| CSIRO Astronomy and Space Science ATCA Users Guide |
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The next two sections describe some necessary considerations for both centimetre and millimetre observations at the ATCA. This section on centimetre observations is also relevant for millimetre observers and should be read by everyone planning to propose for the ATCA. The millimetre section describes some additional requirements specific to millimetre observations.
| Array | 6km | 1.5km | 750m | EW352 | H214 | H75 | |
| Band | |||||||
| 16 cm | 0.01 K | 1 mK | 0.4 mK | 0.07 mK | 0.1 mK | 0.01 mK | |
| 6 cm | 0.02 K | 2 mK | 0.5 mK | 0.1 mK | 0.1 mK | 0.02 mK | |
| 3 cm | 0.02 K | 3 mK | 0.7 mK | 0.2 mK | 0.2 mK | 0.02 mK | |
| 15 mm | 0.02 K | 2 mK | 0.6 mK | 0.2 mK | 0.2 mK | 0.02 mK | |
| 7 mm | 0.04 K | 4 mK | 1.1 mK | 0.2 mK | 0.3 mK | 0.03 mK | |
| 3 mm | 1.6 mK | 3.0 mK | 0.3 mK | ||||
Table 1.2: Continuum brightness temperature sensitivity (12 hr integration) for bandwidth specified in Table table_sensitivity, for various arrays.
| 1.3.1 Calibration | ||
| 1.3.2 Sensitivity Calculations |
There are four classes of data calibration: flux, bandpass, polarization and gain. The ATCA instrumental bandpass, polarization response and gain amplitude are all relatively stable and flux, bandpass and polarization calibration can be done just once during an observation. At centimetre wavelengths these three calibrations can be performed with an observation of B1934-638. This is often called the primary flux calibrator. Atmosphere-induced phase and amplitude gains are more variable and brief observations of suitable calibrator sources (gain calibrators) are usually interleaved with observations of the target source. In addition, observations are made to calibrate aspects of the instrument’s performance: antenna pointing and instrumental delay. Pointing calibration is normally only necessary at short wavelengths 7mm and 3mm, and involves periodic observations of a nearby point source. Differences in signal arrival times at the correlator arise from the geometric delay, which can be calculated, and instrumental delays, which must be measured. Instrumental delays vary slowly and are measured with a single observation of a strong point source at the beginning of an observing session.
| 1.3.1.1 Flux Calibration | ||
| 1.3.1.2 Gain Calibration | ||
| 1.3.1.3 Bandpass Calibration |
All observations must be ultimately related to a compact source in the southern sky whose flux is constant, unpolarised, and known. For frequencies below 30 GHz, PKS B1934-638 is used; this source should be observed at least once a day. Its flux density (following a revision of the flux density scale in August 1994) is 12.6, 5.0 and 2.7 Jy at 2100, 5500 and 9000 MHz, respectively (Reynolds 1994: http://www.atnf.csiro.au/observers/memos/d96783~.pdf) and 1.2 and 1.0 Jy at 17000 and 19000 MHz respectively (Sault 2003: http://www.narrabri.atnf.csiro.au/calibrators/data/1934-638/1934_12mm.pdf). The flux densities are incorporated in the on-line calibration routine in CAOBS, and in the MIRIAD calibration software. For frequencies above 30 GHz, observations of a planet (Uranus is recommended, Mars may be an alternative in the most compact arrays) will be required for primary amplitude calibration (in suitably compact configurations). Suitable software exists in MIRIAD for using planetary calibration data.
The calibrators database gives information on potential sources that may be used to measure the interferometer amplitude/phase calibration. A round-trip phase correction machinery automatically corrects for the changes in electronics path length in real time; therefore, the astronomical gain calibration observations are for taking out the atmospheric path changes and any errors in the baseline parameter determinations. The on-line system temperature calibration takes out the temporal changes in the electronics gain but does not correct for elevation dependence of either antenna (optical) gain or atmospheric opacity.
A series of brief gain calibrator observations are important to correct for these effects. The gain calibrator should be close to the target source. Observers concerned with accurate positions will also need to choose a gain calibrator whose J2000 position is accurately known. It is preferable to make \sim2min calibrator observations once every \sim30min. Less-frequent calibration is possible at 16cm and more frequent calibration may be required at 3cm. At 15mm and shorter wavelengths, it is preferable to calibrate as frequently as possible (consistent with not spending too much time off source), unless the array is very compact.
In summer, or during the day, phase stability can be poor. Phase stability can be monitored for bright compact sources with VIS, or with the ATCA seeing monitor. The ATCA Seeing Monitor describes the system as initially installed. In 2008 a new satellite beacon was adopted, resulting in a change in the frequency from 30GHz to 21GHz.
Observations requiring maximum phase stability (e.g., at wavelengths less than 6cm) should therefore be made in winter, or at night. Most compact sources are variable, but they can be calibrated for the observation against PKS B1934-638.
The most up-to-date calibrator list is available by position or flux-limited online search: http://www.narrabri.atnf.csiro.au/calibrators/.
A spectral-line observation will normally require a bandpass calibrator. At low frequencies, the primary calibrator PKS B1934-638 is usually enough, but at wavelengths of 3cm and shorter a source such as 0537-441, 1253-055, or 1921-293 (usually >5Jy) may be needed. The Compact Array bandpasses are stable, and so a single bandpass calibration is normally sufficient, unless you need high dynamic range.
The general expressions for the flux and brightness sensitivities are given in the document AT/01.17/025, and these general expressions have been used in Table table_sensitivity. The observer has control of the integration time (t), bandwidth (B), observing wavelength (\lambda), number of baselines (N) and synthesized beam size (\theta) only, and with these variables and the system sensitivity (S_{sys}), the expressions reduce approximately to:
rms Flux Sensitivity = \Delta S = 7.55\times10^{-5} S_{sys}(N t B)^{-1/2} (Jy), and
rms Brightness Sensitivity = \Delta T = 1.36\times10^3 \Delta S \lambda^2/(\theta_{\alpha}\theta_{\delta}) (K),
where S_{sys} is in Jy, t in min, B in MHz, \lambda in cm, and \theta is in arcsec. For the full 6 km array, N=15. There is a convenient ATCA sensitivity calculator at http://www.narrabri.atnf.csiro.au/cgi-bin/obstools/atsen8.pl which takes into account system temperature, observing frequency, array, correlator configuration and (u,v)-weighting. It has been updated for CABB observing.
Users Guide last modified on 2011-04-27 15:49:06