Observing Strategy

A tremendous amount could be said about observing strategy. See the article by Joan Wrobel in the book ``Very Long Baseline Interferometry and the VLBA'' (1995: ASP) for a good discussion. That book also contains a lot of information about VLBI and is a good reference for all observers to have. Other important sources of information are the ``VLBA Observational Status Summary'' and ``General Instructions on Observation Preparation'' (which is sent to scheduled users). These and many other useful documents can be accessed on the Internet from the ``VLBA Information for Astronomers'' page. This section of the SCHED  Manual will be limited to a few important concepts to keep in mind while scheduling. It is assumed that the observer already has a reasonable idea of how much time is needed per source, what frequencies to observe, etc. The suggestions here are more oriented toward smooth observing, processing, and calibration.

• Fringe finders: In order for the staff at a VLBI correlator to tell if the correlation has gone well, it is necessary to have occasional scans on sources that will provide strong fringes. A minimum of two such scans should be provided and, for longer projects, there should be one every 4-6 hours at a minimum. The ``VLBA Observational Status Summary'' and ``General Instructions on How to Prepare for Observing'' documents mentioned above have lists of appropriate sources.

  The fringe finders can also be used for ``manual phase cal''. This is the process of aligning the phases and delays on all individual IFs. This is simply done by running the fringe fitting program on the calibrator and applying the results to all data. For this purpose, it helps to have a scan with all antennas at reasonably high elevations simultaneously.

• Setting the flux scale: Setting the flux scale of VLBI images is not trivial. Idealy, one would have a compact source of known flux density that could be used, as 3C286 and 3C48 are on the VLA, to calibrate the flux densities. However, by their nature, sources that are compact to VLBI (essentially none are unresolved) are variable and, therefore, do not make good flux calibrators. There are two ways to deal with this.

  One is to try to obtain the flux density of a compact calibrator at a close enough time to the VLBI observations that variations will not be significant. At the time of the VLBI observations, some of the larger antennas can be asked to measure flux densities of sources being observed (see the TANT command. Better yet, if you have the phased array at the VLA or can get an hour or so of VLA test time, accurate flux densities of your VLBI calibrators can be measured. In order to do this, be sure to include a VLA flux calibrator (usually 3C286 or 3C48) in the VLA schedule. You don't need to record VLBI data on it.

  It is also possible, and perhaps preferable, to rely on the apriori calibration of the VLBI antennas. For this, it is best to look carefully at your data and determine which subset of antennas is giving consistent calibration. With the VLBA at intermediate frequencies, it is possible to get the flux scale right to within a few percent this way. You just have to be sure not to let any antennas with bad weather or other problems contribute to the flux scale. In the AIPS calibration task CALIB, use ANTUSE to limit the gain normalization to the antennas whose gains you trust.

  When using the apriori gains for absolute flux calibration, care must be taken with bandpass calibration and channel selection. The gains are measured using the full bandwidth of the baseband channel - they are based on total power measurements from the baseband converters. Such gains apply to the average across the full baseband. If you attempt to apply such gains to a data set with the edge channels removed, there will be an error of a few percent because those edge channels typically have lower gain (which is partly why you removed them) and the average for the remaining channels will be higher. You can deal with this either by doing the calibration including all edge channels, or by doing a bandpass calibration based on all channels. Bandpass calibration brings all channels to the average level.

  If you are going to rely on apriori calibration, be sure to obtain some data at close to the frequencies at which antenna gains have been measured. These frequencies are given in the vlba_gains.key available by anonymous ftp to ftp.aoc.nrao.edu. This is the file that you will need for VLBA calibration. At those frequencies, the Tsys used in calibration and the gain are based on the same values of Tcal. If you observe at other frequencies, you will be depending on the Tcal values being correct (or at least having the right ratio) at your frequency and the frequency where the gains are measured. That is not assured.

  For any of these methods to work, it is best to include a strong, compact calibrator and to observe it several times to check consistency.

• Setting the relative gains of antennas: Another invaluable use of a strong, compact calibrator is to help ensure that all antennas and all IFs have consistent calibration. After apriori calibration and setting of the flux scale has been done, a model of the calibrator can be used to derive whatever additional gain adjustments are needed. These can be applied to the whole observation to obtain the best possible calibration, short of self-cal.

• ``Readback tests'' and module-swap gaps: The recording media should be stopped occasionally to help prevent major amounts of data loss if something should go wrong and, on Vex controlled stations (non-VLBA), to allow disk bank changes. Two minute gaps every hour or two used to be used for readback tests with the tape systems. Tape is no longer used, but an equivalent data integrity test is likely to be built into the disk based systems in the future. Also, if the disks are not stopped, all the data ends up in one file. If there are problems closing the file, the data can become unreadable. As of early 2008, it is recommended that a gap of 30s or more be inserted every hour or two. For Vex stations (most non-VLBA stations), gaps should also be inserted to allow bank changes. The VLBA can switch between mounted disk banks (modules) on the fly, but the field systems need a pause in the data recording. Such gaps should be inserted every 22 minutes for recordings at 1 Gbps and proportionally less often at lower bit rates. These gaps need to be more than 10s long.

• Phased VLA: The phased VLA has special calibration needs. If the target source is either too weak or too resolved to be used for phasing up the array, special scans on a calibrator must be inserted for this purpose. This can be done with SCHED or VLA OBSERVE (see the section on VLBI at the VLA for details. It can be done by the VLA data analysts, but the schedule must contain gaps of adequate length.

  Also, the effective beam of the phased VLA (or any other phased interferomenter such as Westerbork) is the synthesized beam. This can be less than an arcsecond in the worst cases. This places far greater demands on source positions required for observing than is typical at single antennas. Be sure to provide positions of sufficient accuracy, which can mean 0.1 arcsecond.

  Anyone using the VLA for VLBI should consult the VLBI at the VLA guide.

• Calibrator scans: Except for phase or fringe reference calibrators, there will typically be only 2 or a few more scans on each calibrator of the types discussed above. It is tempting to put these scans at the beginning and end of the scheduled observing time. However this is quite risky as those are the time periods most likely to experience problems. The beginning is especially risky because, if there are any problems at a site, they may not have been fixed yet. The end is ok, except at sites where the main target source has been down for a while and the antenna has been idle. It may not wake up again cleanly for the calibration scans. It is best to take time (a few minutes on each calibrator) out of the middle of the schedule a couple of times to get the calibration data.