Note that much of what is below applies to the old VLA system and has changed drastically with the EVLA upgrade. See the earlier section on VLBI at the VLA for more current information.
SCHED can be used to schedule essentially all types of VLBI observations on the VLA. It can also be used instead of OBSERVE to schedule pure VLA observations involving a limited subset of the special cards that can be in VLA schedules. This includes most continuum observations. This situation is likely to evolve as the EVLA upgrade, which includes a new control system, comes on line, so stay tuned. For more information on the VLA, check out the VLA from the NRAO home page at http://www.nrao.edu. For much detailed information on VLBI at the VLA, follow the links to VLBI at the VLA. The latter document is available in postscript form from the VLA home page.
All observations at the VLA need an observe file that is used by the VLA on-line computers to control the antennas and correlator. In addition, all VLBI observations at the VLA also require a VLBA-style control file for the VME that controls the VLBA data aquisition rack and recorder. SCHED produces both kinds of files.
The VLBA control file, called bq001crd.y by SCHED for an example project with EXPCODE=bq001, is similar to those provided for VLBA stations, except that some VLBA specific commands for receivers, etc. are omitted. This file controls BBC settings, when the recorder starts and stops, and other recording details.
The observe file, called bq001obs.y by SCHED in this example, is the file that is normally created by the NRAO program OBSERVE (can be found from the NRAO home page) and given to the VLA on-line system. While SCHED is intended for VLBI scheduling and this output is provided mainly for projects that use the VLA for VLBI, there is no reason that it cannot be used for scheduling pure VLA projects. In fact, a few parameters of interest only for non-VLBI projects are provided for VLA observers, including the ability to schedule in LST. However, SCHED has only limited ability to provide the special setup cards, namely those that start with “//”.
SCHED can provide “//LO” and “//FI” cards with parameters to set synthesizers to fixed values. The setup file can be used to provide values for the synthesizer settings. If FREQ or DOPPLER is used, the BBC settings will be adjusted to try to reach the right frequency for the VLBI data. Note that this has no effect on the VLA observing frequency. “//LO” and “//FI” cards are provided if, and only if, the required information is present in the setup file or could be obtained for the setup file by SCHED from the frequency catalog. SCHED also provides the “//PM” cards if proper (or planetary) motion is present for the source in the source catalog. Since the epoch of zero offset is given in IAT, IAT-UTC must be provided with parameter IATUTC in the SCHED keyin file. If other “//” cards are needed, or other options are needed on the “//LO” and “//FI’ cards, the regular VLA OBSERVE must be used or they must be edited in by hand. Beware of the fixed formats: if values are put in the wrong columns, the VLA on-line system will not behave as expected!
SCHED checks the ranges of the frequency settings given for the “//LO” cards. But the EVLA has made it possible to reach frequencies far outside the range of the original VLA hardware. Such frequencies can be requested using frequency settings in the style of the VLA, but outside the allowed VLA ranges. In addition, the parrameter EVLA should be given in the setup file to turn off some of the checks of frequency ranges. This is all at a primitive state as of Fall 2007 and will be updated as the EVLA matures.
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.
Below is an example of a file for a VLBI observation that uses the phased VLA. For phasing, a source is used that can also be used as a VLBI phase or fringe fitting reference. All antennas are sent to the reference and VLBI data is taken on it. This is now the prefered style for phasing. Of course the VLAPSRC parameter could also have been used if non-recording scans with only the VLA on the calibrator had been desired.
Note that the main example in the Examples section has a single VLA antenna in it and serves as an example of that type of observing.
SCHED cannot be used for scheduling the EVLA (yet anyway) and the old VLA system has been turned off. So the following section is not of much use now. Once VLBI at the EVLA has been established, the capabilities to schedule EVLA runs with SCHED will be revisited.
This second VLA example demonstrates the use of SCHED to schedule a pure VLA project. The SCHED input for VLA observations is very much like that for VLBI observations. There are some parameters provided, and listed below, to control some of the input information for which the VLBI defaults may not be appropriate. The example is a modified version of an actual schedule used to make the VLA observations described. The modifications were required by changes in the way in which SCHED handles setup files and the parameters VLABAND, VLABW, PEAK, VLAMODE (for pointing scans), and VLAPEAK. Some of these parameters are now read from the setup files, not the main schedule input.
The VLA capabilities in SCHED are for the old VLA system that has been shut down. Stay tuned for instructions related to running the EVLA (VLA).
This section gives a brief description of each of the parameters available to SCHED that are specifically for VLA observations or are especially useful for VLA observations. More general purpose SCHED parameters that are also required for other stations are not mentioned. For details on the parameters, consult the detailed descriptions of each parameter. The first list is of parameters in the main schedule input. The second is of setup parameters.
VLATYPE: Specify the type of observations. Should be provided for non-VLBI observations. Valid options are ’VLBI’, ’CONTINUUM’ or ’LINE’.
VLATSYS and VLANTSYS These turn on and off the Tsys corrections made to the correlated VLA data. This used to be required in order to obtain calibration data for the VLBI observations. That is no longer true so most observations can be made with the Tsys corrections turned on which is now the default.
VLAPEAK controls reference pointing on the VLA. It is a good idea to do reference pointing when observing at 7 mm. See the section on Reference Pointing to see how to get SCHED to insert reference pointing scans automatically.
VLAUSERN: This gives the VLA user number. The default of 600 is reasonable for VLBI but the actual user number should be provided for other types of projects.
VLAMODE: Specify the VLA mode. Common options for VLBI are ’ ’, VA, VX, or VS. See VLA documentation on the many others that are available for non-VLBI observations.
