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High Sensitivity Array (HSA), Global Millimeter VLBI Array (GMVA), and Global cm VLBI

by Justin Linford last modified Aug 06, 2020 by Emmanuel Momjian

VLBA+ Arrays

If your science project involves adding additional telescopes to the 10 standard VLBA stations, it is considered a VLBA+ observation. These observations involve extra steps. First, the observations must be coordinated with the other observatories.

Y1: Adding a single VLA antenna to the VLBA

Y1 observations are the easiest VLBA+ observations to schedule. Because the VLA has 27 antennas, pulling a single antenna out and adding it to the VLA can often be done fairly easily. However, the VLBA schedulers will still need to coordinate with the VLA and determine the best date/time to perform such an observation. The VLA control staff must also ensure that the VLBA recording media is properly mounted at the VLA.

Y27: Adding the phased VLA

Because the VLBA and VLA scheduling staff are all located in the same office building, it is relatively painless to coordinate VLBA observations with the phased VLA. However, the observations must still be scheduled as “fixed date” in order to ensure everything is set up properly ahead of time for the observation.

Adding the VLA to the VLBA involves some special scheduling considerations.

First, all of the rules for scheduling a VLA observation must be followed. See the Guide to VLA Observing for more details. The two items that will have an impact on most observers are:

  1. Dummy scans: The VLA must have a 90 second scan for each frequency setup used before that frequency is used. This 90 second 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. 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).

Second, 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 density scale 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 density scale calibrator scan should have pointing before it. The standard VLA flux density scale calibrators are 3C286, 3C48, 3C147 and 3C138. Pick one with the shortest slew. If you do not include such a 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. [NOTE: If no flux density scale calibrator scan is scheduled, or if the scan is not observed correctly, observers can use gain constraints in the AIPS task CALIB during calibration (e.g., to exclude the VLA in the overall normalization).]

Third, 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 Observation Preparation Tool (OPT).

Details of including the phased VLA in a SCHED keyin file, including examples of setups and scans, can be found in the Building a Schedule File in SCHED chapter of this guide.

Adding other external telescopes

Coordination with observatories outside of NRAO (including Green Bank Observatory, Arecibo, and Effelsberg) requires some lead time. Each observatory has different demands on their available open-skies time. While NRAO will make every attempt to schedule an approved project as proposed, users should be aware that circumstances outside the control of NRAO staff may prevent a project from having access to all requested telescopes (e.g., when hurricane Maria caused damage to Arecibo).

Due to the coordination required for these types of observations, they must be scheduled as “fixed date”. In the case of high frequency observations, NRAO will attempt to schedule a small collection of possible observation times. If the weather forecast is not conducive to observing at the designated frequency, the observation will be delayed to a later date.

Note that the NRAO has no control over the hardware at other observatories. If an observation fails due to a hardware malfunction at one of the non-NRAO telescopes, it may not be possible to re-observe.

GMVA projects

Projects making use of the GMVA are only scheduled during the pre-determined GMVA sessions, as set by JIVE. Observations are coordinated by the Max Planck Insitut fur Radioastronomie (MPIfR). Note that weather conditions may not be optimal during these specified times. Observations will be attempted regardless of weather conditions at any particular telescope. Failed observations may not be re-observed.

GMVA observations are correlated at the MPIfR in Bonn, Germany. The correlation process is much more complicated than a usual due to the very heterogeneous array of antennas used and the impacts of weather on each individual station. Several correlation passes are often necessary for each observation. Therefore, the time to receive correlated data may be longer than a normal VLBA observation, and GMVA data is typically delivered to observers about two months after observing.

Global cm VLBI

Observations requiring the VLBA to participate in Global cm VLBI are coordinated by JIVE. Similar to GMVA observations, the dates for Global cm VLBI observations are determined well in advance of the observations. As such, the observing conditions may not be optimal during the observations. Failed observations may not be re-observed. The time to correlate Global cm VLBI observations is likely to be longer than a typical VLBA observation due to the more complicated correlation process, which is similar to the GMVA.