Frequently Asked Questions

Bandwidth and Noise

What bandwidth should I use for noise calculations?

If you are doing a spectral-line project, you need to determine the noise per channel.  To do this, take the noise over the entire bandwidth (as reported by the EVN Sensitivity Calculator), then multiply by the square root of the number of channels.  (For an example, see Slide 46 in the 2019 VLA and VLBA Proposal Preparation presentation by Emmanuel Momjian.)

If you are are doing a continuum detection experiment, in which you want to cover as much spectral range as possible, use the total (RFI-free) bandwidth. When in doubt, use the total bandwidth you plan to use in making an image.  For high frequencies this is usually the entire bandwidth.

 

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 √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.  For most cases you will want dual or full polarization.

 

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. You should also use phase referencing if you are interested in the absolute position of the target, even if it is bright enough for self calibration.

 

Calibrators

How should I pick calibrators for my project?

The new VLBA Calibrator Search Tool 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. An ideal calibrator source has significant flux density on all the baselines that will be used for a project.  Also, check that the positions of the sources are known to an acceptable uncertainty for your project. If you cannot find recent information on 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).

An alternative search tool is the NASA VLBI Calibrator Search tool, which is particularly useful for those interested in the history of a source's flux density measurements.

 

What calibration sources do I need for polarimetric observations?

Polarimetric observations require three types of calibration:

1. Leakage (D-term) calibration:  To calibrate the D-terms you will need scans on a strong (~1 Jy) calibrator, preferably one with little structure.  Popular D-term calibrators are OQ208 (J1407+2827) and DA193 (J0555+3948).  If an unpolarized source is chosen for the D-term calibrator, only a single scan is necessary.  If the D-term calibrator has any polarized flux, at least 5 calibration scans should be made covering as large a range in parallactic angle as possible.  You can plot the parallactic angle coverage in SCHED. 
2. RCP-LCP delay calibration: Observe at least one 2-minute scan on a very bright (~10 Jy) polarized calibrator.  The goal here is to be able to detect fringes on the cross-hand (RL and LR) correlations so a significant amount of polarized flux is required.
3. Electric vector polarization angle (EVPA) calibration:  Observe at one electric vector polarization angle (EVPA) calibrator (although 2 or 3 EVPA calibrators are recommended).

It is possible that some of these calibrators can be the same source, like using one source for both RCP-LCP delay and EVPA calibration.  For obvious reasons you cannot use an unpolarized calibrator to calibrate the D-terms and use that for the other two calibrations.  See the VLBA Polarimetry page for more details.

 

Overhead

What overhead should I count on for slewing, calibration, etc?

The best way to estimate the total time needed for an observation is to build a dummy schedule in SCHED. Make sure to include scans on any calibration sources.

 

Setup Restrictions

How do current observing setup restrictions impact my proposal?

There are a number of observing constraints with the 2 VLBA observing modes. Most of these become important when preparing the observing script, but the following are of potential interest to proposers as well.

• The VLBA receivers can only be set to certain allowed frequencies. Check the SCHED example setup files for examples of how to tune the frequencies in your obseravtion.
• 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 setting up observations, calibrating and analyzing my VLBA observations?

Users can always use the NRAO helpdesk for assistance with any issues related to any of the telescopes operated by NRAO.  VLBA staff are available to provide assistance with proposal preparation, schedule creation, and data reduction.

VLBA users are also encouraged to visit the NRAO in Socorro, NM, for face-to-face assistance with calibrating and reducing their observations.  Travel support is available. Please note that current NRAO policy requires all visitors to provide proof of vaccination against the COVID-19 virus.

There are also on-line data reduction tutorials.

 

Past Approved Proposals

A Proposal Finder Tool (PFT) may be used to search cover sheets of proposals previously approved for time on NRAO telescopes. See also active triggered proposals, large proposals, and approved Director's Discretionary Time (DDT) proposals.

 

Acronyms

AUI = Associated Universities, Inc. (the managing body of NRAO)
DDC = Digital Downconverter
DDT = Director’s Discretionary Time
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
IEEE = Institute of Electrical and Electronics Engineers
LCP = Left-hand Circularly Polarized
LST = Local Sidereal Time
NRAO = National Radio Astronomy Observatory
NSF = National Science Foundation
OSS = Observational Status Summary (Note: the VLBA and VLA each maintain their own separate OSS.)
PFB = Polyphase Filter Bank
PFT = Proposal Finder Tool
PST = Proposal Submission Tool
RCP = Right-hand Circularly Polarized
RDBE = ROACH Digital Backend
RFI = Radio Frequency Interference
ROACH = Reconfigurable Open Architecture Computing Hardware
RSRO = Resident Shared Risk Observing
SEFD = System Equivalent Flux Density
SRO = Shared Risk Observing
SRP = Science Review Panel
TAC = Time Allocation Committee
TEC = Total Electron Content
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 and Frequencies

One of the most confusing aspects of radio astronomy for new students is the naming convention for the different radio bands. The letter designations for the bands do not appear to make any logical sense. Why is P-band a lower frequency than L-band, but S-band is a higher frequency than L-band? The story most of us are told is that the band letter designations were created during World War II. The Allies were using radar for aircraft detection and tracking, and (of course) radio for communications. To confuse the Axis powers, the Allies gave secret letter designations to the various frequency bands they were using. To further confuse the Axis, those letter designations were not assigned in alphabetical order, and lower-case sub-band designations were often added. This meant that a radio operator who suspected their transmission was being intercepted could tell other operators “switch to Ku”, and the Axis would not necessarily know which frequency to change to in order to intercept the rest of the communication. For radar, it meant that even if the Axis discovered that a particular radar operated at X band, they did not know the specific frequency they would need to broadcast in order to jam it.

There may have been some logic to the letter designation. For example, “S” may have stood for “short wave” and “L” may have stood for “long wave”. “P” is said to have meant “previous”, because the British originally operated radars in the 250–500 MHz range before switching to higher frequencies. “C” is said to have stood for “compromise” (between S and X). Radars that operated at X band were often used for fire control, so “X” is said to be a reference to a crosshair.

Unfortunately, these historical explanations do not seem to be documented anywhere. According to some sources, the US government never declassified the documents relating to the frequency designations.

What is documented is the fact that the International Telecommunications Union adopted official band designations in 1959. However, these designations only covered frequencies up to 40 GHz.

In 1984, the IEEE adopted official radar frequency letter designations for frequencies from 3 MHz to 300 GHz. These designations were updated in 2019.

The table below shows the NRAO frequency band designations, which mostly (but not quite always) follow the IEEE standard. The NRAO P and Q bands are not official IEEE bands; P spans the upper end of the VHF band and lower end of the UHF band, and Q covers a portion of the IEEE V band. The “Associated Wavelength” for each band often depends on who you are talking to, the specific telescope they are talking about, and what their scientific interests are.

NOTE: The VLBA receivers often do not cover the entire frequency range of a given band.  See the Frequency Bands & Performance section of the OSS for details on the VLBA receivers.