LST: Schedule is assumed to be in LST. LST specified without a value assumes the VLA. A station name can be given as the argument, in which case the LST is for that station. When LST is specified, the DAY must be the modified local sidereal day number as found on VLA monthly schedules.
DAY: Must be the modified local sidereal day number as found on VLA monthly schedules if LST is specified. In such cases, YEAR and MONTH are ignored.
IATUTC: Difference between IAT and UTC, which is currently about 30 seconds. This is only used for setting the zero offset epoch for the “//PM” (proper motion) card.
SETUP: Specifies the setup file from which information for the “//LO” and “//FI” cards are taken.
FREQ: and DOPPLER Warning — these parameters cannot be used for non-VLBI observations at the VLA since they are only used to adjust the baseband converter frequencies.
RECord: and NORECord These parameters find one of their main uses in observations that use the VLA, especially the phased array. It is often useful, for both phasing scans and for scans inserted just for calibration of the VLA, to be able to turn the VLBI recorders off. RECord and NORECord can be used to specify which scans should be recorded on the VLBI drives and which should not be.
VLAPSRC: Phasing source to use before a VX mode scan. This is a convenient way to insert VLA phasing scans without having to explicitly include extra scans in a VLBI schedule.
VLARFANT: Reference antenna to use for single dish data source or as phasing reference for phased array. The default should be good, and, if not, operations is likely to need a last minute change, so it is unlikely that users should use this parameter.
VLAINTEG: This parameter gives the correlator integration time. It would normally be used to give a value less than 10 seconds to encourage faster phasing.
The parameters in the setup file that apply only to the VLA are below. They are used to set the band and bandwidths of the VLA and for setting up the “//FI” and “//LO” cards.
The values for all of these parameters, except FLUKESET, VLAIF, and VLAROT,, which are not normally needed, can be determined by SCHED from the frequency catalog. Therefore most projects do not require VLA specific parameters in the setup files.
The setup file parameters are:
FEFILTER: The front end filters are normally 50 MHz wide. If there is strong RFI near the observing frequency, as is common at L and P bands, it may be desirable to use the 25 MHz or 12.5 MHz front end filters. This parameter is used to request the filters to use. It goes on the “//LO” card.
FLUKESET: Which flukeset (1 or 2) to be used. Unfortunately there seems to be no good way to predict this reliably. Usually Flukeset 1 will be used for phased array and normal VLA observations while Flukeset 2 will be used for single antenna VLBI, but this is not always true. The VLA operators should be alerted that Flukeset specified on the “//FI” card might have to be changed. If it is not specified, or is set to 0, a blank will be given and the VLA operators will set it.
VLABAND: Specify the frequency band. Must be one of the defaults without the //LO and //FI cards. Some valid options are VP, VL, VC, VX, VK, PP, LL, CC, XX, UU, KK, HH, or 18. For complete information on the standard bands, see the following section.
VLABW: Specify the back end bandwidth for each of the 4 VLA IFs as a 4 digit number in quotes (SCHED treats it as a character string), with one digit for each IF. The default is ’0000’ for 50 MHz in each IF. The consequent back end bandwidth is 50/(2**n) MHz, where n is the digit specified.
FLUKEA: A Fluke synthesizer frequency. It is usually near 100 MHz. See the document “VLBI at the VLA” for details on what it should be. See Section 2.5 for more information on the frequency settings.
FLUKEB: B Fluke synthesizer frequency. Actually this is the value for the “//FI” card which is twice the actual synthesizer setting. To get the same frequency on the AC and BD IFs, FLUKEB should be FLUKEA plus 100.0.
VLAFEAB: 1st LO for the VLA observations for the AB (RCP) IFs. For standard VLBI projects, the value should be -3.2 for L band, 0 for C band, and 17.5 for K band. Again see VLA documentation for details of other alternatives.
VLAFECD: Same as VLAFEAB but for the VLA’s C and D (LCP) IFs.
VLASYNA: 2nd LO for the VLA A and C IFs. On the “//LO card” this number is rounded to the nearest MHz, but it should be given here exactly. It is usually near 3900 MHz. It must be a value of N * 50 +- 10.1 MHz.
VLASYNB: Same as VLASYNA but for the VLA B and D IFs.
All of the below will change with the EVLA, which can reach just about any frequency. The following is basically obsolete.
A total of 19 standard observing bands are defined at the VLA. All projects should specify one of these bands, even when frequencies will be modified. This ensures that various defaults at the VLA get set properly. Just pick one with similar properties. For VLBI, one of the “V” bands should be chosen.
The tables below describe the bands. The first show the band names along with the synthesizer settings. The column headings are the SCHED names (not necessarily the traditional names in use at the VLA) for these parameters. The second table gives the value that SCHED would want for the VLBI FIRSTLO parameter when using the standard setup. If the frequencies are modified, FIRSTLO would need to be altered. SCHED will insure that FIRSTLO agrees with the frequency that will be obtained with the settings used. The other columns in the second table show the range of sky frequencies covered by the VLA band. The observing frequencies must lie in these bands. Note that the first frequency is that of the edge converted to DC, and then to 600 MHz in the signal sent to the VLBI rack. The other end of the band can be a lower frequency if there is a net lower sideband at this point (the BBC’s can be used in lower sideband mode to get a final net upper sideband).
The VLA band VQ will be changed a few months into 2001. The table below gives the frequencies for the new version. The old version was 80 MHz higher in frequency and missed the SiO line at 43.122 GHz. The current plan is to switch SCHED to the new VQ in January 2001, but have SCHED always write LO and IF cards in the VLA observe file for 43 GHz observing, even when using the standard band. After a few months in which any schedules made with the old scheme are flushed through the system, the VLA on-line system will be switch to the new standard. Some months after that, SCHED will stop writing the LO and IF cards for the new standard VQ band.