1.4 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.

1.4.1 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.

1.4.2 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.

A swap-program may, if possible, be scheduled for 7mm projects — see the following section for details.

1.4.3 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 (http://www.narrabri.atnf.csiro.au/observing/flexsched.html) for details.

A quickstart guide for 3mm observing is available at
http://www.atnf.csiro.au/observers/docs/3mm/quickstart.html.

1.4.4 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 at

http://www.narrabri.atnf.csiro.au/observing/obstools/velo.html.

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

Pointing calibration \rightarrow Paddle calibration \rightarrow Bandpass/Phase calibrator \rightarrow Target source \rightarrow Paddle calibration \rightarrow Bandpass/Phase calibration \rightarrow Target observation.

The CABB web scheduler at

http://www.narrabri.atnf.csiro.au/observing/sched/cabb/.

can be used to construct the observing schedule.

1.4.4.1 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^\circ are best made using hybrid configurations, or 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 Shadowing Diagrams for details.

1.4.5 Calibration

For observations using a narrow bandwidth, a delay calibration is best made using a coarser channel resolution. Note that using a different correlator configuration will not affect your delay calibration as long as the frequencies and bandwidths match those of your target observations.

1.4.5.1 Flux calibration

At wavelengths shorter than a few centimetres, extra-galactic sources generally prove to be too variable to be useful as flux density calibrators. So at these wavelengths, the blackbody emission from the planets are often used as flux standards. The two most commonly used planets for the ATCA at 3mm are Mars and Uranus. Note Jupiter is too large for use as an ATCA flux density calibrator. Neptune would also be a possible flux density calibrator. However Neptune is quite close to and half the strength of Uranus, and so does not offer anything over Uranus.

Uranus is the preferred flux density calibrator for the ATCA. Mars is less suitable for a number of reasons: some models of Mars’ average brightness temperature suggest variations in the temperature of up to 10% as a result of Mars’ diurnal rotation, its distance from the Sun and the viewing geometry of Earth-based observers. Definitive measurements and confirmation of these models are poorly studied in the literature. Depending on its distance and the array configuration, structure on Mars is also readily detected with the ATCA: Mars has hot equatorial and cold polar regions.

Details of planet rise and set times can be determined at
http://www.parkes.atnf.csiro.au/cgi-bin/utilities/planets.cgi.

1.4.5.2 Bandpass calibration

Bright extragalactic continuum sources (such as 0537-441, 1253-055, or 1921-293) are useful for bandpass and delay calibration as well as pointing. A 15 minute integration will usually provide an adequate bandpass, except at the narrowest bandwidths where extra time should be allowed. Refer to the ATNF calibrator catalogue and use the form to select bright sources (by specifying a lower limit to the flux density) near your target source – but be sure to avoid CenA, i.e., 1322-427 at cm wavelengths and other sources with large “defects” (see catalogue webpages for details) at your frequency in your array, and resolved planets.

1.4.5.3 Gain calibration

To find the nearest gain calibrator to your source, use the position and flux-limited calibrator online search engine from the ATNF calibrator catalogue. The list of calibrators interrogated by this engine includes OVRO and BIMA sources, which will be useful for observations of sources north of -30^\circ.

The spatial and time separation of source and phase calibrator measurements has a strong dependence on telescope configuration and the atmospheric conditions. The ATNF online calibrator cycle calculator can be used to estimate of the rate of gain calibrator measurements for a number of different phase decorrelation percentages.

Daytime observations, or observations with long baselines should require much more frequent gain calibration measurements than those made during nighttime observations and with compact configurations.

A technique that can be applied at frequencies and array configurations where decorrelation is severe, is to observe a relatively weak (0.5 Jy) ‘test’ quasar, near the target source, in addition to your gain calibrator.

The test quasar should show up if the gain calibration is adequate and it is possible to determine whether a non-detection is due to a weak source or just bad phases. Another reason to observe a second (not necessarily weak) quasar is to check the reliability of phase referencing. The observation would consist of a mosaic of source, gain-cal, and test quasar that is repeated every 5 minutes.

1.4.5.4 Pointing calibration

Regular pointing checks (every hour or so) are essential because of the small size of the ATCA primary beam at mm wavelengths. Pointing corrections are valid within an area with a diameter of around 15-20 degrees, centred on the azimuth/elevation of the pointing scan.

To refine your pointing during your observation, it is best to choose a pointing calibrator near your target source (within 10^\circ if possible), not only because the pointing changes with position, but also because you will want to observe it throughout your run. Generally, any quasar with a flux density of 100 mJy or more will suffice.

1.4.5.5 Paddle (vane) calibration

At millimetre wavelengths, the atmosphere can no longer be approximated as perfectly transparent. It degrades overall sensitivity in two ways: the atmosphere emits radiation, and so raises the system temperature, and the atmosphere attenuates the astronomical signal. For ATCA at 3mm wavelength, the effect of the atmospheric opacity is corrected for via a measurement of the effective system temperature - the the so-called "above atmosphere" system temperature. More details are available in the MIRIAD User Guide.

The ATCA uses the chopper wheel method (also call paddle or vane calibration) in its 3mm system to determine an above atmosphere system temperature. This involves placing an ambient load (paddle) in front of the receiver horn. This is a standard procedure at 3mm wavelengths for placing the amplitudes on a brightness temperature scale. You will need to include periodic paddle measurements in your schedule file — about every 10–20 minutes. The entire paddle scan takes about 2 minutes.


Users Guide last modified on 2013-02-18 04:17:11