CASA VLBA Calibration and Imaging

CASA, the Common Astronomy Software Applications, is the primary data calibration and imaging package for both the Karl G. Jansky Very Large Array (VLA) and the Atacama Large Millimeter/submillimeter Array (ALMA).

CASA and the VLBA

CASA can be used for calibrating and imaging some VLBA data sets.  However, active development is still underway for some of the basic functions many VLBA users will need in order to properly calibrate their data.

When To Use AIPS

NRAO continues to recommend using AIPS for all VLBA calibration.  In particular, AIPS should be used for the following types of VLBA observations:

  • Polarimetric: Calibration of resolved polarized sources is not yet available in CASA.
  • Astrometric: Pulse-cal tone corrections are not yet available in CASA.
  • Spectral line projects requiring fringe-rate mapping: Fringe-rate mapping is not yet available in CASA.
  • Low Frequency (<4 GHz): Total electron content (TEC) corrections have not yet been verified to work correctly for VLBI observations in CASA.  Also, note that many C-band (4 to 8 GHz) VLBA observations benefit from TEC corrections, especially when the Sun is active.
  • VLBA+Y27 or any other VLBA+ arrays that require the use of antab files: CASA does not support the use of antab files to ingest calibration data (system temperatures, gain curves, etc.).
    • NOTE: There is a work-around for using antab files in CASA thanks to the JIVE CASA VLBI Tools.  However, the tools need to be used carefully to ensure that the data are not corrupted.  See VLBA Scientific Memo #39 for more details.
  • Any VLBA data correlated prior to December 2009 with the Hanning taper applied:  CASA version 6.6.4 and later can load VLBA data correlated with either the legacy hardware correlator or the DiFX correlator.  However, importing pre-DiFX data that used the Hanning taper is not yet supported.  There are also known problems with the oldest VLBA data.  SEVERE warnings will appear when CASA encounters known problems, but not every case has been fully tested.  Users wishing to calibrate pre-DiFX data with CASA should exercise extreme caution.

Examples of When CASA Can Be Used

The following are a few examples of when CASA can be used with VLBA data:

  • A VLBA or VLBA+Y1 observation at 8 GHz or higher of a bright source that can be fringe fit directly (no phase referencing needed) where absolute position is not a concern.
  • A basic phase-referenced VLBA or VLBA+Y1 observation at 8 GHz or higher, continuum or spectral line.
  • A VLBA observation that has been calibrated in AIPS, but users want to do imaging in CASA.

 

Note for Mac Users Regarding CASA Viewer

The CASA Viewer will no longer launch on Macs running OS 13 or OS 14 for CASA 6.5.6 to 6.6.4.  In CASA 6.6.5, the Viewer is no longer included for Mac versions.  As a result, Mac users are no longer able to use tclean in the interactive mode.  For more information on the CASA Viewer, see the Viewer on Mac documentation.  Mac users who need to run tclean interactively can request access to the NRAO Linux-based computing resources via the helpdesk.

 

CASA Resources

The CASA Homepage has information about the most recent CASA release.

The CASA Download page has links for obtaining CASA, as well as information on how to install the software.

The CASA Documentation page has links to several pages, including known issues, memos, and tutorials.  The most up-to-date documentation on CASA tasks and tools is available from the CASA Read the Docs page.

 

CASA VLBA Documentation

A complete tutorial, including a data set, for calibrating and imaging a phase-referenced 8 GHz VLBA observation of an AGN: VLBA Basic Phase-referencing CASA Guide

A guide to calibrating basic VLBA observations with CASA: VLBA Scientific Memo #38

A guide to calibrating VLBA+Y1 observations with AIPS or CASA: VLBA Scientific Memo #39

 

Some Useful Publications

CASA, the Common Astronomy Software Applications for Radio Astronomy: The CASA Team 2022

Development of VLBI Processing Capabilities For CASA: van Bemmel et al. 2022

Imaging ICRF Sources with CASA: Hunt et al. 2021

rPICARD: A CASA-based calibration pipeline for VLBI data: Janssen et al. 2019

EVN CASA tutorials (and VLBA AIPS tutorials): VLBI CASA Guides

 

Getting Assistance

Users who decide to calibrate their data with CASA and encounter problems can contact VLBA staff via the NRAO helpdesk.  When submitting a helpdesk ticket, users should select "VLBA" in the Topics section and select "CASA Data Reduction" from the Department drop-down menu.  It is also very helpful if users make it clear they are dealing with VLBA data in the Subject line of the ticket, so it can be assigned to the appropriate staff members as quickly as possible.  For example, instead of entering "Missing Tsys", users could enter something like "VLBA - Missing Tsys".

Please note that users who are attempting to use CASA for a type of observation listed under the "When To Use AIPS" section above may be told they need to use AIPS instead.  In this case, users are reminded that VLBA and AIPS staff are also available to assist users with installing and running AIPS.

 

Providing Feedback

Users who have feedback regarding CASA for VLBA calibration and imaging are encouraged to send an email to casa-feedback_at_nrao.edu.  These emails will be handled by the CASA development team.  Please note that this email is not a helpdesk system and will not generate helpdesk tickets.  Users who encounter bugs or other problems should report them via the NRAO helpdesk.

 

Recent Developments

Capabilities recently added to CASA:

 

Areas of Active Development

The following tools are currently being developed by JIVE and NRAO CASA software engineers and scientists:

  • Total Electron Content (TEC) corrections
  • Support for antab files
  • Support for VLBA phase-cal tone corrections

 

Acknowledgments

NRAO thanks the JIVE CASA developers for their continuing dedication to providing the tools necessary for VLBI calibration and imaging in CASA.

The CASA development team is a diverse group based at the NRAO, the European Southern Observatory (ESO), and the National Astronomical Observatory of Japan (NAOJ).

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