VLBA 7GHz Flux Density Scale

The NRAO recently discovered that gain values for the 6-8 GHz portion of the C-band were not being attached to FITS-IDI files.  

Previously, the gain at the upper end of the C-band receiver was very similar to the gain at the lower end of the receiver band. Hence, one set of gain values was sufficient for all frequencies in that receiver to obtain flux density scale calibration to within 10% uncertainty. However, recent measurements of the gains indicate that there is now a significant difference in gain between the lower and upper portions of the receiver band.

Compounding this issues, the 6-8 GHz gain at the Pie Town and North Liberty stations changed significantly starting on 2020-02-11.  This change was due to a focus problem related to new control hardware. This focus issue was corrected on 2023-04-05.

Using the incorrect gain values results in incorrect flux density values for observations using the upper end of the C-band receiver. The incorrect focus at PT and NL also results in a loss of sensitivity on all baselines to those stations. The sensitivity loss is unrecoverable, but the flux density scale can be corrected using the new gain values for the appropriate time interval and frequency tunings.

Investigations by NRAO staff show that, for 6-8 GHz observations, the combination of using the incorrect gain values and the poor focus at two stations leads to flux density values (as measured in the image plane) that are incorrect by factors of between 30% and 50%.  Using the correct gain values results in calibrated flux densities that are within 10% of the expected values, which is consistent with the stated uncertainty of the VLBA flux density calibration.

While the focus issue has been fixed, VLBA staff are continuing to work on getting the correct gain values attached to the FITS-IDI files.

The focus correction will also impact observations between 4 and 6 GHz, but on a much smaller scale and only for observations after 2023-04-05.

In order to provide users with the correct gain values, NRAO staff have created antab files with the appropriate gain values for C-band observations:

NOTE: The current gain values are preliminary.  Users should check back often for updated values.

The processes for correcting the gains using AIPS or CASA are described below.

Users are encouraged to contact VLBA staff via the NRAO helpdesk if they have any problems or concerns.

Correcting the Gain Values Using AIPS

To import the correct gain values in AIPS, use the ANTAB task.  Set the INNAME and INCLASS parameters to the file that needs to be updated (or use getn to get the appropriate file).  Set CALIN to the provided antab file, and set GCVER to the GC table to be updated (usually, GCVER 1).  Type "go antab".

Note that running ANTAB will not create a new GC table, but will update the current table with new values. Once ANTAB has finished, use the PRTAB task to inspect the GC table and ensure that the gain values were correctly updated. 

If an error occurs when running ANTAB and a message appears saying something like "KEYIN: ERROR, UNKNOWN PARAMETER MK", this means the observation did not include that station (in this example case, MK was missing). The antab file will need to be edited to remove the station that was not included (comment out those lines using "!").

Correcting the Gain Values Using CASA

The new gain values must be attached to the FITS-IDI file before creating the Measurement Set with importfitsidi. This can be done using the JIVE VLBI tool append_gc.py, which is available from the JIVE casa-vlbi GitHub repository. 

NOTE: Take extreme care when using append_gc.py!  It requires having astropy installed in the CASA environment. If astropy is not properly installed, running append_gc.py may permanently corrupt the FITS-IDI file. NRAO strongly recommends making a back-up copy of the original FITS-IDI file before attempting to attach the corrected gains.

Before running the append_gc.py script, it is necessary to define some paths to the directory containing the script:

export PYCAPATH=/full/path/to/script/directory/
export PYTHONPATH=/full/path/to/script/directory/:\$PYTHONPATH

Once the paths are set, run append_gc.py. 

When running the script on a personal computer, enter the command:

casa --nogui -c $PYCAPATH/append_gc.py --replace antab-filename FITS-IDI-filename

If the script will be run on an NRAO computer, it is best to use the VLA pipeline version of CASA, which includes astropy. Define the specific CASA version to use with the "-r" flag:

casa -r 6.4.1-12-pipeline-2022.2.0.64 --nogui -c $PYCAPATH/append_gc.py --replace antab-filename FITS-IDI-filename

There may be messages such as "Antenna MK not present in FITS-IDI file", but the script will simply skip antennas that did not observe. 

Once the gain values are attached, convert the FITS-IDI file to a Measurement Set with importfitsidi and create the gain calibration table with gencal. Check that the values in the gain calibration table are correct by either plotting them with plotms, or inspecting the table with browsetable.  If the gain values are correct, proceed with the calibration as normal.

For more information on calibrating VLBA data with CASA, see the VLBI CASA Guide Tutorials.

Self-calibration

Regardless of which software package is being used for calibration, one additional step will be useful for those whose data were affected by the focus issue at NL and PT. After the data have been calibrated using the correct gains, users should do an amplitude and phase self-calibration step to adjust for the reduced sensitivity at the antennas with bad focus positions. To do this, create a model (image) of a calibrator source excluding the problematic antennas. Compute the self-calibration solutions based on that model, then apply the solutions to all antennas.

4 to 6 GHz Observation

Observations at the lower end of the C-band receiver (4-6 GHz) during the early part of the 2020-02-11 to 2023-04-05 time period may also have incorrect gains for PT and NL due to the focus error. The antab file provided for 2020-02-11 to 2023-04-05 contains gain values for this frequency range.  In addition, observations in the 4--6 GHz range during this time period should perform self-calibration as described above. The impact on 4-6 GHz observations will be much smaller, typically less than about 10%.

Observations between 4 and 6 GHz after 2023-04-05 will have incorrect gains attached to the FITS-IDI file due to the impact of the focus change. The result of these incorrect gain values is anticipated to be relatively minor; the flux density in the image plane is expected to be incorrect by a factor of ~10% to ~20%.  Observers with C-band observations after 2023-04-05 should make use of the appropriate antab file provided above.

Users whose observations are centered near 6 GHz are encouraged to be very careful when choosing which gains to apply to their data.

VLBA staff are available via the NRAO helpdesk to help address any other issues, provide assistance with correcting data, and answer questions.