Array Options

There are five different array options: The first is the standard VLBA involving only its 10 dedicated stations. The other four options are known as "VLBA-plus", which involve adding additional antenna(s) to the dedicated VLBA stations: VLBA + single dish VLA (Y1), VLBA + three VLA antennas (Y3), the High Sensitivity Array (HSA), and Global VLBI.

Standard VLBA

The standard VLBA includes 10 identical 25-meter antennas located at sites across the United States:

  • Saint Croix (SC) in The US Virgin Islands
  • Hancock (HN) in New Hampshire
  • North liberty (NL) in Iowa
  • Fort Davis (FD) in Texas
  • Los Alamos (LA) in New Mexico
  • Pie Town (PT) in New Mexico (the closest VLBA site to the VLA)
  • Kitt Peak (KP) in Arizona
  • Owens Valley (OV) in California
  • Brewster (BR) in Washington
  • Mauna Kea (MK) in Hawaii

For live webcam images of the VLBA sites, visit the VLBA Virtual Tour. 

For additional information on the VLBA sites, see the VLBA Observational Status Summary (OSS).

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VLBA + Single Dish VLA (Y1)

A single dish VLA antenna (Y1) can be used in conjunction with the VLBA. The inclusion of such a dish would provide a short (~50 km) baseline to the VLBA station at Pie Town (PT). 

<< Previous: 2.1 Standard VLBA   Next: 2.3 VLBA+Y3 >>

VLBA + Three VLA Antennas (Y3)

VLBA + Y3 is currently offered under the VLBA Resident Shared Risk Observing (RSRO) Program only.

This document describes the particular case of three VLA antennas (Y3) being used in conjunction with the VLBA‡, with each of them correlated with the VLBA antennas and among themselves. This VLBA + Y3 mode has been partially commissioned through RSRO projects, but needs further commissioning. See the VLBA Observational Status Summary (OSS) for more details about RSRO projects.

The intention of Y3 in the context of this document is to provide VLBI with additional short spacings when combined with the VLBA. However, the use of Y3 limits the bandwidth on the VLA antennas. Relative to Y1, Y3 has 3 times the collecting area but only 1/4th the bandwidth due to recording-data-rate restrictions. Therefore, while it adds multiple baselines and enhances the uv coverage, from the sensitivity aspect Y3 is roughly equivalent to adding another half-antenna.

For proposals requesting Y3, we note that:

  1. Y3 is only available for A-configuration when the outer antenna baselines are useful when combined with the VLBA.
  2. Proposals must clearly demonstrate why single VLA antenna (Y1) is insufficient for the science proposed, because Y3 projects require significantly more resources than Y1. Note that Y3 does not appreciably increase the sensitivity for continuum observations over Y1 because of the bandwidth limits on Y3 noted above.
  3. If there are no restrictions on scheduling and the full array is not required for the project then Y3 will be observed in the moves from from BnA (or B) to A and from A to D configurations when the outer antennas are at their A-config locations. 
  4. If a Y3 project must be observed during full A configuration, e.g., the project is triggered, then proposals are strongly encouraged to also include a proposal for how to use the remaining VLA antennas in a regular (non-phased) VLA mode to support the proposed VLBA+Y3 science. These should be submitted as joint VLBA/VLA proposals, with the VLBA proposal including the Y3 and the VLA proposal for the remaining 24 antennas. Because most VLA proposals for A-configuration require the full resolution of the array there will, in general, be very few other projects that can be carried out with the 24 VLA antennas not being used for the Y3 experiment, therefore proposals that do not include a request for the remaining 24 VLA antennas must have a very strong justification for not using the entire array. Note again, that if the project can be scheduled in the moves between configurations this is not required.

‡There is another Y3 concept that has not yet been tested nor commissioned and we note it here for completeness. This can also be proposed under RSRO. In this other mode, known as Y3-IP, three VLA antennas are used to enable interferometric reference pointing (IP) on the three VLA antennas when observing at high frequencies (> 15 GHz) along with the VLBA. During the observing session, when not doing IP: a) the three antennas can be phased up, summed, and recorded as a single VLBI element, giving approximately three times the sensitivity of a single VLA antenna, or b) the data from one of them can be recorded for VLBI if a wide field of view is critical for the science. Since this does not need to use the ends of the arms, in fact short baselines would be preferred, this type of Y3 does not include the restrictions detailed above.

 

<< Previous: 2.2 VLBA+Y1   Next: 2.4 HSA >>

High Sensitivity Array (HSA)

High Sensitivity Array

For projects that require the maximum sensitivity, users should consider submitting proposals for the High Sensitivity Array (HSA). Currently, users may add the following telescopes to the VLBA to improve sensitivity:

  • Effelsberg: Effelsberg has often been used in conjunction with the VLBA, and is frequency agile at 5 GHz and above. For more information about including Effelsberg with the HSA we refer to the Effelsberg HSA page and the Effelsberg VLBI page.
  • Green Bank Telescope: The Green Bank Telescope is also commonly used with the VLBA, and is frequency agile (with some limitations) at all its bands.  See the VLBI at the GBT page for more information.
  • Very Large Array:  The Very Large Array (VLA) is available in phased-array mode (Y27).  Please see the VLBI at the VLA, and the HSA/GMVA/Global VLBI and Building a Schedule chapters of the Guide to Observing with the VLBA pages for further considerations.

