| CSIRO Astronomy and Space Science ATCA Users Guide |
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During a normal observation, many different calibration tasks need to be performed. This section describes all these different tasks, and how best to schedule them.
| 2.2.1 Flux Calibration | ||
| 2.2.2 Bandpass and Array Calibration | ||
| 2.2.3 Gain and Pointing Calibration | ||
| 2.2.4 Polarisation Calibration | ||
| 2.2.5 Calibration Time Guidelines |
Flux calibration is required to translate the arbitrary gain scaling that is produced by an observation to an absolute flux scale. The most effective way of doing this is by observing a calibrator that has a known flux (on the absolute flux scale) and comparing it to the sources that you are observing that have unknown fluxes.
For the ATCA, there are currently only four flux calibrators, and of those only two are regularly used.
For frequencies between 1 GHz and 25 GHz, the preferred flux calibrator is PKS 1934-638. It has a known, stable flux, and conveniently has no linear or circular polarisation. The flux models for 1934-638 are described in the memos:
For frequencies higher than 25 GHz, the preferred flux calibrator is the planet Uranus. Its flux is known to vary with time, but it does so in a way that is understood and can be modelled. The planets Mars and Neptune can also be used, but Uranus is preferred because its angular size is smaller than Mars (making it easier to observe with typical Compact Array baselines), and it is brighter than Neptune (which would require a longer scan to provide the same signal-to-noise level).
To be as effective as possible, the flux calibrator should be observed when it is at the same elevation as the target source, and at as high an elevation as possible. Doing this means that any gain-elevation dependence is reduced, and the effect of airmass is also reduced. At low frequencies (below 7 GHz), these requirements are not as important as they are for higher frequencies, where the atmospheric effects become a large factor. Indeed, many (if not most) centimetre observers simply make a scan on 1934-638 at the beginning of their observations, and this is usually good enough to get a flux uncertainty of only 10%.
Because 1934-638 is a point source (at least at the resolution of the Compact Array), it can be observed the same way as any other calibrator. That is, a scan on 1934-638 should be included in the schedule, and should be preceded by a pointing scan when it is observed with the 15mm receiver.
Observing the planets is a little more complicated. Because they are not point sources on even the most compact of ATCA baselines, they cannot be used as a pointing reference. A nearby source must therefore be used to determine the pointing offsets before observing the planet. Also, because the planets will substantially fill the primary beam of the ATCA antenna, the apparent system temperature of the antenna will be increased while a planet is being observed. Because of this, if a paddle scan is made while observing a planet, the system temperature will not be correctly determined, and the flux scaling will be in error. When observing with the 3mm system therefore, the paddle scan should be made while observing the same nearby pointing reference calibrator.
To make a good observation of Uranus, follow the procedure:
The procedure is similar for the other planets. Note that because the phases of the nearby calibrator are not used by the Miriad flux calibration task MFBOOT, it is not absolutely necessary to bracket the planet with a calibrator observation. It can however be useful to have that data to diagnose potential problems, should they arise.
The ATCA receivers do not have perfectly flat responses to incoming radiation as the frequency changes. To determine how the the receiver responds as a function of frequency, it is necessary to make an observation of a bandpass calibrator.
A bandpass calibrator can be any source that has sufficient flux to enable the receiver response to be determined without thermal noise contributing significantly (ie. the observation must have a high signal-to-noise ratio). With CABB, this requirement is easily met by all but the weakest calibrators. It is useful however to choose a very bright source as the bandpass calibrator, because only a short integration on such a source is required to get a good bandpass calibration. Generally, one of the following sources is used for bandpass calibration, and which one is chosen is usually dependent on the LST at which the observation is made.
Because of their brightness, these calibrators are also useful for the initial setup of the array (determination of array delays and phases), and as such they are also often referred to as “setup calibrators”. It is recommended that one of these calibrators be used for this purpose.
As the receiver response does not change significantly as a function of time, it is usually sufficient to make an observation of a bandpass calibrator at the very start of the allocated time. Conveniently, if one of these bandpass calibrators is used as a setup calibrator, then the bandpass calibration observation can be made immediately following the completion of the setup without further slewing overhead. The amount of time spent integrating on the bandpass calibrator depends on its flux, but generally 5-10 minutes is sufficient at all frequencies. It is necessary to observe the bandpass calibrator with each frequency configuration that will be observed, as the bandpass solution will be frequency dependent. If the observations are at 3mm, a paddle measurement should be made immediately before the bandpass observation.
