Introduction to the VLBA

This page is a guide to using the VLBA, aimed specifically at inexperienced users, but also useful as a repository of most important links for more experienced observers. Information contained in this guide is intended to address the more-than-90% of all observations that might be classified as "standard" and are relatively simple to make.

Introduction

The Very Long Baseline Array (VLBA) is an interferometer consisting of 10 identical antennas, separated by distances from 200km to transcontinental 8600 km (with the longest baseline between Mauna Kea, Hawaii and St. Croix, Virgin Islands). The VLBA is controlled remotely from the Science Operations Center in Socorro, New Mexico. Each VLBA station consists of a 25m antenna and an adjacent control building. During observing runs the received signals at each VLBA station are independently amplified, digitized, and recorded on fast, high capacity recorders, and then sent to the correlator in Socorro where they are (cross)correlated.

The VLBA observes at wavelengths of 90 cm to 3 mm (312 MHz to 96 GHz) in nine discrete bands plus two narrow sub-gigahertz bands, including the primary spectral lines that produce high-brightness maser emission. VLBA can be used to observe and image a variety of compact radio sources having brightness temperatures higher than ~10⁵K. The array’s continuum sensitivity can be dramatically improved by combining the VLBA array with other telescopes: with GBT, VLA, and/or Effelsberg to create High Sensitivity Array (HSA), or with the existing VLBI-available telescopes in Europe and in the USA to create Global cm VLBI or Global mm VLBI Array (GMVA).

About 50% of available observing time is reserved for open skies observing, while the remaining time is dedicated to observations sponsored through the US Naval Observatory that provides a major contribution to the operation of the VLBA.

The pathway from proposal to final product is similar to the VLA one, and to telescopes operating in most wavebands. To help the novice VLBA users, below we list the basic steps, indicate who has the responsibility of carrying out each step ("PI" stands for the Principal Investigator), and provide direct links to further, more detailed documentation. The users are also forwarded to the VLBA Observational Status Summary (OSS) where a complete description of all capabilities of the VLBA available for observing semesters are provided (updated for each proposal call).

Why use the VLBA?

A wide range of scientific programs, from the classic observation of jets in active galactic nuclei (AGN), to position measurements of gamma-ray bursts (GRBs), and movies of supernovae (SNe) and extended stellar atmospheres are possible with this integrated telescope which provides images and astrometry on milliarcsecond scales.

Precision astrometry is a VLBA science centerpiece. The relative astrometric accuracy of ~10 micro-arcseconds achievable with the VLBA is comparable to what the Gaia satellite is designed to achieve for most stars in its catalog. The VLBA has been used to determine the 3D structure of the Milky Way by measuring parallaxes with 10 mas accuracy or better, which helps to quantify the detailed distribution of luminous and dark matter in the Milky Way. It is also used to measure pulsar parallax distances and proper motions.

The VLBA provides excellent angular resolution, allowing to probe parsec-scale structures at the centers of distant galaxies. There the VLBA plays a major role in the observation and monitoring of the dynamics of the central regions of AGN.

The VLBA is valuable in the age of multi-messenger astronomy. The NASA Fermi Gamma-ray Space Telescope has been in operation for over 10 years, and VLBA contributes through coordinated observations with Fermi casting light on the highly energetic processes in the Universe. The VLBA is also used to follow-up on transient events like Novae, GRBs, or neutron star mergers (one of the gravitational waves origins).

The simplest observations are those of strong continuum sources at centimeter wavelengths; these are even easier than VLA observations of similar sources, since a-priori amplitude calibration is used, and the source serves as its own phase calibrator. Observations of weak sources are readily accomplished using the technique of phase referencing, exactly analogous to the standard observing technique at the VLA.

Detailed information on general radio interferometry can be found in the Synthesis Imaging in Radio Astronomy Lectures, and the Very Long Baseline Interferometry (NRAO Summer School 2018) and Astrometry lectures (NRAO Summer school 2010) are particularly relevant to VLBA.

Proposal Selection

The refereeing and time-allocation process is common to VLBA/HSA, VLA and GB proposals and is summarized on the Proposal Evaluation and Time Allocation page (direct link to TAC timeline, see also Section 15 of the VLBA OSS).

Schedule Preparation

VLBA schedules are prepared using the SCHED software, which has an extensive on-line SCHED documentation. Most users start with a template input (or "key-in") file, then modify it as needed. SCHED also can compute the times at which sources are up at different stations, and can plot the (u,v) coverage for a particular draft schedule; these capabilities may be useful in writing the proposal, as well as in scheduling.

Preparation of schedule files is nominally the responsibility of the PI and most important steps are highlighted in the following sections. If in need of assistance please send a question to the NRAO Helpdesk and select the "VLBA Department".

Observing strategy

All the required information to successfully prepare VLBA observing file can be found in the SCHED documentation. Main points to remember during the schedule file preparation are listed below.

Frequency Setup Files. These are setup files that set up the frequencies at which PI wishes to observe. Standard frequency setup files for default VLBA frequencies, that will work for most continuum observations, are available in the SCHED documentation. Observers who wish to use non-standard setups (frequencies or otherwise) should consult the SCHED documentation (Section 3.6) or NRAO Helpdesk.

Fringe phase, delay and rate calibration. VLBI data require phase, delays, and depending on the observing frequency sometimes also rates, to be calibrates. An initial a-priori correlator model at each station is first applied. Hardly ever these models will be sufficient, and schedule needs to accommodate for further calibration setup that will aid in later data reduction (fringe fitting). Options include the utilization of: (1) pulse-cal system, (2) phase referencing, (3) strong phase calibrator combined with target self-calibration during final data reduction, or some combination of these methods.

