VLA/VLBA Polarization Calibration Page

Steve Myers & Greg Taylor

NRAO, Socorro

Last update: 2 Sep 2010

Table of Contents
Contact Information
Data By Year
Data for 1999
Data for 2000
Data for 2001
Data for 2002
Data for 2003
Data for 2004
Data for 2005
Data for 2006
Data for 2007
Data for 2008
Data for 2009
Other Resources
Calibration Guide
Tip Curves
Source Models
Obs Status Summary
  News and Status:
  • New (2010-9-2): After the VLA was switched off in January 2010, the regular POLCAL observing program was halted to focus on EVLA commissioning. Starting in May 2010, we resumed POLCAL observing in a limited fashion under the TPOL0003 project name. You can find data from this project using the standard Data Archive tool, e.g. https://archive.nrao.edu/archive/archiveproject.jsp .

    Note that at first these will only be run occasionally, on a monthly schedule at best. If you have VLBA observations that need a particular POLCAL observation, please contact us at the NRAO Helpdesk https://help.nrao.edu to see if we can schedule an appropriate TPOL0003 run.

    WARNING: We will not be automatically processing and posting the POLCAL results (as we used to with the VLA) for the foreseeable future. You will need to download and process the TPOL0003 EVLA data for yourselves. We apologize for this inconvenience but all our resources are focused on EVLA commissioning at this time.

    Note that we have two "standard" TPOL0003 schedules that we would like to run regularly:

    using J1331+3030 as primary
    using J0137+3309
    If at all possible, use these calibrators so we do not have to modify the schedules!

  • (2010-1-13) The VLA has been turned off and we are discontinuing regular POLCAL observations and processing. These will start to resume as EVLA commissioning progresses.

  • (2009-8-1) As we transition from the VLA to EVLA systems, the auto-processed POLCAL results are becoming unstable. If values in the 2009 table look suspect, you should download the data from the archive and process it yourself rather than relying on our pipelined values.

  • (2008-8-27) The polarization calibration program is still being observed. There is currently only a fairly short backlog. If you want special sources observed, or have questions about this program, contact the analysts or smyers at nrao (e.g. if there are specific dates and sources you are interested in).

  • (2007-7-1) As of July 2007, the modcomp system at the VLA was turned off, and we no longer get automated processing of tip curves. In the interim, you will have to use the WX table based option (the default) in FILLM, or guesstimate the optical depth from plotting TANT vs. Elevation from the TY table.

  • (2007-7-1) There is now a mix of EVLA and VLA antennas, which may calibrate differently. There is no sign of problems in the Polcal database, but be alert.

General note on data quality: This program is observed as a series of snapshots of the main calibrator plus various targets, and several scans of a leakage calibrator. The data is reduced semi-automatically using AIPS (procedures) with some scientist intervention. The goal of the program is to calibrate the polarization electric vector position angle (EVPA) for a network of sources suitable for VLBA observation (compact structure). Flux densities for these sources are produced and posted but THESE FLUX DENSITIES ARE NOT SUFFICIENTLY ACCURATE TO BE USED AS PRIMARY FLUX DENSITY SCALE CALIBRATORS. The fluxes are provided as a check on data quality (errant fluxes may indicate problems due to weather for that observation or other problems) and as a guide to how strong that calibrator is. Note also you should be careful in just picking one observation to use, be sure to compare to nearby observations to make sure that this day is not errant.

The EVPA and flux densities are reference to the primary VLA calibrators 3C48 (0137+331) and 3C286 (1331+305). There is an entry in the HTML tables for which one was used on a given day.

If you want to see what data is available from this program, you can use the new NRAO Data Archive Tool typing in POLCA as the project name [ query] for data since Sept 2004 (when the program became POLCA) or TESTT [query] for data from Sept 1999 to Sep 2004 (look for the CXKQ data as there is some ambiguity with other programs with similar name in the archive). You can download the data using this archive tool and analyze it yourself if desired. Note that the POLCA data is immediately publically accessible.