As the antenna track the source around the sky, the conditions between the source and the antenna will vary, and as such, the gain of the system will change. These changes will be related to atmospheric conditions, gravitational deformation of the dish, ground spillover, etc.
To determine how the gain changes with time, it is convenient to observe a very simple source that is not expected to change over the period of the observation. More precisely, by observing a source that looks the same irrespective of time or (u,v) coordinates, changes in gain can be determined directly. Therefore, gain calibrators should be unresolved, with a stable flux over the observation timescale.
The ATCA has a large list of gain calibrators, spread over the viewable sky. This list can be queried using the web interface at http://www.narrabri.atnf.csiro.au/calibrators/. This interface queries the database in one of three ways:
Currently, two separate databases can be queried in the same way. The “Old” database consists of C007 monitoring observations of all the calibrators, primarily with the original ATCA correlator. This database does not always have the latest set of C007 observations, so is usually out of date. The “New” database is based on a pipeline reduction of all calibrator observations made with the CABB correlator, and on manual reduction of C007 observations as soon as possible after each run. Although this new database is still under development, most recent flux values can be considered as reliable.
The ideal gain calibrator would be very bright – to maximise the signal-to-noise ratio of the gain calibration – and very close to the target source. A gain calibrator that is too far away from the target source may not be sampling the same atmospheric conditions that exist between the source and the telescope, which might potentially lead to large phase errors in the gain solution, particularly in bad weather and at high frequencies. It is generally recommended that the gain calibrator be within 10 degrees of the target source.
Because of these restrictions, the gain calibrator also presents itself as the ideal candidate for pointing calibration. At high frequencies, where the primary beamsize is relatively small, it is very important to ensure that the telescope is pointing as accurately as possible. The ATCA has a facility to correct the pointing in a localised area of sky, by means of a pointing scan. This scan requires that it be executed on an unresolved, bright source – exactly the same criteria as for the gain calibrator. It is recommended that at frequencies above 10 GHz (or at all wavelength bands shorter than and including 15mm) periodic pointing scans should be made on the gain calibrator. Ideally, the time between pointing scans should be less than 1 hour.
The ATCA uses a dual linear orthogonal feed system to measure the incident radiation. Ideally, these two feeds would be completely independent of each other, but in reality this is not the case, and radiation that is measured by one feed can also mix into the other; this effect is called “leakage”. Polarisation calibration is primarily concerned with measuring this leakage effect.
To accurately determine the leakage parameters (see the technical memo “AT Polarisation Calibration”: http://www.atnf.csiro.au/observers/memos/d97b7f~1.pdf), it is necessary to sample a weakly polarised, unresolved calibrator at a large range of parallactic angles. Fortunately, observations of a gain calibrator will generally meet this requirement. Along with the determination of the relative leakage terms, it will also be possible to determine the gain calibrator’s relative Stokes Q and U quantities, and its polarisation position angle.
Dave Rayner’s “Circular Polarization User’s Guide” is a useful guide on how to accurately measure circular polarisation with the ATCA (http://www.atnf.csiro.au/observers/memos/circpolguide.pdf).
Many decisions need to be made when scheduling an observation. These decisions relate to how long observing procedures require, and how often they should be performed. This section will give a summary of these time considerations, in a “frequently-asked-questions” format.
Barring hard-to-solve problems occuring, an experienced observer should allow up to 20 minutes of initial calibration time per frequency combination. A less experienced observer may require longer if things do not go exactly to plan.
This depends on the conditions under which the observations are performed, such as atmospheric seeing, the array size and the central observing frequency. The ATCA provides a calculator at http://www.narrabri.atnf.csiro.au/calibrators/calcycle.html to advise observers on how often to visit their phase calibrator.
For this calculator, input the central observing frequency and the maximum baseline of interest (ie. don’t include the 6km antenna if its data will be discarded). For the seeing monitor RMS phase, give a nominal value of 300 microns if the observations will be performed in winter, and 700 microns if they will be in daytime during summer; for other times, choose a value between those two extremes. For the phase screen speed, choose 5 m/s. For the Kolmogorov exponent, leave the default 0.83 unless the 6km antenna will be required, in which case enter 0.33 instead.
For example, for an observation at 19 GHz, with a seeing RMS of 300 microns, in a 750m array, the calculator determines that if the gain calibrator was visited every 2 minutes, the observations would have an RMS phase of 15^\circ, be decorrelated by 3%, and have a maximum dynamic range of just 282. However it also determines that this is as bad as it gets, so that a 10 minute visit interval would give the same results. Since 2 minutes is a very frequent visit schedule, it would seem that self-calibration would be preferable for high dynamic range observations.
