequencies can be anywhere in the range covered by the selected feed-horn except at 12 and 3mm where the two frequencies must be less than 2700MHz apart. Note that the second frequency has a smaller number of filters available, and can only observe at 128, 64 or 16MHz bandwidth. (The 16MHz filters were installed in 2005.)

In order to convert the received radio signals to lower frequencies, the radio signals are mixed with signals produced by a part of the CA known as the local oscillator. Four local oscillator signals provide four IF outputs to enable the dual-frequency, dual-polarisation operation. You can also switch frequencies at the end of each integration cycle (typically ten seconds). To change to a pair of frequencies covered by a different feed-horn requires a rotation of the turret and takes about twenty seconds. However, to avoid excessive wear of the turrets, there is a limit of 4 rotations in an hour. Thus time sharing between a number of frequencies is limited only by signal to noise ratio, wear and tear on the equipment, your imagination, and the off-line software.