| CSIRO Australia Telescope National Facility ATCA Users Guide |
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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).
All six antennas of the Compact Array are equipped with 12mm receivers covering the frequency range 16–26GHz. The 12mm system shares a common dewar with the 7mm and 3mm receivers, 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 12, 7 and 3 mm receivers have separate feed horns, observations cannot be simultaneous in the 12, 7 and 3 mm bands.
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.
The inner five ATCA telescopes (i.e., excluding CA06) are outfitted with a 3mm receiver and can observe in the range 83.5 to 106GHz. A noise diode has been added to the 3mm system on CA02 (only) to aid with 3mm polarization 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
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 bandpass calibration \rightarrow Paddle calibration \rightarrow Phase calibrator \rightarrow Target source \rightarrow Phase calibration \rightarrow Paddle calibration \rightarrow Target observation.
Either the command line of ATCASCHED, or the web version
http://www.narrabri.atnf.csiro.au/observing/sched/
can be used to construct the observing schedule.
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 http://www.narrabri.atnf.csiro.au/observing/shadowing/ for details.
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.
Generally, planets are the most reliable mm primary flux calibrators, the best being Uranus. Details of planet rise and set times can be determined at http://www.parkes.atnf.csiro.au/cgi-bin/utilities/planets.cgi
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 http://www.narrabri.atnf.csiro.au/calibrators 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.
To find the nearest phase calibrator to your source, use the position and flux-limited calibrator online search engine from the ATNF calibrator catalogue http://www.narrabri.atnf.csiro.au/calibrators/ 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 http://www.narrabri.atnf.csiro.au/calibrators/calcycle.html can be used to estimate of the rate of phase calibrator measurements for a number of different phase decorrelation percentages.
Daytime observations, or observations with long baselines should require much more frequent phase 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 phase calibrator.
The test quasar should show up if the phase 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, phase-cal, and test quasar that is repeated every 5 minutes.
Regular pointing checks (every hour or so) are essential because of the small size of the ATCA primary beam at 3mm.
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 1Jy or more will suffice.
This is a standard procedure at 3mm wavelengths for placing the amplitudes on a temperature scale. You will need to include periodic paddle measurements in your ATCASCHED file — about every half hour should be sufficient. The entire paddle scan takes about 2 minutes.