Facilities > VLBA > Documentation > Manuals > VLBA Observational Status Summary 2021B > Offered Capabilities during the Next Semester

Offered Capabilities during the Next Semester

by VLBA SUS — last modified Dec 21, 2020 by Frank Schinzel

The Call for Proposals

The most recent Call for Proposals summarizes the General Observing (GO) capabilities being offered for the Very Long Baseline Array (VLBA), the High Sensitivity Array (HSA), and Global VLBI arrays such as the Global 3mm VLBI array (GMVA).

In addition to these general capabilities, NRAO continues to offer shared risk observing options for those who would like to push the capabilities of the VLBA beyond those offered for general use. These are the Shared Risk Observing (SRO) and Resident Shared Risk Observing (RSRO) programs.

Details about what is being offered for each program is given below. If you have any questions or problems with any link or tool, please submit a ticket through the NRAO Helpdesk.

General Observing (GO)

VLBA Summary of Capabilities

The VLBA provides ultra-high angular resolution for astrophysical studies including:

  • Non-thermal continuum emission, including polarimetry, from active galactic nuclei (AGN), Galactic micro-quasars, pulsars, and other sources.
  • Maser emission lines of OH (1.7 and 6.0 GHz), CH3OH (6.7 and 12.2 GHz), H2O (22 GHz), SiO (43 and 86 GHz) and other molecules, and numerous thermal absorption lines, in a variety of Galactic and extragalactic circumstances.
  • Multiple-phase-center surveys across the primary beam.
  • Parallax and proper motion via differential astrometry of a variety of stars, star-forming regions, and nearby extragalactic objects, at accuracies as good as 10 microarcsec.
  • Absolute astrometry at accuracies of ~200 microarcsec to expand the International Celestial Reference Frame.

The VLBA operates two data systems, a Polyphase Filterbank (PFB), and a Digital Downconverter (DDC). These are described in detail in the Roach Digital Backend (RDBE) section which also includes suggestions for selecting the optimal observing system for various scientific goals. In general we strongly suggest using the DDC mode when possible. This allows 4096 Mbps (4 Gbps), plays better with eTransfer, and has been seen to be more reliable in operation.

The VLBA will continue recording at a maximum rate of 4096 Mbps (4 Gbps), and we expect to be able to support this rate for most of the open-skies observing time.

The GO capabilities being offered are:

CapabilityDescription
4 Gbps recording rate
  • Available for the 6cm, 4cm, 2cm, 1cm, 7mm, and 3mm receivers

  • 1024 MHz polarization-summed bandwidth

  • Requires the DDC data system

  • Available for standard VLBA, global VLBI, VLBA+Y1, and HSA observations that include Y27, GBT, and Effelsberg

  • We expect to be able to support this recording rate for most of the available open-skies observing time
  • NOTE: 90cm, 50cm, 21/18cm, and 13cm bands require recording rates of 2 Gbps or less due to their limited bandwidths

S/X Simultaneous Observations
VLBA + Y1 (single VLA antenna)
Multiple Phase Centers
  • Standard 10-station VLBA observations are limited to about 600 phase centers at 2 Gbps (512 MHz polarization-summed bandwidth) with a single correlator pass for dual polarization products

    • 600 phase centers at 4 Gbps may require 2 correlator passes for dual polarization products

  • Multiple correlation passes can achieve an unlimited number of phase centers

  • NOTE: Full polarization reduces the maximum number of phase centers per correlator pass by a factor of 2

Flexible Frequency Setup with DDC Data System
  • 1, 2, 4, or 8 data channels with bandwidths anywhere from 1 MHz to 128 MHz

    • NOTE: All data channels must use the same bandwidth within an observing scan

  • Recording rates from 4 to 4096 Mbps

  • Data channels may be placed nearly anywhere in the IF

  • See Linford & Brisken (2020) for more details on frequency setups

Flexible Spectral Resolution
  • Up to 4096 spectral channels per data channels for routine DiFX processing

    • Any number that can be factored as 2n × 5m can be specified

    • The total number of spectral channels (summed over all channels and polarization products) should be no more than 132,096 for compatibility with AIPS

  • Minimum spectral channel spacing of 2 Hz

Spectral Zooming
  • During correlation, allows the selection of a narrower frequency window to have a large number of spectral channels

Pulsar Modes
  • Binary gating, matched-filter gating, and pulsar binning correlation modes for pulsar observations

Proposals requiring significant additional correlator resources, such as multiple phase centers per field or multiple pulsar phase bins, should consider mechanisms to support the correlation without adversely affecting the throughput of other projects. These should be entered in the technical justification portion of the proposal.

HSA Summary of Capabilities

The HSA comprises the VLBA, phased VLA, GBT, and Effelsberg telescopes. All of the HSA stations are equipped with instrumentation compatible with the VLBA observing capabilities described in the Roach Digital Backend (RDBE) section. Ongoing special considerations for the HSA telescopes are documented in the HSA.

HSA observations with a recording rate of 4096 Mbps is offered as General Observing.

VLBI observations combining the VLBA with any one or more of the other three HSA stations can be requested in a single HSA proposal. However, separate proposals must be submitted for any non-VLBI use of any requested telescopes.

GMVA Summary of Capabilities

Complete information on the GMVA is available at the GMVA website.

The GMVA will record at the highest bitrate which instrumentation and resources permit. Currently all telescopes will record at 4096 Mbps (except the KVN, which will record in a 1024 Mbps mode). Note that the availability of NOEMA (Plateau de Bure) for GMVA Session II in 2020 and later sessions is not confirmed.