The choice of the method will depend on the science application, or the brightness of the target source. The pulse calibration data are stored in the PC table of your visibilities. Although fairly standard, some science applications (e.g. spectroscopy) should not use the pulse cal system; these schedules may opt to not use the PC table, or may need to disable the pulse cal generators altogether and instead opt for manual phase calibration in their schedules. For the manual phase calibration if the target source is strong enough, the PI may opt to observe a near-by strong phase calibrator and perform self-calibration in the data reduction stage. But for astrometric applications, or if target is weaker than approximately 30mJy at cm wavelengths, and 100-200 mJy at 22 and 43 GHz, the phase referencing method will be required. Phase referencing is very similar to the phase calibration done at the VLA and is described in detail in Chapter 17 of ASP Conference Series Volume 82 "Very Long Baseline Interferometry and the VLBA", and in VLBA Scientific Memo No. 24. For more information see Section 12 of the VLBA OSS.

Amplitude Calibration. Standard amplitude calibration of the VLBA uses continuously measured system temperatures (T_sys) via switch-noise system and predetermined gain curves. Therefore no additional calibration measurements are required during an observation, unless amplitude errors of less than ~5% are required. Please see Section 11 of the VLBA OSS and SCHED documentation (Section 2.2) for more information.

Specialized observing modes (polarimetry, scpectroscopy, pulsar observing). Apart from total intensity (Stokes I) continuum observations, VLBA can also perform observations in more specilized modes including polarimetry and spectroscopy. Detailed information on polarimetric observations can be found in the Polarimetry section of the OSS and VLBA Scientific Memo No. 26, and the VLA flux/polarization monitoring program that can be used to calibrate the polarization position angles. For details on setting up spectroscopic observations see the Spectral Line Observations section of the VLBA OSS. VLBA also supports the more advanced mode of pulsar observing, details of which can be found in the Pulsar Observationssection of the VLBA OSS.

Playback Gaps. Recording gaps of at least one minute should be scheduled at approximately once per one hour of observing. This protects against losing large amounts of data due to playback problems. For more information see the SCHED documentation (Section 2.2).

Schedule Submission & Dynamic Scheduling

At present, most VLBA observations are done dynamically, with time allocation made approximately 1-2 days in advance, depending on weather forecasts and telescope availability. Once the PI submits the schedule, observations are performed by the NRAO staff and the observer/PI will automatically receive the observing log once the observation is complete. For detailed information on the schedule submission and dynamic scheduling see the Dynamic Scheduling Guide and SCHED documentation (Section 2.2).

Correlation, Data Validation and Distribution

NRAO staff will perform the data correlation on the VLBA DiFX correlator, confirm the quality of the correlator output, and distribute the data to the observer. Correlation and data-distribution parameters are derived from the original schedule file. The default data distribution method is ftp from the data archive. The observer should request a project access key from the NRAO Helpdesk. The proprietary period of data ownership for VLBA observations is 12 months from the time the correlated data are released.

Calibration

Users should check the AIPS website for AIPS Cookbook and any relevant news affecting the software and data reduction with AIPS (PDF or PS versions in the AIPS Cookbook are also available). For detailed instructions on all aspects of calibration, see A Step-by-Step Recipe for VLBA Data Calibration in AIPS (html) of the AIPS Cookbook for details. Users who would like to request assistance should specify it in their original observing proposal, or else contact the NRAO Helpdesk and select "VLBA Department", at least 3 weeks before the observation.

Tables that can be used to perform much of the calibration of VLBA data are extracted from VLBA monitor data and appended to the correlator output FITS files. There is no longer any need for observers to prepare input files for various types of calibration, unless telescopes in addition to the VLBA, VLA, GBT, and Effelsberg are used. The supplied tables are as follows:

• TY table: system temperatures measured every 1-2 minutes
• GC table: a-priori VLBA telescope gains
• PC table: extracted pulse-calibration amplitudes and phases
• FG table: flagging information from individual telescopes
• WX table: weather information from stations

Note that the non-VLBA telescopes may not have all desired information (e.g. FG or PC tables).

For more information about application of these tables, see Operations Memo No. 34.

Final imaging and analysis

Final imaging and analysis of the scientific data is generally the responsibility of the observer, although considerable assistance from NRAO is available.

Within AIPS, task IMAGR is typically used for imaging, with CALIB used for self-calibration cycles. See Chapter 5: Making Images from Interferometer Data (html) in the AIPS Cookbook for details. The DIFMAP package, developed at Caltech, also can be used for data editing, self-calibration, and imaging.

Support (including financial support)

NRAO offers possibilities for scientific collaborations with NRAO staff, as well as support in data reduction and publishing results. 

• Data Reduction: Financial support is available for observers affiliated with U.S. institutions who wish to visit NRAO to reduce their VLBA data. Details about the financial support, as well as information on visiting DSOC in Socorro, NM and on requesting scientific collaborations with NRAO staff are available from the Other Info for Observers page. Proposers should bear in mind that NRAO staff will not do all the VLBA work for them, but instead will work collaboratively to help teach them VLBI techniques while working on the scientific program.
• Scientific Results and Publications:Timely preparation of scientific papers based on VLBA observations is the responsibility of the investigators. NRAO will provide assistance with page charges for observations which are based wholly, or in part, on VLBA observations; this financial assistance is available only for investigators based at U.S. institutions, and the relevant information can be found on the Other Info for Observers page.
• Graduate Students:NRAO has a pre-doctoral fellowship program that permits students to spend 2 years at an NRAO site working on their dissertation using data from the VLBA (or other NRAO telescopes). In addition, NRAO's summer-student program has a number of slots for graduate students. Students in both programs are paid stipends during their tenure at NRAO. Information is available about these and other NRAO student programs.