Frequently Asked Questions

by Justin Linford last modified Aug 24, 2020

Observing with a certain cadence

My science objective requires that the observations be done with a specific cadence. How do I schedule these?

If your project requires that the observations are separated by a very specific amount of time (e.g., exactly 28 days), they should be scheduled as “fixed date” observing. We recommend that you contact the VLBA scheduling officer as soon as possible to arrange your observations. It is also a good idea to prepare your SCHED keyin files well in advance of the observations in order to avoid any delays.

Note that fixed date observations will take place regardless of weather conditions or other constraints. Therefore, there is a possibility that these observations will take place under less-than-optimal conditions.

It is recommended to schedule repeating observations with a flexible cadence, if the science goals allow for it. For example, instead of requiring an observation every 30 days, the request could be for every 30 +/- 3 days. This allows the observations to be scheduled dynamically and allows the schedulers to avoid poor weather conditions, antennas down for maintenance, and other conditions that can impact an observation. This type of flexibility is especially important for projects with B priority.

All cadence requirements should be included in the Dynamic Constraints section of the keyin file.

Polarization Products and Noise

How does the number of polarization products affect the noise?

Choices are single polarization (RR or LL), dual polarization (RR and LL), and full polarization (RR, RL, LR, and LL). If the emission is unpolarized, dual polarization gives twice the amount of independent data as single polarization does, and therefore decreases the rms noise by a factor of √2, as the sensitivity calculator will show. Full polarization does not improve this any further since RL and LR do not contribute to Stokes I.

Phase Referencing

When should I use phase referencing?

If there is any chance that your science target will not be bright enough for self calibration, you should use phase referencing.

Calibrators

How should I pick calibrators for my project?

The VLBA Calibrator List  will provide you with a list of potential calibration sources.  It is generally a good idea to check for recent information on these sources to ensure that variability is not an issue at your desired observing frequency.  Also, check that the positions of the sources are known to an acceptable uncertainty for your project.  If you cannot find recent information calibrators at your desired frequency, you can request extra observing time to identify a good calibrator (see the Technical Justification section regarding whether targets can be self-calibrated).

What calibration sources do I need for polarimetric observations?

Polarimetric observations require scans on at least one leakage (D-term) calibrator, and at least one electric vector polarization angle (EVPA) calibrator (although 2 or 3 EVPA calibrators are recommended).  Several (at least 5) calibration scans should be made covering as large a range in parallactic angle as possible.  Also, at least one 2-minute scan should be scheduled on a very bright (~10 Jy) calibration source to permit the calibration of the RCP-LCP delays.  See the VLBA Polarimetry page in the VLBA OSS for more details.

Setup Restrictions

How do current observing setup restrictions impact my project?

The VLBA can only be tuned to certain allowed frequencies.  Check the SCHED example setup files for examples of how to tune the frequencies in your observation.  For details on creating setups, see Linford & Brisken (2020).

If a target is observed at multiple frequencies during a single observation, there will be increased calibration overhead since it takes a finite amount of time for the telescope to switch receivers. For phase reference observations, each target source scan at a certain frequency has to be bracketed by gain calibrator scans at that frequency.

Assistance with VLBA Observations

Can I get help calibrating and analyzing my VLBA observations?

The NRAO is now offering assistance with certain VLBA programs. Currently, this offer is for standard VLBA observations only (no HSA, GMVA, or Global VLBI). This assistance program is intended for new or novice users. If you wish to request assistance, please check the text box on the bottom of the Proposal Cover Sheet in the PST and provide a brief statement about why the extra help is needed.

Even if a project was not awarded assistance under this new program, any VLBA observer can always contact the VLBA staff via the NRAO helpdesk if they have any questions or encounter any problems.

CASA for VLBA Calibration

Can I use CASA to calibrate my VLBA observations?

CASA releases 5.3 and later contain tools for VLBI calibration. However, these tools were primarily developed for use with the EVN and EHT. Some tools may not work properly with VLBA data. Users who wish to calibrate VLBA data in CASA are strongly encouraged to use CASA 5.7 because it will have the most current bug fixes and patches. CASA 6.1 can be used, but it lacks the ability to plot fringefit solutions due to plotcal being deprecated in the CASA 6.X versions. VLBA staff are currently testing the CASA VLBI tools.

Tutorials for VLBI calibration in CASA were developed by Anita Richards (University of Manchester) and Jack Radcliffe (SARAO / University of Pretoria) for the Development in Africa with Radio Astronomy project. There are two VLBI data reduction tutorials: one for EVN continuum observations, and one for EVN spectral line observations.

The EVN Data Reduction Guide provides a more general overview of the calibration process and compares AIPS tasks with related CASA tools.

In addition to the main CASA program, VLBA users will also need a script to convert the VLBA gain curve table into a format usable in CASA. This script (gc2.py, written by Mark Kettenis) is available from JIVE’s CASA VLBI Github repository.

Acronyms

AIPS = Astronomical Image Processing System
BBC = Baseband Channel
CASA = Common Astronomy Software Applications
DDC = Digital Downconverter
DDT  = Director’s Discretionary Time
EHT = Event Horizon Telescope
EVN = European VLBI Network
EVPA = Electric Vector Polarization Angle
GBT = Green Bank Telescope (officially, the Robert C. Byrd Green Bank Telescope)
GMVA = Global Millimeter VLBI Array
GO = General Observing
GST = Greenwich Sidereal Time
HSA = High Sensitivity Array
ICFR3 = International Coordinate Reference Frame 3
IEEE = Institute of Electrical and Electronics Engineers
IVS = International VLBI Service
JIVE = Joint Institute for VLBI in Europe
LCP = Left-hand Circularly Polarized
LST = Local Sidereal Time
NASA = National Aeronautics and Space Administration
NRAO = National Radio Astronomy Observatory
OPT = Observation Preparation Tool (for creating VLA schedules)
OSS = Observational Status Summary
PFB = Polyphase Filter Bank
PFT = Proposal Finder Tool
PST = Proposal Submission Tool
RCP = Right-hand Circularly Polarized
RDBE = ROACH Digital Backend
RFC = Radio Fundamental Catalog
RFI = Radio Frequency Interference
ROACH = Reconfigurable Open Architecture Computing Hardware
RSRO = Resident Shared Risk Observing
SEFD = System Equivalent Flux Density
SNR = Signal-to-Noise Ratio
SRO = Shared Risk Observing
SRP = Science Review Panel
TAC = Time Allocation Committee
TEC = Total Electron Content
TLA = Three Letter Acronym
TJ = Technical Justification
ToO = Target of Opportunity
UHF = Ultra High Frequency
VHF = Very High Frequency
VLA = Very Large Array (officially, the Karl G. Jansky Very Large Array)
VLBA = Very Long Baseline Array
VLBI = Very Long Baseline Interferometry

NRAO Bands: Letter Codes, Frequencies, and VLBA Receivers

VLBA Bands

Band
Designation
Frequency
Range
Associated
Receiver
P 312 - 342 MHz 90 cm
UHF 596 - 626 MHz 50 cm
L 1.35 - 1.75 GHz 20 cm
S 2.2 - 2.4 GHz 13 cm
C 3.9 - 7.9 GHz 6 cm
X 8.0 - 8.8 GHz 4 cm
Ku 12.0 - 15.4 GHz 2 cm
K 21.7 - 24.1 GHz 1 cm
Q 41.0 - 45.0 GHz 7 mm
W 80.0 - 90.0 GHz 3 mm

See the Frequency Band & Performance section of the VLBA OSS for more details.