Dropping the seeing RMS to 100 microns (which would be a good winter day) improves matters significantly, allowing a dynamic range of 874 with no decorrelation and a 5^\circ RMS phase for any visit interval greater than 2 minutes.
It is important however not to extend the interval too far, as then the atmosphere being sampled by the gain calibrator will be different to that the source has sampled during that time. For cm observations, one might need only visit the gain calibrator once or twice per hour in good conditions, while for mm observations, it is unwise to visit the calibrator less frequently than once every 15 minutes.
It is therefore advisable that while making a schedule, input conservative estimates into this calculator to determine the phase calibrator interval. However, if high dynamic range images are required, the schedule should be flexible enough that the interval could be shortened in case the conditions degrade during the observations.
The primary aim of a gain calibrator observation is to measure the atmospheric phase, so any gain calibrator observation must have a sufficient signal-to-noise ratio for this to be determined. For example, for an observation at 19 GHz, a signal-to-noise ratio of 50 is required to reach a seeing RMS of 300 microns. For that same observation with CABB, the continuum RMS would be 0.24 mJy/beam in one minute. Thus any gain calibrator with a flux greater than 12 mJy would suffice if it were observed for one minute.
The general guideline here is that one should observe the brightest possible nearby gain calibrator for 1-2 minutes per visit to get a good measure of phase.
This consideration is largely governed by the flux accuracy required for the observations to be successful, and the strength of the primary flux calibrator at the observing frequency. If a flux accuracy of 1% is required, then the aim should be to make an observation with a signal-to-noise ratio of 100. For a observation with 2 GHz bandwidth, this is relatively easy, as even a 5 minute observation of 1934-638 at 50 GHz gives a signal-to-noise ratio in excess of 1000.
The sensitivity calculator should be used to estimate the observing time required to achieve the desired flux accuracy. However, it is never a bad idea to pad the required time out a little to ensure that enough good data will remain after flagging.
For continuum observations, it is not normally necessary to observe the bandpass calibrator for more than 5-10 minutes, and only one observation is required.
It has been observed that the CABB bandpass is not perfectly stable over the course of a normal 12 hour observation, and for this reason it is advisable to observe a bandpass calibrator more than once per observation, if bandpass accuracy is very important (eg. for spectral line studies). Visiting it three times (once at the start, once at the end, and once sometime near the middle) during a 12 hour observation should be sufficient to obtain a reliable bandpass solution. Alternatively, provided the gain calibrator is reasonably bright, the periodic visits to it may provide the information to accurately determine how the bandpass solution changes over time.
The integration time spent on the bandpass calibrator will have an effect on the accuracy of the flux that is measured after reduction. It is necessary to obtain each correlator channel’s flux to the same accuracy as required for the flux calibrator, ie. obtain a signal-to-noise ratio of 100 per channel if the required flux accuracy is 1%. Since any source can be a bandpass calibrator, the sensitivity calculator should be used to find out the RMS noise per channel for your observations, and then a calibrator found from the calibrator database that provides enough flux to meet the signal-to-noise ratio requirements. For example, a typical CABB observation has 2048 channels, and at 50 GHz, the RMS noise per channel is about 35 mJy for a 5 minute observation. For a signal-to-noise ratio of 100, a calibrator with flux greater than 3.5 Jy is required, and a search through the calibrator database identifies 18 such sources. Of course, if a primary flux calibrator is strong enough to be a bandpass calibrator as well, then observing time can be conserved by not having a separate bandpass calibrator.
When changing to or from any receiver except 7mm, expect a 20 second overhead while the turret rotates. To go to or from the 7mm receiver requires a movement of the millimetre package translator, which will take approximately 2 minutes, and if the turret needs to rotate, it will do so only after the translation is complete, thus adding another 20 seconds to this. Hardware limits will prevent turret rotations occurring more frequently than once every 15 minutes.
Allow for an overhead of 20 seconds while switching frequencies that use the same receiver.
To maintain pointing accuracy, a pointing scan should be scheduled at least once per hour. A pointing scan will only improve accuracy for sources within 20 degrees of where the scan was made, so if the sources being observed cover a wider area than this, multiple pointing scans will need to be made to maintain pointing accuracy for all sources.
To get accurate fluxes, it is a very good idea to perform a paddle scan at least once every 10-20 minutes, and more often if the weather is bad, or volatile. A paddle scan will take approximately 90 seconds.
Users Guide last modified on 2011-04-27 15:49:06