Furthermore, it is expected that phased ALMA will participate in some GMVA observations during ALMA Cycle 8 (Oct 2020 - Sept 2021; see the ALMA Cycle 8 Pre-announcement). There are likely to be ~43 ALMA antennas available for GMVA observations; but the phased sum used for VLBI may be constructed only from those that lie within a circle of radius 0.5 km. GMVA session dates for 2021 are not yet fixed but Session I in 2021, which is traditionally in the period March - May, would provide an opportunity for GMVA + ALMA observing.

Shared Risk Observing (SRO)

The VLBA Shared Risk Observing (SRO) program allows observers access to capabilities that are essentially commissioned, but are not well tested. If proposers are in doubt as to whether their proposed setups are Shared Risk or Resident Shared Risk, they are encouraged to get in touch with NRAO staff (e.g. by using the NRAO helpdesk) well before the proposal deadline. We emphasize the "shared risk" nature of the SRO program. Since observers will be attempting to use capabilities that are not well-tested, NRAO can make no guarantee of the success of any observations made under this program, and no additional commitment is made beyond granting the hours actually assigned by the peer review process.

VLBA SRO: Baseband Data Copy

The raw data recorded on the station recorders can be copied to user-supplied media for correlation at a different location that has the capability of reading VLBA baseband data (in VDIF format). For a successful proposal requesting this VLBA Shared Risk capability, the following items should be addressed in the proposal:

  • If correlation is needed to be performed elsewhere; e.g. a higher time resolution than can be provided by the VLBA DiFX correlator
  • The use of baseband copy should be fully justified in the proposal, either in the technical justification, or the science justification of the proposal
  • The acceptance and consideration of the request of baseband copy is subject to VLBA resources available
  • Proposers are expected to provide their own media storage, which is compatible with the copy mechanism

The use of baseband copying may be requested by selecting the "Baseband Copy" checkbox under "Special Features" in the VLBA Resource in the PST.

Resident Shared Risk Observing (RSRO)

The VLBA Resident Shared Risk Observing (RSRO) program provides users with early access to new capabilities in exchange for a period of residency in Socorro to help commission those capabilities.

RSRO proposals should be submitted using the NRAO Proposal Submission Tool in response to a regular proposal call. The proposal should include a scientific justification, as for normal proposals, which will be peer reviewed as part of NRAO's time allocation process. Selecting "VLBA RSRO" from the "Observing Mode" menu on the Resources page makes an "RSRO Comments" text-entry facility available for describing the technical resources required. A description of the personnel who will be involved in the residency along with their expertise and proposed dates of residency should also be included in the technical justification.

We emphasize the "shared risk" nature of the RSRO program. Since observers will be attempting to use capabilities under development and in the process of being commissioned, NRAO can make no guarantee of the success of any observations made under this program, and no additional commitment is made beyond granting the hours actually assigned by the peer review process.

Proposals for any area of user interest are welcome; several desirable developments are listed below.

Many pulse cal tones per channel

The DiFX software correlator has the capability to extract every pulse cal tone in each channel (up to 128 in the most extreme case), compared to just two channels, which is the historical norm. AIPS now has recent support for this in the form of increased capability of existing tasks and several new ones. Many pulse cal tones may offer several new calibration or data assessment techniques. Users seeking early use of this capability are invited to propose for a RSRO project with goals of validating the new software and developing/documenting best practices for its use.

Improved troposphere model

The Vienna Mapping Function formulation used to characterize the tropospheric delay has been implemented for the DiFX correlator. Use of this updated model is thought to improve phase referencing at high frequencies and the precision of relative astrometry. A RSRO applicant should expect to spend a few weeks in Socorro working with staff to evaluate the effectiveness of the new model, leading to it being commissioned as a feature available to all observers.

Rapid response capability

Dynamically scheduling the VLBA in response to a trigger should allow observing to begin within 5 to 10 minutes, in opportune situations. Establishing a general rapid-response capability that would cover all conceivable cases is not considered feasible, so this RSRO program would allow the definition of triggers and responses suitable to a specific case of interest to the proposing user group. The work required would include setting up software infrastructure and operations procedures for automated preemption of ongoing array activity, subject to prioritization constraints.

L/P Dual-Band Observations

This mode allows for simultaneous L-band (20cm) and P-band (90cm) observations with the VLBA. However, we note that the P-band data acquired in this dual-band mode will experience some sensitivity degradation (~33%) compared to data obtained in single-band P-band observations.

Y3 Observing with the VLBA

Similar to adding a single VLA antenna (Y1) to the VLBA to provide short spacings, three VLA antennas (hence Y3) may be added to the VLBA. This is needed to enable interferometric reference pointing on the VLA antennas when observing at high frequencies (> 15 GHz), or to record the data from 3 VLA antennas independently for correlation with the VLBA (e.g., one VLA antenna from each arm, especially when the VLA is in its most extended configuration).

Recording Wide-band VLA Visibilities in parallel with Y27 VLBI

In addition to recording VLA phased array (Y27) data on the Mark 6 unit for VLBI purposes, this mode will deliver wider bandwidth data through WIDAR for the VLA itself. This would be useful to optimize the sensitivity of the VLA observations as a secondary (VLA only) science product, or for pulsar observations to derive the parameters that are needed for a proper correlation of the VLBI data at a later time (e.g., pulsar gating).

Participation in the VLBA RSRO Program

The primary requirement of the RSRO program is that there be at least one expert from each participating group will to reside in Socorro and to contribute to commissioning, while incurring as little overhead from VLBA staff as possible. Limited support for accommodation in the NRAO Guest House for participants in the RSRO program may be available.  See the RSRO Considerations section in the Guide to Proposing for the VLBA for additional details about participating in the RSRO program.