Facilities > VLBA > Observing > VLBI at the VLA: Scheduling Hints

VLBI at the VLA: Scheduling Hints

by Amy Mioduszewski last modified Jun 29, 2016
VLBI at the VLA: Scheduling Hints

Introduction

This page describes scheduling the VLA when it used as a VLBI element.  For more general information see the associated  VLBI at the VLA page.  When the VLA is part of a VLBI observation it will be scheduled with NRAO's SCHED software.  SCHED is the main software package for scheduling VLBI observations.  Versions of SCHED 11 and later must be used for the phased-VLA.  Probably the most useful part of this page are the examples.

Questions and concerns should be directed to the NRAO Helpdesk.

The VLA as the VLA

Even though the VLA is a VLBI element you still must follow the rules for VLA scheduling which can be in the Guide to VLA Observing.  The three items that will effect most observers are:

  1. Dummy scans: The VLA must have a 1 minute scan for each frequency setup used before that frequency is used.   This 1 minute does not have to be on-source. The best practice is to put the dummy scans at the beginning of schedule to get them out of the way and so they can be in the possible slew time at the beginning of the schedule.
  2. Scans cannot be longer than 10 minutes.  This is a CASA limitation, but is enforced for all schedules.  Also it is a good idea not to have super long scans because if recording fails in a scan it is likely the entire scan is lost.
  3. Pointing: The VLA must do reference pointing at all frequencies higher than 15 GHz.  See the high frequency observing strategy guide for details but below are the most important notes.  The VLA:
    • should perform the reference pointing at X or C band and 1s integrations to
      determine pointing solutions for the high frequencies; the default X band setup is
      recommended for this.
    • must repoint if the antennas move ~15 degrees, so after about an hour and/or the telescopes move to a different source.  This changes if you are near zenith (see VLA documentation).
    • needs at least 2.5 minutes on source to point, this can be tricky at the start of a schedule since you do not know where the telescopes are moving from.  It is generally recommended that you assume there will be a very long slew (~10 minutes).

Flux Calibration

In order to improve the amplitude calibration of both the VLBI data and the VLA only data, it is good practice to include a few minute scan on a standard VLA flux calibrator in a phased-array observation.  This scan does not need to be phased nor have to be observed by the VLBA, however, if the observations are at a frequency requiring pointing at the VLA then the flux calibrator scan should have pointing before it.  The standard VLA flux calibrators are 3C286, 3C48, 3C147 and 3C138.  Pick one with the shortest slew.  If you do not include a flux calibrator scan, you are depending solely on the VLA switched power which is only good to ~10%.

The amplitude calibration will either be attached to the data, or the observer will receive an ANTAB style text file that includes the amplitude calibration for the VLA as a VLBI station.  This can be loaded with the AIPS task ANTAB into the TY and GC calibration tables attached to the correlated VLBI data.

Autophasing the VLA

In order to get the highest sensitivity from the VLA, the VLA antennas must be autophased before they are summed.  The phasing takes out the delay and phase differences between antennas.  In order to autophase the array the corrections are determined by observing a point-like calibrator, then these can be applied to a weaker or resolved target source.  If your target is bright and unresolved (with the VLA) then it can be used as its own autophase calibrator.  The autophase source should be strong, unresolved at the VLA and near the target.  How strong and near the autophase source should be depends on the frequency of observing, rough guidelines can be found on the Guide to VLBI at the VLA page.  The old VLA calibrator manual is a good place to look for autophase sources.  You can also use the calibrator search in the VLA Obs Prep Tool (OPT).

Autophasing in SCHED

It will probably take about a minute to autophase.  Every scan with the VLA in it must have a "vlamode".  There are three modes that will be use most often:

  1. vlamode = ' ', used when the VLA is not determining nor applying the autophase.  E.g., dummy scans, pointing scans.
  2. vlamode='va', determine autophase.
  3. vlamode='vx', apply the autophase determined in the last vlamode='va' scan.

Reference pointing for the VLA in SCHED

See the high frequency observing strategy guide for details on how to and when to reference pointing at the VLA. In SCHED the command "vlapeak"  controls determining and applying pointing at the VLA, there are two modes used most often:

  1. vlapeak= 'determine', determine reference pointing solution.
  2. vlapeak='apply', apply last reference pointing solution.

Frequency setups

Frequency setups compatible withV LBA PFB and DDC (4- and 8-channel) observing systems are available for the VLA.  For more information see the table of phased array modes.  The VLA's LO system is much more flexible than the VLBA's DDC system.  See the RDBE section of the OSS for a description of the DDC/PFB.  When putting together a frequency setup for a VLBA+VLA observation, start with the VLBA setup and just copy most of that setup for the VLA.  The VLA setup should be correct in the channel (subband in VLA speak) width, edge frequencies,  net sideband,  IF channels and FE (front end specifications), but the details of the LO do not matter.  Possible major differences between the VLA and the VLBA:

  1. The VLA is always dual polarization, even in shared risk modes.
  2. The VLA is always net upper side band (netside in SCHED).
  3. The IF channels (ifchan in SCHED) on the VLA are always A, C, B, D
  4. The Front End (FE) must be specified.  This tells the telescope which receiver to use.

