Calibration Considerations

A Note on Terminology: The VLBA Observational Status Summary (OSS) now uses the term “data channel” to describe a subband with a single polarization. However, the Proposal Submission Tool (PST) still uses the term "baseband channels" and SCHED still uses the term “baseband converter” or “BBC”. To be consistent with the VLBA OSS, this document will use the newer terminology of “data channel”. Please keep in mind that “data channel” and “spectral channel” are NOT the same; each data channel is further divided into spectral channels.

Flux Density Scale Calibration

Because the VLBA uses system temperatures and gain curves to determine the flux densities of targets, users are not required to schedule scans on flux density calibration sources.

Observations with the High Sensitivity Array (HSA; see Chapter 2.2) using the phased VLA should schedule time on a VLA absolute flux density scale calibrator (3C286, 3C138, or 3C147), although it is not required.  Without a scan on one of these calibrators, user's will need to rely on the VLA's switched power for flux density scale calibrations, which is only good to about 10%.  As additional benefit, the VLA flux density of all the sources can be independently determined and the observer can determine how much (if any) flux is resolved out on the long baselines.

For more details on VLBA flux density scale calibration, see the Amplitude Calibration section of the VLBA OSS.

Fringe Finding

In order to properly calibrate the phases and delays in your observation, you will need at least one scan on a bright (and preferably compact) target. In general, it is recommended that this scan last about 2 minutes.  Users should chose a source that has strong signal on all baselines at their observing frequency.  The NRAO maintains a list of recommended fringe finders. Considering the importance of the fringe finder scans to successful calibration, it is strongly recommended that user schedule these scans near the middle and/or end of the observation. Note that fringe finder sources can often also be used for bandpass calibration (see below).

If an observation lasts longer than about 4 hours, it is recommended to observe a fringe finder at least twice (if the fringe finder is also the bandpass calibrator, the same fringe finder should be observed both times). 

Bandpass Calibration

In order to properly calibrate the bandpasses for each data channel, you will need at least one scan on a bright target. This can often be the same target as the fringe finder.

Self Calibration

Bright VLBA targets can be self-calibrated (models generated after initial calibration can be used in an iterative process to improve the calibration). If you know that your science target(s) will be bright enough to self calibrate, make a note of this in the Technical Justification. Note that self calibration does not preserve the location data of a target, so it is not recommended for astrometric projects.

Phase Referencing

If your science targets are not bright enough for self calibration or you need to preserve the location information of the targets, it is likely that you will need to transfer the phase solutions from a calibration source to your science targets. This is known as “phase referencing”.  Note that phase referencing requires a substantial amount of additional observing time because of the large number of scans on the phase reference calibrator(s). Users who require phase referencing should consult Wrobel et al. (2000) and Reid & Honma (2014) for more details on the strategies involved.

Choosing a phase reference calibrator involves 2 steps.

First, determine the baseline sensitivity for your observing frequency. The OSS has a table of the VLBA baseline sensitivities for each frequency band. However, note that the table assumes you will be using the maximum possible data rate for the observing frequency and that you will be combining all of the data channels together for the calibration. If you want to calibrate each data channel separately, you will need to use the EVN Sensitivity Calculator to determine the baseline sensitivity in a single data channel (see details at the end of this section).

Second, look for calibrator sources near your science target.  There are multiple search tools that will return lists of nearby candidate phase reference calibrators based on the coordinates of a science target.  NRAO recommends the new VLBA Calibrator Search Tool or the NASA VLBI Calibrator Search tool.

