| CSIRO Australia Telescope National Facility ATCA Users Guide |
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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 (see Largest well-imaged structure). In scheduling these, slew time becomes an important consideration; these may be calculated from the information given in Mosaicing, or more easily, by running ATCASCHED. A single short observation in an E-W array will give a 1-dimensional strip distribution; 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 6 cm at least 8 short observations well distributed in hour angle are needed to reduce confusion to the noise level. At 13 and 20 cm, short observations are unlikely to ever reach the noise limit.
Observing Time 6 or 3km 1.5km 0.75km
25 days 6'[a] - -
12 days 4.5' 6' -
4 days 160'' 4' 8'
2 days 115'' 160'' 5'
1 day 80'' 115'' 230''
6x 1-10 min[b] 30'' 50'' 100''
1-10 min[c] 20'' 40'' 80''
[a] For other wavelengths, scale sizes by \lambda/6cm
[b] Well distributed in hour angle
[c] One-dimensional information only
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Largest well-imaged structure at 6cm wavelength
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 primary and secondary 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 secondary 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. Procedures for 3mm polarization observations are still being established.
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 20, 13, 6 and 3 cm, the instrumental polarisation is about 1.6%, 9%, 1.6% and 3% of the apparent total intensity at the half power point, respectively. At 20 and 6 cm this error is almost purely linearly polarised (there is no circularly polarised component), whereas at 13 and 3 cm the circularly polarised component is somewhat less than 1% at the half-power point.
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. Mosaicing smears out the off-axis response still further, by as much as an order of magnitude.
At 20 cm, instrumental polarisation has significant frequency dependence, showing variations of several percent at 1327 and 1444 \pm 5 MHz. Leakages of more than 10% occur at 4550 \pm 10, 5328 \pm 10 MHz and 8780 \pm 10 MHz. Data near these frequencies may need to be flagged.
Objects which of similar size or larger than the primary beam will need to be mosaiced. 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 mosaiced without necessarily losing (u,v)-coverage. In mosaicing mode, data is not recorded when antennas are driving between fields. 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. Mosaicing mode may also be useful for observing large numbers of nearby sources, as the observing overheads are reduced.
Section @ref{s:mosaic-file} 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). A program (MFPLAN) is available in MIRIAD to help select configurations and determine optimum observing frequencies.
The flux density 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 reconfiguration a pointing solution is determined at night when thermal effects are least; these typically show rms errors of 10 arcsec. These solutions degrade with thermal effects, especially in summer where an rms of about 30 to 60 arcsec is more likely.
A reference pointing mode is available. In this mode, a 1 Jy calibrator, about 5^\circ to 10 ^\circ away from the target will hold the pointing to 10 arcsec rms. A bright (say 5 Jy) calibrator at 2^\circ to 3 ^\circ from the target will reduce the errors to about 2 to 5 arcsec rms. Each reference pointing pattern takes typically 18 integration cycles. Reference pointing should be reserved for wavelengths of 12mm and below. See the Reference Pointing Guide for details.