Observing > Call For Proposals > Call For Proposals: Semester 2016A > Very Long Baseline Array (VLBA), High Sensitivity Array (HSA), & Very Long Baseline Interferometry (VLBI) Proposals

Very Long Baseline Array (VLBA), High Sensitivity Array (HSA), & Very Long Baseline Interferometry (VLBI) Proposals

by Davis Murphy last modified Jul 16, 2015 by Mark Claussen

The 3 August 2015 deadline applies to all types of Very Long Baseline Array (VLBA) and High Sensitivity Array (HSA) proposals requesting time in semester 2016A (1 February 2016 – 31 July 2016) or multi-semester proposals. It also applies to global mm VLBI proposals for the Spring 2016 (May 19-25), or later, sessions. Please see the instructions for submitting VLBA, HSA, and global VLBI proposals. Requests for resources beyond just the VLBA – i.e., the inclusion of HSA or Global 3mm VLBI (GMVA) stations – need to be quantitatively justified in the proposal.

VLBA Observing Capabilities

The VLBA provides ultra-high angular resolution for observations of non-thermal continuum emission, maser 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 absorption-line studies of numerous thermal spectral lines. The VLBA operates two data systems. In the following summary, an "IF" is one of the four 512 MHz signals carried on cables from the antenna's vertex to the control building; a "channel" refers to a single contiguous frequency range of any bandwidth, observed in a single polarization, that is sampled, filtered, and recorded as a separate entity. The two data systems comprise the following:

  • The Polyphase Filterbank (PFB) observing system provides sixteen 32 MHz channels with a fixed 2048 Mbps recording rate. The channels can be selected flexibly between two VLBA IF inputs. Channel placement is restricted to 32 MHz steps along the frequency axis.
  • The Digital Downconverter (DDC) observing system is considerably more flexible than the PFB. As many as eight channels can be arbitrarily selected from up to four VLBA IFs and placed at 15.625 kHz steps on the frequency axis with bandwidths ranging from 1 MHz to 128 MHz by multiples of 2. Extremely narrow bands can be accommodated by observing at 1 MHz bandwidth and selecting a narrower range using the DiFX correlator's spectral zoom mode. Bandwidths for DDC observations must be uniform across the entire observing array and throughout the entire duration of the observation. Channels cannot span either of two zone boundaries, within the IF band, at 640 and 896 MHz. Use of 128 MHz bandwidth is limited to 4 (or fewer) channels by the 2048 Mbps recording rate limitation.

Wideband science is possible using either the PFB observing system at its fixed 2048 Mbps data rate, or the DDC system at 2048 Mbps or lower rates. Further details are available in the VLBA Observational Status Summary. It is anticipated that the pool of recording media will support the highest data rates for approximately half of all observing hours. Spectroscopic and other narrow-band observations will generally be best supported by the DDC system. Inputs to either data system can come from any of the four VLBA IFs. Typically only two are available, in opposite polarizations; some receivers support less common modes, such as dual-polarization dual-frequency. The four-IF capability of the DDC allows these modes to be exploited.

Note that proposals requiring significant additional correlator resources, such as multiple phase centers per field, should address mechanisms to support the correlation without adversely affecting the throughput of other projects.

Available VLBA Observing Time

Because of the high demand for Galactic time from ongoing Large programs, proposers should be aware that the available time on the VLBA is distributed non-uniformly across the sky and across the semester. The plot below shows the estimated time available for scientific observations, as a function of Pie Town LST and GST; the number of available hours per GST hour is the number of days (or passes) in the semester for which a particular LST is available . The very low availability at 21 - 05 GST, in particular, is because of the high subscription rate due to large Galactic parallax projects which are observed in parallax "season", late February through the end of April. 




VLBA Filler Project Challenge

NRAO continues to solicit proposals for one or more Large projects for up to 750 hours per semester of "filler" time on the VLBA. To be eligible for this large time allocation with FILLER status the project should be flexible enough to be scheduled:

    • with non-ideal weather conditions;
    • with less than the full complement of antennas;
    • with a target list of source positions around the sky; and
    • with short duration or variable length scheduling blocks.

Teams must provide tools that allow VLBA operations, with minimal effort, to create schedules for arbitrary blocks of time of one hour or longer when such time becomes available during dynamic scheduling. Large proposals for VLBA filler time will be subject to the usual NRAO Large Proposal Policy. Multi-semester proposals will be considered.  Types of projects that might use VLBA filler time include: surveys of many sources, astrometry, geodesy, deep integrations spread over many sessions, and long term monitoring. It is rare for fewer than 6 antennas to be functional and have good observing conditions; high frequency projects that can use a reduced array are therefore viable.  Regular proposals that can utilize the same sort of VLBA filler time are also encouraged.

VLBA Resident Shared Risk Observing Program

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. For example, the phased-VLA system was developed through RSRO programs.  We encourage innovative ideas for new VLBA capabilities from the user community.  Some suggestions from NRAO staff are  included at the VLBA RSRO program page, along with details for submitting RSRO proposals.

Proposers should be aware that RSRO capabilities are generally not approved at priority A, owing to the level of risk associated with these observations.

High Sensitivity Array (HSA)

The HSA comprises the VLBA, phased VLA, GBT, Effelsberg, and Arecibo telescopes.  All of these are equipped with instrumentation compatible with the VLBA Observing Capabilites described above.  Additionally, we are pleased to offer access to the Large Millimeter Telescope Alfonso Serrano (LMT) in Mexico for use at 3mm in combination with the VLBA, and optionally the GBT, through the VLBA Resident Shared Risk Observing program described above. Observing in conjunction with the LMT is available from 1 February through 30 June 2016.

