You may not wish to make detailed high resolution images but instead, for example, survey a large number of compact or simple sources. Under these circumstances, if the sources are strong enough, short observations will suffice (Table 1.5). In scheduling these, slew time becomes an important consideration; these may be calculated (see Section 1.7.3), or with the use of the CABB scheduler. A single short observation in an E-W array will give a 1-dimensional (u,v)-coverage; two or more short observations will give some 2-dimensional information. In such short observations consequent higher sidelobe levels will exacerbate problems caused by confusing sources in the primary beam. Because the Compact Array has poor (u,v)-coverage, in contrast to its good sensitivity, this confusion makes it difficult to reach the receiver noise limit when observing weak sources or searching for detections, particularly at lower frequencies. Current experience suggests that at 6cm at least 8 short observations well distributed in hour angle are needed to reduce confusion to the noise level. At 16cm, short observations are unlikely to ever reach the noise limit.
Two orthogonal linear polarisations are measured simultaneously. The position angle of the polarisation splitter is stationary with respect to the alt-az–mounted antennas and so rotates on the sky. A suite of tasks in Miriad is available for polarimetric calibration of Compact Array data. No specific calibrators, other than the usual flux and gain calibrators, are needed for polarisation calibration. An observation of B1934-638 is essential when only making short observations. Small drifts with time can be corrected for using the on-line measurements of the XY phase differences to an accuracy of 0.5 degrees at all bands. Best polarimetric calibration results when the gain calibrator is observed at a good sampling of different parallactic angles.
Miriad is routinely used to calibrate data in all bands, with consistent results being achieved over several months in the cm bands. The on-axis instrumental polarisation is typically below 2-3%. After calibrating for instrumental polarisation, we are currently able to reduce on-axis instrumental effects to 0.1%, and better with some care.
The off-axis polarisation increases roughly as the square of the distance from the pointing centre at least up to the half-power point. At 16 and 4cm, the instrumental polarisation is about 3% of the apparent total intensity at the half power point, respectively. This error is almost purely linearly polarised (there is no circularly polarised component). Because the ATCA antennas have an alt-az mount, the off-axis response varies with parallactic angle, and will be smeared out by a factor of a few by a long synthesis. This smearing is a function of declination. The Miriad task offpol can be used to simulate off-axis polarimetric response of a long synthesis observation. Mosaicking smears out the off-axis response still further, by as much as an order of magnitude.
At 16cm, instrumental polarisation is largely independent of frequency. The leakages are now flat to better than 2% across the entire usable band. More detail on the polarisation properties of the ATCA 16cm receivers can be found in the receiver commissioning report.
Objects which are of similar size or larger than the primary beam will need to be mosaicked. In such cases, the recommended spacing of pointing centres is half of the primary beamwidth. The Compact Array antennas and control system allow for rapid switching between pointing centres — as frequently as once per integration cycle. This enables a source to be rapidly mosaicked without necessarily losing (u,v)-coverage. The antenna acceleration limit is 800 deg/min/min, and the slew limit is 38 deg/min in azimuth and 19 deg/min in elevation. Mosaicking mode may also be useful for observing large numbers of nearby sources, as the observing overheads are reduced.
There is also a facility for “on-the-fly" mosaicking, whereby the antennas are continuously driven. This type of mosaicking may be especially useful for large-area mosaics at low frequencies, where the drive times between mosaic pointings are significant.
Section 2.3.4 explains how to set up mosaic files. The Miriad User Guide describes how to reduce a mosaic data set.
As (u,v)-distance is proportional to frequency as well as baseline length, different (u,v)-spacings can be obtained not only by varying the antenna configuration, but also by varying the frequency.
Additional (u,v)-coverage can be obtained in the mm bands by observing at multiple frequencies. Observing at two frequencies has the added advantage of increasing sensitivity, as they can be observed simultaneously. Observing more than two frequencies requires time sharing. While this will not improve the sensitivity further, it can significantly improve the (u,v)-coverage. However, there is a trade off between gaps in the tangential and radial directions in the (u,v)-plane. Typically two or three pairs of frequencies, observing each setting for 10 minutes, is a good compromise.
When you use both bandwidth synthesis and two or three configurations, and require (u,v)-coverage to 6km, the best choice of configurations is not two or three 6km arrays, but a combination of 6km with 1.5km and 750m arrays (all arrays using the 6km antenna). The flux density of most sources will often vary significantly between different frequencies and, furthermore, this variation itself (i.e., the spectral index) will vary across the source. This complicates the task of combining data from the different frequencies when you want high-dynamic-range images. However software is available in Miriad to account for the spectral variations in the imaging, deconvolution and self-calibration steps. These algorithms solve for, or use, both a basic flux-density image and a spectral-index image. For typical spectral indices, they are appropriate for frequency ratios less than about 1.25.
After each antenna reconfiguration a pointing solution is determined. Generally, rms errors of around 5 arcsec are achieved at this time. These solutions degrade with thermal (and other) effects. By the time observations are undertaken, rms errors of about 10 arcsec (or more) are likely. There is a significant offset (of up to 30 arcsecs) between the mm and the cm receivers.
A reference pointing mode is available which is recommended for mm observations. In this mode, a 1 Jy calibrator, about 5° to 10° away from the target will hold the pointing to 10 arcsec rms. A bright (say 5 Jy) calibrator at 2° to 3° from the target will reduce the errors to about 5 arcsec rms. A reference pointing scan generally takes 2-3 minutes to complete. See the Reference Pointing Guide for details.