At high observing frequencies, it can be difficult to point antennas with sufficient accuracy to keep the target source in the beam. On tactic to improve this situation is known as reference pointing. The idea is to peak up the pointing on a source prior to the VLBI scan. The source can be the target source, if it is sufficiently strong. Otherwise it can be another source. It is best to find a source as close as possible to the target, but it may be necessary to go tens of degrees to find one that is suitable. The reference pointing is commonly done at a lower frequency than the interferometer observations for improved SNR.
Reference pointing is commonly used at the VLBA for observations at 3mm. The actual pointing observations are typically done at 7mm where the sensitivity is greater and the beam is a larger. The pointing offsets are determined by the on-line system by fitting total power measurements made while the antenna is moved over a pattern that includes the nominal on-source, half power, and off-source positions. Often this is done on strong SiO maser sources. One minute should be allowed for completion of this pattern after the antenna reaches the pointing source. Once a pointing offset has been determined, it will be used until another is determined, or the project changes. It cannot be turned off. Optimal time intervals between pointing scans and maximum offsets to pointing sources are not yet known. But pointing every half hour to hour on sources within about 20 degrees should be ok.
Because total power mode is being used for VLBA pointing, sources must be very strong -- more than about 10 Jy for continuum sources or about the same flux density averaged over the observing bandwidth for line sources (peak Ta of about 2 K). Very few continuum sources are strong enough, so most appropriate sources are SiO masers observed with restricted bandwidth, usually 2 MHz, centered on the line. See the sample pointing command file $SCHED/catalogs/peak.cmd, provided with the SCHED catalogs and discussed below, for a list of possible targets. Most of the SiO line sources are variable. The ones in peak.cmd were thought to be good at the time the file was made. Information on masers can be found at the SEST web site. Some information on continuum sources at mm wavelengths can be found at the OVRO.
Reference pointing is commonly used for 7mm observations on the VLA. It is done interferometer mode at X band (4cm) and so can only be done when more than one antenna is available to the observer. VLBI projects with only one VLA antenna (Y1) cannot use reference pointing. Since interferometer mode is used, only on-source and half-power pointing positions are needed. But the pattern is slower, taking about 2.5 minutes. See the discussion of VLAPEAK for more details. Intervals between pointing observations should again be half to one hour and the source should be within about 15 degrees of the target. Continuum calibrators above about 0.5 Jy at 8 GHz are appropriate, so there are far more available than for the VLBA.
Reference pointing scans can be inserted explicitly by the observer or SCHED can be requested to attempt to do the job automatically. For explicit scan insertion, the user specifies a scan with times and a source. Other factors such as the setup can also be specified. The parameters PEAK and/or VLAPEAK will need to be set and the user should consult the documentation on those parameters. SCHED can be requested to fill most of the required parameters using the same information used for the automatic scan insertion discussed below. See the discussion of the parameter POINT for information on how to control this semi-automatic mode.
Explicit insertion of pointing scans can be a pain and can completely dominate the work involved in scheduling high frequency observations. Therefore SCHED has a mode where it can do the work. This mode is invoked with the AUTOPEAK command and involves the use of a special set of input parameters either from a separate file, the PEAKFILE, or from in-stream commands, in the main SCHED input, contained between PEAKINIT and ENDPEAK. A standard version of this file is called peak.cmd and is available in the SCHED catalogs subdirectory. That file can be used as an example of the format. It is possible to watch details of the process by which SCHED chooses pointing sources by setting the parameter PKWATCH. Be warned that this can produce a lot of output to the sched.runlog file.
There are several examples that demonstrate the use of reference pointing. eg3mma.key sets up the pointing scans without any help from the automatic features in SCHED. eg3mmb.key demonstrates use of the external peak command file. eg3mmc.key produces the same results but using PEAKINIT and no external file. egvla7mm.key shows the insertion of peaking scans on the VLA. These examples only include a few VLBA stations and the VLA. Others will be added to demonstrate scheduling of more global observations such as those of the CMVA.
The peaking control information is organized around groups of antennas and lists of possible pointing sources. Up to 5 groups of antennas can be specified. For full automatic pointing, separate scans will be added for each group (this can add quite a few scans). For each group, SCHED finds the source in the pointing list that can be reached most quickly from the target source, that is above a specified minimum elevation at all antennas in the group. Pointing will only be added for scans observing at a frequency above a specified cutoff and only when there is enough of a gap in the schedule to fit one or two scans plus the slew to the pointing source from the previous VLBI source and the slew to the next VLBI source.
The input parameters for the PEAKFILE are:
As with all types of SCHED input, end each group with a ``/''. The order of parameters within a group (between ``/''s) is not significant.