The phased VLA (Y27) will be available for HSA observing in semester 2016A, in C, CnB, and B configurations (see the VLA section of this Call for Proposals for applicable dates). HSA proposals can request the phased VLA in conjunction with the VLBA and other HSA telescopes, subject to availability of matching observing systems (see below). As a general capability, the phased VLA is available as two independently-tunable VLA sub-band pairs, one polarization pair (RCP+LCP) in the A0/C0 baseband pair and the other (RCP+LCP) in the B0/D0 baseband pair. Any matching bandwidths available on the VLA as well as the VLBA DDC data system described above can be used.  Since they have not been extensively tested, other sub-band configurations that match the VLBA's PFB and DDC-8 modes are available as shared risk.  VLA phasing and VLBI observing must be carried out at the same total bandwidth. Sub-band bandwidths of 16 MHz to 128 MHz (by multiples of 2) are available as a general capability.  Sub-band bandwidths narrower than 16 MHz may work if the phasing source is strong enough, but are expected to be of limited use. The restrictions are fewer for the VLA than for the VLBA or other stations, so the HSA guidelines should be followed. The VLA must be set up to match the VLBA; mixed modes are not allowed.  Further details are available in the document VLBI at the VLA, and in the VLBA Observational Status Summary.

Observing with a single VLA antenna (Y1) in conjunction with the VLBA will only be available through the VLBA Resident Shared Risk Observing program.

The GBT is equipped with the same instrumentation described in the VLBA Observing Capabilities section, and is able to support all the observing configurations described there (but see the note on the 6 cm receiver below). Further details may be found in section 5.7 of the GBT Proposer's Guide. All proposals to use the GBT as part of VLBI must include time to set up the telescope (pointing, focus, etc.) prior to the start of the observation. This can take 0.5-1 hour depending on the frequency (see Chapter 7 of the GBT Proposer's Guide, and the GBT Observer's Guide, for further information).

The GBT's 6 cm receiver is similar to the VLBA's new system, but does differ in the conversion to circular polarization. Recent tests have seen substantial polarization leakage between the RCP and LCP channels. Improvements and further testing are under way. However, for this Call, we will consider only proposals for total-intensity observations.  Such proposals should request full dual-polarization modes for both observation and correlation, and careful calibration of the leakage terms should be included in the data analysis.

The Effelsberg and Arecibo HSA stations have also installed the same wideband equipment as at the VLBA and GBT, but their full implementation is not yet complete.  The following table summarizes the availability of the various observing systems for HSA stations for 2016A.

HSA station Observing system
PFB DDC-4 DDC-8
VLBA Y Y Y
GBT Y Y Y
Y27 SRO Y SRO
Arecibo Y N N
Effelsberg Y Y Y

DDC-4 and DDC-8 support 2IF observing modes with a maximum of 4 and 8 channels, respectively. DDC-8 can also observe 4IF modes for cases where they are supported by the stations' receivers and IF transmission systems. Tested 4IF modes available at present with the HSA include only the 6 cm systems on the VLBA and GBT (see also the temporary restriction described above for the GBT).

Combinations marked "SRO" are available as "Shared Risk" observing, which means these are capabilities that have been verified to work with certain set-ups, but have not been extensively tested. Proposers should be aware that SRO capabilities are generally not approved at priority A, owing to the level of risk associated with these observations.

The LMT will be offered as a station of the HSA for 3mm observations with the VLBA for observing dates 1 February 2016 through 30 June 2016 in semester 2016A.  The GBT may also be requested with the VLBA and LMT. Access to VLBI using the LMT is provided through the VLBA RSRO program, and VLBA RSRO should be selected as the resource in the Proposal Submission Tool. For VLBI the LMT can record a single-polarization 2048 Mbps mode compatible with the VLBA, using a dual-polarization 3mm receiver (RCP/LCP) with a tuning range exceeding that of the VLBA.  Proposers should use the VLBA capabilities to define their resource request when proposing to use the LMT for VLBI.

Global 3mm VLBI Array (GMVA)

VLBI proposals for observing at 3mm wavelength using the VLBA, GBT, Effelsberg, Pico Veleta, Plateau de Bure, Onsala, and Yebes telescopes should be submitted by 3 August 2015 through the NRAO Proposal Submission Tool.  Additionally, the LMT will be offered for inclusion in the GMVA on a "best effort" basis.  The LMT can be selected using the "Other Stations" text field in the PST.  Successful proposals will be considered for scheduling in the May 2016 (or later) session. As noted above, at some GSTs, the available time on the VLBA in the May 2016 session may be limited due to prior commitments (primarily in the Galactic Plane).  Also, although the GBT can participate in 3mm VLBI during the daytime, its sensitivity could be several times worse due to thermal deformations from solar heating.  To maximize the sensitivity for continuum observations, the GMVA will record at the highest bit rate that the telescope instrumentation and resources permit. All telescopes will record at 2 Gbps; the only exception being Plateau de Bure, which will record in a compatible 1 Gbps mode.

For further details on proposing please consult the relevant administrative and technical information hosted at Bonn.

Proposal Preparation

Proposal preparation and submission for the VLBA, HSA, and GMVA are via the Proposal Submission Tool (PST) at NRAO Interactive Services. Use of the PST requires registration in the NRAO User Database.

Information about VLBA capabilities can be found in the VLBA Observational Status Summary. Questions may also be directed to the NRAO Helpdesk.