Things to consider:

1. Distance from the science target: The closer the calibrator is to the science target, the better. The uncertainty in the position of the science target increases with the separation between the calibrator and the science target. Also, closer calibrators require less slew time, which can reduce the total time of the observation and/or allow for faster “nodding” between the calibrator and science target (very important for higher frequency observations). For calibration purposes, NRAO recommends that observers use a phase reference calibrator within about 5.7 degrees of the science target to ensure that atmospheric effects can be properly calibrated out. At frequencies below about 5 GHz, ionospheric effects become more dominant and it is recommended that users pick a phase reference calibrator within about 4 degrees of the science target. At frequencies below 2 GHz, NRAO recommends using a calibrator that is within the primary beam while observing the science target, if at all possible (see Chatterjee 1999).
2. Flux density: In order to successfully fringe fit, the phase reference calibrator must have a signal-to-noise ratio (SNR) of at least 7 at your observing frequency. It is ideal to have an SNR of 7 on all baselines as this allows each baseline to be calibrated independently. However, as long as the SNR is at least 7 for all baselines from a reference antenna (usually a station near the middle of the array such as FD, PT, or LA), solutions will be found for the longer baselines. A higher SNR is better, so if there are multiple calibrators at roughly the same distance from your science target, using the brightest one would usually be better.
3. Absolute position uncertainty: In general, you want the lowest positional error possible. Reid & Honma (2014) state that an accuracy of <10 mas is necessary, but <1 mas is preferred.

You will need to balance all three considerations and pick a phase reference calibrator that is most appropriate for your science goals. It is almost always best to pick the calibrator closest to your science target. It may be tempting to use a very bright calibrator that is 1 degree further away than a weaker one, but that increases the risk of decorrelating the signal (phase solutions derived from the calibrator may not be the appropriate solutions to apply to the science target because the atmospheric/ionospheric effects are different for the two sources). If there are few bright calibrators near you science target, it would be better to propose for a higher data rate rather than risk atmospheric or ionospheric effects ruining the observation.

A good rule-of-thumb for basic continuum observations at mid-frequencies (1-10 GHz) with 32 MHz of bandwidth per data channel is to pick the calibrator nearest your science target with a flux density of at least 50 mJy and positional uncertainty <1 mas. However, it may be that the closest source to your target with low positional uncertainty is only 20 mJy. As long as your baseline sensitivity is ~2.8 mJy or lower (which may require that you combine at least some of your data channels during calibration, especially for the longer baselines), that source should work as a phase reference calibrator.

If you have any questions about selecting a phase reference calibrator, please contact the NRAO helpdesk and a member of the VLBA staff will assist you.

REMINDER: Users can contact the NRAO helpdesk to arrange for short observations of candidate calibrators at any time.  Additionally, proposers can (and probably should) request extra observing time to identify the best phase reference calibrator(s) for their science target(s). Checking candidate calibrators is especially important for observations at higher frequencies (>10 GHz) where not many calibrators are well-characterized. Keep in mind that compact sources are inherently variable at all frequencies. Even if you have selected what you believe to be a good calibrator from the search tools, it is a good idea to obtain short observations of a few candidates to make absolutely certain that you have selected the best possible phase reference calibrator for your project. The request for extra time to find calibrators is made in the Technical Justification section of the proposal.

Using the EVN Sensitivity Calculator to estimate the baseline sensitivity in each data channel:

1. Select only 2 of the standard VLBA stations (even for HSA projects). For example, Hn & Nl.
2. Select your observing band and set the data rate to your total data rate divided by the number of data channels.
   Example 1: For a total data rate of 4096 Mbps, there will be either 8 data channels, each with 128 MHz of bandwidth. Therefore, the data rate in for each data channel is 4096 Mbps/8 = 512 Mbps.
   Example 2: When using the polyphase filterbank, the total data rate is 2048 Mbps with 16 data channels each with 32 MHz of bandwidth. Therefore, data rate for each data channel is 2048 Mbps/16 = 128 Mbps.
3. Set the integration time to the calibrator scan duration. This should be no longer than the fringe-fit interval: 1 minute for 300-700 MHz, 2 minutes for 1-9 GHz, 1 minute for 12-15.4 GHz, 30 seconds for 21.7-24.1 GHz, 20 seconds for 40-45 GHz, 10 seconds for 80-90 GHz. (Scan durations in the range of 30 to 90 seconds are commonly used at frequencies between 1 and 9 GHz.)
4. Click “GO” and the Calculator will report the baseline sensitivity in the box under the integration time.