Check the OSS for any limitations or special instructions for each HSA telescope.  Also, make sure to use the EVN sensitivity Calculator to estimate the rms noise for your observation.

 

Important note on possible HSA expansion: After the loss of the 305-m Arecibo radio telescope, the NRAO has looked into the possibility of having other radio telescopes be part of the HSA. Considering that there are currently no agreements to include other stations, the HSA continues to be the addition of the GBT, the phased VLA, and/or Effelsberg to the VLBA in the semester 2022A.

 

Special considerations for HSA proposing and observing

  1. Proposals: Proposals should be sent to NRAO only, through the Proposal Submission Tool, which can be accessed via NRAO Interactive Services. Click on VLBA/HSA when creating the proposal.
  2. Field of View:  The field of view of the HSA is more limited than for the VLBA, because of the smaller primary beams of all the HSA telescopes, especially the phased VLA (Y27). For the latter, note that the larger the extent of the array, the smaller its field of view.    
  3. Scheduling: Use NRAO's SCHED software to make the observing schedule, available from anonymous ftp at: ftp.aoc.nrao.edu The SCHED manual has several example files to get you started. Please be sure to use the latest version of the software, or to run SCHED on the final schedule file submission.

    For scheduling note that:
    • Effelsberg has station name 'EB_VLBA'
    • GBT has station name 'GBT_VLBA'
    • The phased VLA has station name 'VLA27'
  4. Slew speeds:The Effelsberg, GBT, and VLA slew speeds are slower than for VLBA antennas. The GBT slew rate also decreases during cold weather.  Be aware of this when writing your schedule, and be sure to examine the output from SCHED. SCHED has the latest information on slew speeds, including acceleration and deceleration, for all telescopes.
  5. Pointing: Since the Effelsberg and GBT antenna beamwidth is ~4 times smaller than those of VLBA antennas, periodic pointing checks are essential, especially during short wavelength observations. Observers should leave frequent gaps in their VLBI schedules (~10 mins each) to allow the telescope operators to make these checks. They should be about every hour or two at 4cm, and more frequently at shorter wavelengths. See the VLBI on the GBT page for more specific information on how and when to schedule pointing checks.
    The VLA also requires pointing at frequencies lower than the VLBA, for the VLA however the pointing must be scheduled by the observer in SCHED.  See VLBI @ the VLA:Scheduling Hints for examples more information.
  6. Submitting Schedules: The submission deadline is two weeks before the observation. The input file for SCHED, incorporating any non-standard setup files, should be e-mailed to vlbiobs@nrao.edu.  If you have problems with your schedule contact vlbiobs@nrao.edu.  If any changes are made to schedule after the deadline the support personal at all the telescopes must be contacted.
  7. Frequencies: While the GBT has frequency coverage equal to the VLBA, changing receiver bands takes 5-10 minutes. For more details on changing frequencies at GBT see the VLBI on the GBT page.
    The VLA and VLBA have similar frequency bands, but the VLA receivers generally have a wider tuning range.
    See Instructions for Joint VLBA + Effelsberg Observing for information about frequency availability and changing between frequencies at Effelsberg.
  8. Non-Standard Frequency Setups: Observers using non-standard frequency set-up files (i.e. not one supplied with the current version of SCHED) should contact the technical friends at the telescopes. These are  at the GBT and Uwe Bach at Effelsberg. Contact should be made well before the observations so that any special files needed for tuning can be made in good time.
  9. Correlator: All observations will be correlated at the VLBA correlator in Socorro.
  10. Archive: All observations will be archived on-line and password protected during the 1 year proprietary period. Request a project access key from the data analysts. See the NRAO data policy for further details.

 

<< Previous: 2.3 VLBA+Y3   Next: 2.5 Global VLBI >>

Global VLBI

Global VLBI

Users interested in the maximum possible angular resolution, or extremely long observations in a single track, should consider submitting a Global VLBI proposal. Global VLBI falls into two categories:

NOTE: submissions by email, fax, or paper will not be considered for any Global VLBI opportunity.

GMVA

GMVA projects are correlated at the Max Planck Institute for Radio Astronomy in Bonn, Germany. Users interested in GMVA should submit proposals via the NRAO Proposal Submission Tool (PST) at the usual VLA/VLBA deadline: The Wednesday closest to February 1 and August 1 by 1700 Eastern Time. New users are strongly encouraged to contact the VLBA and/or GMVA staff to get assistance in preparing their proposal. 

Global cm VLBI

Global cm VLBI involves combining the VLBA with the European VLBI Network (EVN). Proposals for Global cm VLBI must be prepared and submitted using the EVN NorthStar Tool. The EVN Northstar tool will automatically send copies of Global cm VLBI proposals to NRAO for assessment.  Submission deadlines for Global cm VLBI do NOT follow the normal NRAO schedule.  Instead, the proposal submission deadlines are are February 1, June 1, and October 1 by 23:59:59 UT.  Additional guidelines for EVN and Global cm VLBI can be found at the EVN Guidelines for Proposal Submission and the Using the EVN website.

 

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