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1.6. Millimetre-wave observations (15mm–3mm)

Technical details of the ATCA mm systems are presented in MMICS for the AT mm-wave receiver system by Gough et al. (Proc. 12th European Gallium Arsenide and other Compound Semiconductors Application Symposium, 2004, pp.359-362) and Cryogenically Cooled mm-wave Front Ends for the Australia Telescope by Moorey et al. (Proc. European Microwave Conference, 2008, pp.155-158). This section builds on many of the concepts described in the previous section on centimetre wavelength observations, which should be read before reading this section.

1.6.1. 15mm Observations

All six antennas of the Compact Array are equipped with 15mm receivers covering the frequency range 16–25 GHz. The 15mm system shares a common dewar with the 7mm and 3mm receivers, and the separate feed horns at the top of the dewar are moved to the Cassegrain focus of the Compact Array antennas using the rotator positioning system. Because the 15, 7 and 3 mm receivers have separate feed horns, observations cannot be simultaneous between the 15, 7 and 3 mm bands.

1.6.2. 7mm Observations

The ATCA can observe within the frequency range 30–50GHz. Feeds and receivers were installed into the existing mm-wave receiver packages on all 6 antennas in 2007, with the 7mm upgrade jointly funded by the ATNF and NASA's Deep Space Network to enable the ATCA to participate in occasional spacecraft tracking.

The wide 7mm band presents difficulties in signal down-conversion, as the aliased signal from the first down conversion stage can also be within the 7mm band. Although image rejection filters are used, variations in receiver gain across the band, combined with the frequency-dependent performance of the filters themselves, can result in appreciable signal levels being added to the observing band. Generally, this aliased signal does not cross-correlate and is present as a contribution to the noise level. However, in some circumstances, notably as the delay rate drops to zero (e.g., around source transit on north-south baselines), the aliased component can cross-correlate and be visible as “beating" on some baselines. This data must be flagged during the data processing.

1.6.3. 3mm Observations

The inner five ATCA telescopes (i.e., excluding CA06) are outfitted with a 3mm receiver and can observe in the range 83 to 105 GHz. A noise diode has been added to the 3mm system on CA02 (only) to aid with 3mm polarisation calibration. Aliasing effects similar to those described at 7mm can also arise in the 3mm band.

A limited form of flexible scheduling is operational for observations at millimetre wavelengths. Consult the document Flexible Scheduling at ATCA for details.

A quickstart guide for 3mm observing is also available.

1.6.4. Procedure for mm observations

Observations in the mm band are run from a schedule file in the same way as cm-band observations. However, due to the effects of atmosphere stability, mm observations require bandpass and phase-calibration checks at a much more frequent rate.

Note that as ATCA does not Doppler track, it is important to have a good estimation of the magnitude of the Doppler shift for the line of interest. This is particularly so for the higher frequencies available to the 3mm system. The sky frequency can be calculated using an online calculator.

A simple phase reference observation takes about an hour to run, including calibration. The following procedure is typical:

Pointing calibration → Paddle calibration → Bandpass/Phase calibrator → Target source → Paddle calibration → Bandpass/Phase calibration → Target observation.

The CABB web scheduler can be used to construct the observing schedule. Warnings for mm observations

Elevation angles close to the array horizon should be avoided because of the increased opacity and poorer atmospheric stability. Therefore, observations of objects with a declination further north of -50° are best made using hybrid configurations (i.e., one using the N-S spur) in order to achieve sufficient (u,v)-coverage.

Observations using very compact telescope configurations should be made with caution, as telescope shadowing can be a significant problem, especially for sources at declinations which do not achieve elevations high above the horizon. See Section H.2 for details.