Sending schedules to NRAO

Send the schedules to vlbiobs@nrao.edu as you would for any other VLBI observation.

Examples

The SCHED User Manual has many example schedules, in particular see jvla.key and any DDC examples which will help you set up the VLBA DDC.  Here I present a few examples of frequency setups and sequences of phasing up and pointing.

Frequency Setup Example #1: 8 GHz

setinit = rdbe_ddc_8540_dual.set /
! top section is for the VLBA DDC
dbe      = rdbe_ddc
sideband = U
netside  = U
bits     = 2
bbfilt   = 128
nchan    = 4
pol      = dual
firstlo  = 7900.0
bbsyn=640., 640., 768., 768.
station  = vlba
/
! bottom section is for the VLA
sideband = L
netside  = U
pcal     = off
ifchan   = A, C, B, D
fe(1)='4cm' fe(2)='4cm' fe(3)='4cm' fe(4)='4cm'
firstlo  = 9436.0
bbsyn=896., 896., 768., 768.
dbe = widar
station = vla27
/
endset /

Frequency Setup Example #2: 22 GHz

setinit = rdbe-128-k.set /
! Again, top section for the VLBA DDC
dbe      = rdbe_ddc
sideband = U
netside  = U
bits     = 2
pcal     = off
bbfilt   = 128
nchan    = 4
pol      = dual
firstlo  = 21300.00
bbsyn    = 512.0,512.0, 640., 640.
station  = vlba
/
! bottom part for the VLA
pcal     = off
nchan    = 4
sideband = U
netside  = U
bbfilt   = 128.0
ifchan   = A, C, B, D
firstlo  = 21300.00
bbsyn    = 512.0,512.0, 640., 640.
dbe      = widar
station  = vla27
/
endset /

Scans example #1: Simple phasing up including Flux calibrator scan

norecord
setup=rdbe_ddc_8540_dual.set
stations =  VLA27
!! dummy scan for VLA
source='3C273'  dur = 60  vlamode='' /

!! now observe with everybody
record
stations = VLBA_BR, VLBA_OV, VLBA_KP, VLBA_FD,
VLBA_PT, VLBA_LA, VLBA_NL, VLBA_HN, VLA27

!! Phase up with vlamode='va'
source='3C273' dur = 1:00  gap = 0 vlamode='va' /

!! Apply phased with vlamode='vx'
!! VLA scans cannot be longer than 10 minutes
!! D array so only need to phase up every ~20 minutes
source='M87' dur = 10:00 gap = 0 vlamode='vx' /
source='M87' dur = 10:00 gap = 0 vlamode='vx' /

!! Phase and apply loop
group=3 repeat=5
source='3C273'  dur = 1:00  gap = 0 vlamode='va' /
source='M87'    dur = 10:00 gap = 0 vlamode='vx' /
source='M87'    dur = 10:00 gap = 0 vlamode='vx' /

!! Now lets look at a VLA flux calibrator
source='3C286' dur = 7:00 gap = 0 vlamode='' /

Scans example #2: Pointing and Phasing the VLA at beginning of schedule

start=22:00:00
stations= VLA27
norecord

!! Dummy scans for the VLA, one minute for each setup (yes
!! even for pointing setups).
setup = rdbe-128-x.set  !x-band pointing dummy scan for the vla
source='0234+285' dur = 1:00 vlamode='' /
setup = rdbe-128-k.set  ! k-band dummy scan for the vla
source='0234+285' dur = 1:00 vlamode='' /

!! Point at the VLA, the pointing scan is so long because
!! this is the start of the experiment and the VLA might
!! have to slew a long time to get to the pointing source.
!! Later in the schedule when you have to point again and
!! you know the slew times, you can usually make this shorter
!! The VLA needs 2.5 minutes on-source to point
setup = rdbe-128-x.set  ! vla pointing setup
source = '0234+285'  dur = 10:00 vlamode = ''  vlapeak = 'determine'  /

!! While the VLA points, VLBA will start observing.
setup = rdbe-128-k.set
stations=VLBA_HN,VLBA_NL,VLBA_FD,VLBA_PT,VLBA_KP,
VLBA_OV,VLBA_BR,VLBA_MK,VLBA_LA,VLBA_SC
nopeak
start = 22:00:00       
source='J0437+24' dwell=12:00 record /

!! VLA is back from pointing, now start phasing
stations=VLBA_HN,VLBA_NL,VLBA_FD,VLBA_PT,VLBA_KP,
VLBA_OV,VLBA_BR,VLBA_MK,VLBA_LA,VLBA_SC,VLA27

!! Scan to phase up the VLA with vlamode='va', 70 seconds
!! are used to account for slewing. The VLBA is included
!! in this scan because it is better not to force subarrays
!! unless absolutely necessary. VLA pointing is applied with
!! vlapeak='apply'. The VLBA does not need to point at 22GHz.
source = '0234+285' dur = 1:10 gap=0 vlamode = 'va' vlapeak='apply' /

!! Scan to apply phases to the VLA with vlamode='vx'.
source = '0234+285' dur = 3:30 gap=0 vlamode = 'vx' vlapeak='apply' /