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1.4. Choosing Angular and Frequency Resolution

1.4.1. Image Complexity, Angular Resolution and Observing Time

A synthesis imaging telescope such as the Compact Array provides a great deal of flexibility when deciding how to image an object. In addition to the well-known trade-off between observing time and sensitivity, we have trade-offs with maximum resolution and with the sampling interval in the visibility plane.

The choice of angular resolution is fairly straightforward. Increased angular resolution (resulting from longer baselines) leaves the point-source sensitivity constant, but decreases brightness sensitivity in proportion to the beam area (see Table 1.4).

The choice of (u,v)-plane coverage is more difficult. The amount of independent information needed to specify the image should not exceed the number of independent (u,v)-plane samples. But unfortunately neither of these “independents” is easily defined. At one extreme, consider making a high resolution image of a complex source filling the entire primary beam. In this case full (u,v)-coverage is obtained by using all baselines between 30m and 6000m in increments of 15m. Although this was theoretically possible (it takes 25 separate array configurations), it was never attempted and, since the decommissioning of the second 6km station, is no longer actually possible!

If on the other hand the image is smaller than the primary beam then the Nyquist sampling interval is larger and the number of (u,v)-samples required is reduced. The observing time can then be reduced either by decreasing the number of configurations or the amount of hour angle coverage, depending on the array configuration. For east-west arrays, reducing the total number of configurations is the more practical option. If the source is large but partly empty it can be considered to have a size corresponding to its area.

For ordinary observations Table 1.4 gives the maximum sizes of structures that can be reliably imaged for typical sets of observing configurations. This is only a rough guide since the actual coverage needed depends on details of the 2-dimensional brightness distribution, on the actual distribution of baselines, and on the type of deconvolution. Additionally, other techniques such as mosaicing and multi-frequency synthesis, can be extremely effective at improving (u,v)-coverage.

A further consideration is the minimum spacing available. This is never less than 30 m and for a given configuration can be much larger. This acts as a high-pass filter removing all Fourier components less than the minimum spacing. If this is a serious problem, short baseline information (e.g., from a single dish) can be added separately during processing.

1.4.2. Array Configurations and Baselines

Antenna 6 sits permanently on station W392. Maximum baselines to this antenna (the last 5 columns) are shown for all configurations in Appendix H, but form part of a designed array only for configurations 6.0A to 6.0D. The antennas can be moved to, and set up on, a limited number of fixed stations. Because of this and other physical restraints, the shortest spacing available is 30 m, the longest is 6 km, and the minimum grating increment is 15 m. From 2006 October, the standard set of configurations has been:

  • 1 hybrid configuration with a nominal maximum baseline of 75 m (H75)

  • 1 hybrid configuration with a nominal maximum baseline of 168 m (H168)

  • 1 hybrid configuration with a nominal maximum baseline of 214 m (H214)

  • 2 east-west configurations with a maximum baseline of about 375 m

  • 4 east-west configurations with a maximum baseline of about 750 m

  • 4 east-west configurations with a maximum baseline of about 1500 m

  • 4 east-west configurations with a maximum baseline of about 6000 m.

Snapshot observations using the Northern spur in hybrid configurations will generate a two-dimensional sampling of the (u,v)-plane. The Northern spur was installed so that good coverage of the (u,v)-plane was achievable for observations which are limited to hour-angles near transit. Observations at mm wavelengths should be limited to higher elevations to avoid large atmospheric opacities. Hybrid arrays are also useful for observations of northern sources which are similarly restricted in hour-angle range.

The 6 km antenna may also be added to any of the shorter configurations, but in these cases the distribution of array spacings is bi-modal.

The predetermined set of configurations offered for forthcoming observing terms will assist in planning multi-configuration proposals. Details of previous configurations and those being offered in coming semesters can be found on our website, or see Table 1.3.

Note that proposals requiring two or more configurations will usually be allotted two or more widely separated times; therefore, expect to make two or more observing visits, or conduct the second observation using remote observing. If you are not allotted all the configurations requested, you should re-apply in the next term, as the proposal will not be automatically reconsidered.

The overall philosophy is that, in each semester, there will be a 6km, 1.5km, and 750m configuration, and for these baselines, the full set of 4 configurations will generally be covered in three semesters. Each semester will also contain at least one 375m configuration. Analysis of user preferences for the past few years show some configurations to be more generally useful (e.g., providing better single-configuration (u,v)-coverage), and it is desirable to offer these more frequently. Millimetre observing conditions are optimal in the (southern hemisphere) winter, so arrays of 214m or smaller will be offered mainly in the April semester.

Table 1.3. Array configurations that will be offered in future semesters.

ArraySemester
2016OCTS2017APRS2017OCTS2018APRS2018OCTS
6A
6B   
6C    
6D   
1.5A   
1.5B    
1.5C   
1.5D   
750A   
750B   
750C   
750D   
EW367  
EW352
H214 
H168  
H75   


Besides the predetermined configurations, you can request any standard or non-standard configuration in any term. When writing your application, your scientific justification should include a very convincing argument of why you need to use a special array configuration, rather than one of those offered for the term. If you realise the need for such a “wildcard” request for a significant amount of observing time (e.g., more than 5 x 12h), you can enhance the probability of it being scheduled if you contact ATNF well in advance of the deadline (preferably even before the call for proposals announcement). The wildcard can then be advertised as a potential additional configuration for the term, which may then lead to other proposers requesting it, and making its scheduling viable.

For those who wish to improve their (u,v)-coverage by re-observing on different days with different antenna configurations, specific sets of configurations combines well, e.g., 6A, 6C, 1.5B and 1.5D — for more details see Appendix H.

An interactive tool, the Virtual Radio Interferometer (VRI), is available to assist users in exploring the (u,v)-coverage of standard (and non-standard) configurations.

It is strongly recommended that the proposer gives a clear indication of the maximum extent of their sources. You should also specify the maximum and minimum baselines, and the number of configurations (days) needed.

1.4.3. Bandwidths and correlator

The CABB now provides a variety of frequency resolutions. The available resolution options are more commonly denoted by the CABB configuration that is used to provide them. When development of CABB is complete, four correlator configurations will be offered:

Currently available modes
  • CFB 1M-0.5k: 2048 channels per 2048 MHz continuum IF (1 MHz coarse resolution), and 2048 channels per 1 MHz zoom band (0.5 kHz fine resolution).

  • CFB 64M-32k: 32 channels per 2048 MHz continuum IF (64 MHz coarse resolution), and 2048 channels per 64 MHz zoom band (32 kHz fine resolution).

    There is also a “hybrid" mode available which provides one continuum 2048 × 1 MHz IF (with no zoom bands), and a second continuum 32 × 64 MHz IF (with up to sixteen 64 MHz zoom bands).

Modes available in the future
  • CFB 16M-8k: 128 channels per 2048 MHz continuum IF (16 MHz coarse resolution), and 2048 channels per 16 MHz zoom band (8 kHz fine resolution).

  • CFB 4M-2k: 512 channels per 2048 MHz continuum IF (4 MHz coarse resolution), and 2048 channels per 4 MHz zoom band (2 kHz fine resolution).

It should be noted that the correlator routinely computes double the number of channels as is recorded. This second set of channels is offset from the available set by half the channel width, and are there to provide flexibility in choosing the zoom band frequency, as well as to ensure that it will always be possible to avoid observing a spectral line at the edge of a channel. It is also possible to combine a number of overlapping zoom bands that are separated by half a continuum channel into a seamless single zoom band.