Offered Capabilities during the Next Semester

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 mm 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 receiver bands, frequency and tuning ranges are described in the Frequency Bands and Performance section. 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 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. However, the PFB mode provides more accurate amplitude calibration and should be used if <10% flux density accuracy is required.

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:

Capability	Description
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	Up to 4 Gbps recording rate NOTE: the dichroic will reduce sensitivity slightly at both frequency bands See the Frequency Bands & Performance page for details
VLBA + Y1 (single VLA antenna)	Provides a very short (~50 km) baseline Can help fill in gaps between VLA – A configuration and VLBA resolutions Up to 4 Gbps recording rate
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 × 5^m 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, fast dumps, 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.

Any requests requiring more than one correlator pass (e.g. more than 300 phase centers at 4 Gbps), integration times less than about 10 ms, or the output data size approaches 1 TB, must be extremely well justified.

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.

It is expected that phased ALMA will participate in some GMVA observations during ALMA Cycle 10 (~Oct 2023 - Sept 2024; it is anticipated that the ALMA Cycle 10 Call for Proposals will be open in 2023 March). GMVA session dates for 2024 are not yet fixed but Session I in 2024, which is traditionally in the period March - May, should provide an opportunity for GMVA + ALMA observing. In ALMA Cycle 10, ALMA expects to support 3mm and 7mm VLBI observations. Spectral line VLBI is supported for observations with GMVA + ALMA at both wavelengths.

Restrictions on GMVA+ALMA proposals in Cycle 10:

GMVA observations with ALMA will be limited to a fixed recording mode, which currently provides 4096 Mbps on all baselines.
Direct phasing of the ALMA array is limited to targets with a correlated flux density > 0.5 Jy at 3mm or > 0.35 Jy at 7mm, contained within an unresolved core on ALMA baselines up to 1 km. Direct phasing on the science target ("active" phasing) thus puts a lower limit on the brightness of the science target.
For weaker sources, the option of "passive" phasing was introduced in Cycle 8. In this mode, the ALMA array is periodically phased on a bright calibrator close in angular distance to the science target. (This mode has been in use for VLBI at the VLA for many years.) There are thus no restrictions on the flux density of science targets using passive phasing (aside from SNR considerations on VLBI baselines). The properties of the phasing calibrator must meet the same criteria as for actively phased observations, and it is recommended that the phasing calibrator lie within an angular separation of no more than 6 degrees from the science target at 3mm or 10 degrees at 7mm. Proposers must specify the phasing calibrator in their proposal; consult the ALMA calibrator catalog.
In order to make a clean linear-to-circular polarization transformation of ALMA recordings, any target source must be observed at each frequency for a duration of at least 3 hours (breaks for calibrators permitted) to sample a range of parallactic angles.

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.

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. Currently, this only supports standard VLA 8-bit continuum modes with a 2-GHz bandwidth.

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: in particular, the following restrictions are noted (other limitations are also possible):

No more than 5TBy per station per month can be requested for a given project.
A subset of stations can be selected.
Channel and/or polarization selection on data copy is not supported at this time.
Time selection is possible, with limitations.

Proposers must provide properly formatted, compliant, hard drives in advance of correlation.

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. Due to the ongoing pandemic, the RSRO program participants may choose to contribute remotely to help with the commissioning of these 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 effort along with their expertise and availability 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.

3 Gbps L-Band Observations

This mode allows recording and correlating L-band (20cm) observations with the VLBA with an aggregate data rate of 3 Gbps. Currently the recommended standard is 2 Gbps at L-band with the PFB system, while using 4 Gbps recording with the DDC system will result in a bandwidth that is wider than the useful span of the L-band receivers. The 3 Gbps recording utilizes the DDC with six 128 MHz data channels in dual polarization. This RSRO mode would deliver a bandwidth that is 1.5 times wider than that of the recommended standard. However, interested proposers will need to evaluate the net gain in sensitivity considering the impact of RFI on the increased bandwidth and the system temperature measurements used for calibration.

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).

VLBA real time correlation

The VLBA will be able to support observations where the data are transferred directly over the internet and correlated in real time in Socorro, NM. Such observations are currently limited to the VLBA only (no HSA, or GMVA, or Global cm VLBI), and are initially offered with a per-station maximum aggregate bit rate of 128 Mbps. Furthermore, such observations will be limited to a single observing setup within a given observing block, i.e., the number of data channels and their bandwidth do not change in a given observing session, but tuning or band changes are supported.

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 contributing 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.