| CSIRO Australia Telescope National Facility ATCA Users Guide |
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Obtaining high quality data depends on making a schedule that observes the program sources along with suitable calibrators frequently enough to satisfy the program’s science goals. This section will describe common goals, and provide some advice on how to best schedule such observations.
If many sources need to be imaged in a single 12 hour observation, and complex structure is not expected in these sources, then observing in snapshot mode may be the best option. This mode is also known as making “cuts” across the source.
To make a snapshot schedule, determine how many cuts are necessary within in the time alotted for the experiment, as this constrains how long the schedule should run for. For example, if the experiment is given 12 hours of observing time, and each source needs four cuts, then the schedule must not run for more than 3 hours before repeating.
This repeat time must then be divided up between the sources in the schedule. Care must be taken though to ensure that weaker sources are given more integration time than stronger sources. Also, time must be reserved for calibrator observations and other overheads such as slewing and receiver translation.
The CABB system allows two 2 GHz wide bands to be observed simultaneously, where the band centres are spaced up to 6 GHz apart (band-permitting). In and of itself this capability qualifies as multi-frequency synthesis. However, this capability does not cover the entire usable frequency range of the mm receivers, and so it may still be desirable to perform a frequency switching experiment.
Making a frequency switching schedule is as simple as duplicating scans and changing their frequencies to cover the desired range. Frequencies within any particular band can be changed every cycle if so desired, however there is an overhead of one or two cycles as the request for LO frequency changes propagates through the system. Changing bands often results in a turret rotation, and performing a turret rotation more often than every 15 minutes is not permitted. Note also that when changing to or from the 7mm receiver, the mm package must be translated, and this will take approximately 2 minutes.
For more information, see:
Reference pointing should be used for observations where pointing accuracy is important, including for mm observations where the global pointing errors are a significant fraction of the primary beam size.
To schedule a reference pointing scan, choose a bright source within
20^\circ of the program source, and make a scan on that source
with the settings ScanType=Point and Pointing=Update.
This Update option uses the most recent solution, and has the
advantage of giving corrections with respect to the previous solution,
which can be used to estimate the real pointing accuracy when the same
region of the sky is observed for a long period.
After doing a reference pointing scan, ensure that Pointing is
set to Offset for all other scans on any calibrators and program
sources. This will invoke the latest pointing solution.
For reference, a pointing scan will take about 5 minutes to complete,
regardless of what is given in the ScanLength field. Pointing
solutions are only valid in a small area of sky surrounding the azimuth
and elevation where the pointing was made, and a new pointing solution
should be obtained roughly once every hour during observations.
Solar system objects differ from most astronomical sources in that the proper motions are large enough that the object position changes in the course of an observation. When observing such objects, the schedule file should be created in absolute time mode to ensure that CAOBS correctly tracks the object. CAOBS has the ability to phase track an object with a non-sidereal rate. Note, however, the pointing centre used during a scan does not track the source (the pointing centre used is the mid-position of the object during the scan). Thus the scans must be short enough that there is not significant motion of the source within the primary beam during a scan (the scheduler does not check this).
To schedule an observation of a major planetary object, simply put
its name in the Source field of a scan. During the observations,
CAOBS will interpret this name and calculate where the planet, Sun
or Moon is and point the array towards it.
The scheduler will not however calculate where the planet will be during
the scheduling process, which means that drive time calculations will
not be valid unless the correct coordinates are input by the user.
To get these coordinates, use the MIRIAD task planets.
For minor bodies, CAOBS cannot compute a position, so the correct
position must be provided by the user. In absolute time mode, the scheduler
treats source names beginning with the @ symbol (eg. a source name of
@hbopp) as pointing to an external ephemeris file (eg. a file
$ATCA_EPHEM:hbopp.eph).
Ephemerides for minor solar system bodies can be computed before scheduling using JPL’s Horizons on-line system, at http://ssd.jpl.nasa.gov/?horizons.
Only the telnet (terminal-based) access method is described here, but there is also a web-interface method available from the web page above that is self-explanatory.
Horizons asks the user a series of questions. To generate an ephemeris file for ATCA observations:
Ephemeris, then
Observe, then Geo to select geocentric RA/Dec ephemerides.
default output, and at the
Select table quantities prompt, enter 1,20 to get RA, Dec,
distance and velocity of the object.
q and the main menu will be displayed
again. Then select [M]ail and enter the email address to send the
ephemeris to.
Ctrl-D (the terminal hangup signal).
The ephemeris file produced by Horizons needs to be massaged into the format that the scheduler requires. This is best achieved with the EPHFORMAT command available on XBONES.
$ATCA_EPHEM:file.eph where file is the name entered at the
second prompt.
When planning to observe a spectral line, there are further things to consider. One of the most important things to consider is where the line to be observed will fall within the bandpass, ie. which channel(s) it will appear in. There are several channels in CABB that are always flagged, and several more that are often contaminated with self-generated interference, and these should be avoided when observing spectral lines.
The CABB correlator always flags channels 513, 1025 and 1537 (multiples of 512 MHz). The central frequency specified in the schedule file will fall in the centre of channel 1025.
Self-generated interference will likely be present in channels 129, 157, 257, 641, 769, 1153, 1177, 1281, 1409, 1793 and 1921. All these channels can be easily flagged with the online correlator software and the interference does not leak into the adjacent channels.
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 phase 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 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 aim of a phase calibrator observation is to measure the atmospheric phase, so any phase 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 phase 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 phase 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 (http://www.atnf.csiro.au/observers/docs/at_sens/) 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.
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. 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.
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 10 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 minutes, and more often if the weather is bad, or volatile. A paddle scan will take approximately 90 seconds.