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VLBI at the VLA - old system

Note that many of the details of what is below applies to the old VLA system which is no longer in use.

For much detailed information on VLBI at the VLA, follow the links to the VLBI at the VLA guide.

There are two very different modes in which VLBI can be done at the VLA. These are single dish and phased array observations. For single dish VLBI observations, the IF signals from one antenna of the array are sent to the VLBI recording equipment (VLBA DAR and recorder). Other than having to worry about the VLA's rather complicated LO system, such observations are very similar to observations at other VLBI observatories. The phased array observations, on the other hand, have some very special needs. For such observations, the signals from all VLA antennas are summed in the VLA correlator and the summed signal is sent to the VLBI equipment. This gives a sensitivity for the ``antenna'' that is increased by about the number of phased antennas, if the array is phased properly. Array phasing is accomplished by the on-line control system. Output phases from the correlator are used to derive adjustments to the phase of the LO at each antenna that, when applied, will cause the next correlator phases to go toward zero. The system can phase up on a calibrator and then freeze the phases on a target source when the target source is either too weak, resolved, or confused for successful phasing.

The type of VLA observation is distinguished by the VLAMODE. VLAMODE = VS indicates single dish observing. VLAMODE = VX indicates phased array observing, but using the phases from a previous phasing scan. VLAMODE = VA indicates phased array observing with phasing on the VLA A and D IFs and is the usual active phasing mode. VLAMODE = VB is similar, but with phasing on the VLA B and C IFs. VLAMODE = VR or VL indicates phased array observing using, respectively, the VLA A and B or C and D IFs. As might be deduced by this, because of hardware limitations, the VLA can only phase one of the A and C IFs and one of the B and D IFs. The VLAMODE can be changed on a scan-by-scan basis. See the description of the setup parameter IFCHAN for more information on routing VLA IFs to VLB IFs. VS mode (single dish) can be mixed with the phased array modes, but the number of changes back and forth should be limited. Normally this would only be done when atmospheric calibration, geodetic-like (DELZN) sections are included in a phase referencing project.

The reference antenna can be controlled with parameter VLARFANT. That will be the antenna from which data are recorded in single dish mode and the reference antenna (whose phases are not changed) for the phased array modes. Normally users should not worry about setting VLARFANT because the default is usually good, and if not, you will not know in time and the change needs to be made by operations.

For successful phasing of the array, a source must be greater than about 0.1 Jy (see the guide referenced above for details) and have a position that is good to a fraction of the VLA synthesized beam (enhanced sensitivity is only obtained over this area). It must have small structure phases and not have other sources in the primary beam that might confuse the phasing algorithm. The position accuracy is especially important if a calibrator is being used to phase the array for observations of another source. Adding phasing sources is tricky, because it is desirable to spend a minimum amount of time on them, but if they are missed, the rest of the data will be bad. It is possible to try to influence the speed of phasing by using VLAINTEG to adjust the correlator integration time. The default is 10 seconds. Use of a shorter time may speed phasing but is done at the expense of added noise.

There are two ways to deal with VLBI observations that require phasing on a source different from the target source. The best is probably to simply schedule VLBI scans, with the proper VLAMODE, on the phasing source. These observations can then also be used to assist the VLBI calibration, assuming an appropriate calibrator has been chosen. The other scheme is to insert calibration scans into the VLA's schedule file (the OBSERVE file), but not the VLBI control file, for the phasing operation. SCHED provides some tools to simplify this operation. It can also be done using the VLA OBSERVE program. This is done by the VLA analysts for some programs.

For VLBI observations at the VLA, both an OBSERVE file and a VLBA style control file are needed and can be created by SCHED. All of the controls provided for managing recordings for the VLBA also work for the VLA. SCHED also has special capabilities for scheduling VLA observations as described below.

For observations that require phasing on a calibrator, the array must be phased in auto-phasing (VLAMODE = VA) mode prior to VLBI observations of the target source in extended-phasing (VLAMODE = VX) mode. These calibrator observations constitute additional scans that must be in the VLA observe file, but that no other VLBI observatory or the recorder control file for the VLA needs to know about. (Note that Westerbork needs something of the sort, but they have never requested special information in SCHED output files).

If it is desired to add phasing scans on a calibrator on which VLBI data will not be recorded and that will not be observed at the other VLBI observatories, the parameter VLAPSRC can be used. If a VLAPSRC is given, SCHED will automatically insert phasing observations in the VLA OBSERVE file at appropriate times. Note that VLAPSRC must be a source in the same catalogs that are searched for SOURCE and DOPSRC. SCHED will add a phasing scan to the observe file which will end either 1 minute before the main VLBI scan starts, or 3 minutes after the previous VLBI scan ends, whichever is later. If the latter option is used, SCHED complains if less than 2 minutes of the VLBI scan remains. Also with the latter option, the VLBA-style control file will still start the recorder when the main VLBI scan was supposed to start. This keeps recording at the VLA and other sites synchronized, although it can lead to the need to flag data that won't have VLBI fringes. SCHED will not add a phasing scan if the VLBI scan is in VLAMODE=VA or if VLAPSRC is the same as SOURCE. This avoids the need to keep respecifying VLAPSRC whenever a VLBI calibrator is observed in mode VA and prevents the addition of unnecessary extra scans when the VLBI schedule calls for observations of the phasing source.

It is highly recommended that the NRAO OBSERVE program be used to check the VLA observe file for slew and dwell times on the phasing sources. The scan stop times may have to be adjusted with OBSERVE to obtain an adequate amount of on-source time for phasing. It is especially critical to be careful about slew times for sources near the zenith where long azimuth slews may be needed to go a short distance on the sky. SCHED only recently acquired to ability to calculate slew times and this has not yet been extended to the VLA phasing scans. This is an area of future construction.

SCHED is also able to deal with reference pointing using the PEAK command. Refer to the description of PEAK for details. The VLA-only example below uses this capability.

next up previous contents
Next: An Example Up: Old VLA Previous: Old VLA   Contents
Craig Walker 2014-06-17