2.1 Scheduling Strategy

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.

2.1.1 Snapshot 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.

2.1.2 Frequency Switching and Multi-frequency Synthesis

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 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.

As stated above, the centre frequencies of the two IFs may be placed up to 6 GHz apart. Doing so may have undesirable consequences however, as any band that is placed at the lower end of the 4-12 GHz CABB window will suffer from poor image rejection. By default, closely spaced IF pairs will be moved to the top of this CABB window, but if the two IFs are separated by \sim6 GHz, then the IF at the lower end may experience increased noise levels.

There are some further restrictions to the frequency pairs that can be observed simultaneously:

  • A frequency using one receiver package cannot be observed simultaneously with a frequency using another receiver package. The receiver packages are: 16cm, 6/3cm, 15mm, 7mm, 3mm.
  • A frequency that requires an LO configuration that produces an upper sideband (USB) IF cannot be observed simultaneously with a frequency that requires an LO configuration that produces a lower sideband (LSB) IF. This restriction is important in the 7mm and 3mm bands.

    At 7mm, the USB/LSB switch occurs at 40 GHz. Both centre frequencies must either be above 41 GHz or below 40 GHz in this band.

    At 3mm, the situation is rather more complicated:

    • If the highest central frequency is > 100.6 GHz and the lowest central frequency is > 97.8 GHz, then the band will be USB, and sensitivity will be optimised.
    • If the lowest central frequency is < 97.8 GHz and the highest central frequency is > 100.6 GHz, then the band will be LSB, but the LO will be driven past its rated limit, reducing the sensitivity.
    • If the lowest central frequency is < 97.8 GHz, and the highest central frequency is < 100.6 GHz, then the band will be LSB, and sensitivity will be optimised.
    • If the highest central frequency is < 100.6 GHz, and the lowest central frequency is > 97.8 GHz, then the band may either be LSB or USB, and will be automatically configured by CAOBS to optimise sideband rejection.

For more information, see:

  • The MIRIAD Users Guide, particularly “Multi-frequency Synthesis Observing Strategies”. This gives a very good description of frequency switching and the ramifications it has on the reduction of the data.
  • Multi-frequency Synthesis with the ATCA, Sault, R.J. 1992, ATNF Technical Document Series, 39, 3019
  • Multi-frequency Synthesis techniques in radio interferometry, Sault, R.J. & Wieringa, M.H. 1994, A&A Supp, 108, 585
  • Multi-frequency Synthesis, Conway, J.E. & Sault, R.J. 1995, in Workshops on Very Long Baseline Interferometry and the VLBA, eds. J.A. Zensus, P.J. Diamond, P.J. Napier, ASP Conf. Series

2.1.3 Reference Pointing

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 Offpnt or Offset for all other scans on any calibrators and program sources. Using Offpnt will allow CAOBS to find the most appropriate recently-obtained pointing solution based on azimuth and elevation, whereas Offset will only ever invoke the latest pointing solution.

For reference, a pointing scan will take about 2-3 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.

2.1.4 Mosaicing

Mosaicing is normally used if the source or field to be imaged is large compared to the FWHM of the ATCA primary beam. Mosaicing mode can also be useful when a large number of nearby sources are to be observed, as observing overheads are reduced. Other applications include holography and source surveys using one dimensional cuts. In interferometry adjacent pointings are not independent and so we can get fundamentally better images by processing the different pointings together. Necessary information about the pointing grid pattern to use for proper Nyquist sampling and the time to be spent on each position to optimise tangential uv coverage can be found in Chapter 21 of the MIRIAD Users Guide (http://www.atnf.csiro.au/computing/software/miriad/userguide/userhtml.html).

For close-packed mosaicing (pointing centres every half-beamwidth), drive times are determined by the acceleration limit of 800^{\circ}/min^2. Half the drive time is spent accelerating and half decelerating. For drive times much further than 108’ in azimuth or 27’ in elevation, the drive times are dominated by the slew rate limits of 38^{\circ}/min in elevation and 19^{\circ}/min in azimuth. For example, a 16’ drive for a 20cm mosaic will take, depending on the elevation and azimuth of the source, approximately 2.2 seconds. Data taken during the off-source period is blanked in the correlator using a predicted drive time. As drive times can be a significant overhead, care should be taken to not move the array more than necessary.

Mosaicing is less useful if frequency switching is desired, as no frequency changes can occur during the mosaic scan. Since slew times are often much greater than the time it takes for the correlator to switch frequencies, the overheads involved in repeating a mosaic scan for each separate frequency might quickly negate the normal benefits of mosaicing.

2.1.5 Observing Solar System Objects

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:

  • telnet horizons.jpl.nasa.gov 6775
  • First select an object by giving the source name (followed by a semi-colon for minor bodies) or by source number.
  • In response to prompts, select Ephemeris, then Observe, then Geo to select geocentric RA/Dec ephemerides.
  • Then give the start time of interest, in UTC, in the format suggested by the prompt. At the next prompt give the end time, and then the time increment; typically 1 hours is more than adequate, and the scheduler uses simple linear interpolation of the ephemeris values. The ephemeris supplied to the scheduler must start at least two time increments before the scheduled start time, and must extend beyond its end time.
  • Accept default output, and at the Select table quantities prompt, enter 1,20 to get RA, Dec, distance and velocity of the object.
  • Horizons will then display the ephemeris in the terminal. To get it to email the ephemeris, press q and the main menu will be displayed again. Then select [M]ail and enter the email address to send the ephemeris to.
  • To log out of Horizons, press 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.

  • First, copy the ephemeris from the email sent by Horizons into a plain text file on XBONES
  • Run the command EPHFORMAT with no arguments.
  • At the first prompt, give the name of the text file with the Horizons ephemeris (including file extension). At the second prompt, give a short descriptive name for the output ephemeris file (excluding any file extension). EPHFORMAT will then write out the appropriate ephemeris file to $ATCA_EPHEM:file.eph where file is the name entered at the second prompt.

2.1.6 Spectral Considerations

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.


Users Guide last modified on 2013-02-18 04:17:11