- Steve Myers

  Older News:
  • On 2005-06-09 I discovered a bug in our AIPS procedures that affects Q-band A and B configuration data calibrated off of 3C48. The effect of this error is that the Q-band polarization angles (last column of the tables) can be significantly in error, effectively shifted against 3C48. This error can be as high as 30deg (smaller in B-config, very small in C and D so far). I have fixed the data from 2004-06-27 onward and I am working backwards to determine the extent of the error. Note that in general, my guess is that the Q-band polarization angles in A and B config are only good to about 10 degrees or so due to resolving the primary calibrators. This is under investigation, possibly a better procedure can be devised. If you have any questions or concerns about specific data, contact me.

  • As of July 2004, I Stokes source models for primary calibrators 3C48, 3C138, 3C147 and 3C286 are available with AIPS. These can be found in your AIPS distribution in the TEXT/STARS directory and can be loaded into your catalog within AIPS using the CALRD task. See the 30Jun04 AIPSLetter for details. These models are also available from this site in the Images/Current subdirectory. (2005-5-26)

  • These pages are being overhauled, especially this page, the Gains pages and the Images pages. Please bear with us while we chase broken links. (2003-12-16)

  • The K-band subreflector rotations have been reoptimized as of Mon 13 Jan 2003, so current posted gain curves are invalid. See note on the Gain Curve Page.

  • A list of VLA memoranda dealing with polarization issues is available here. This is somewhat out of date and unfortunately most memoranda are not available on the web, only in hardcopy form at NRAO libraries. (2002-9-18)

  • See previous news items for older items.


See VLBA Scientific Memo 26 (PS 115KB) for more on this Polarization Angle Monitoring Program.

Starting in September of 1999 we began a VLA polarization monitoring program for a number of compact calibrators, widely distributed in Right Ascension, primarily to enable better calibration of the VLBA. Each source is observed about once or twice a month and reduced semi-automatically. Flux densities and polarization angles are tied to observations of 3C48, 3C138, or 3C286. The calibrators in this dataset should be useful as EVPA (polarization E-vector angle) calibrators for VLA and VLBA polarization observations. Note that the angles given in this table are R-L Phase differences as CLCOR desires, not the EVPA itself. The "Flux" column is derived from GETJY after using SETJY versus 0137+331 (3C48) or 1331+305 (3C286) to establish the scale, and thus can be used in SETJY. Note that the angles derived for 0521+166 (3C138) in B-configuration agree with those given in the polarization calibration guide, and the flux density seems stable.

Be sure to read the table notes for cautionary information. Currently only C (5 GHz), X (8.5 GHz), K (22 GHz), and Q (43 GHz) bands are being monitored. At K and Q bands, there are significant excursions in the flux density calibration due to problems with pointing, gain curves, or poor weather for a given session. We are looking into improving the reliability of these measurements, so bear with us.

In addition to the selected sources, other possible calibrators were observed during the course of the program. The entire database can be found in the Master Calibration Database.

So far, we have identified five stable polarization angle calibrators (up to Q-band) other than 0137+331 (3C48) , 0521+166 (3C138) and 1331+305 (3C286) : 0423-013 , 0854+201 , 1310+323 , 1751+096 , and 2136+006 . Some of these are relatively flux-stable also. In addition, the source 2355+498 , although essentially unpolarized, has proved to have a stable flux density and thus could serve as a secondary flux calibrator. Surprisingly, the very bright sources 3C273 (1229+020) and 3C279 (1256-057) seem to have relatively stable EVPA at high frequency despite being highly variable in flux density! CAUTION: There are significant systematic deviations in the derived flux densities for some of these sources, and use of these for flux calibration could be done with extreme care. Check with us if you plan to do this and are concerned.

Some further info:

The most recent versions of AIPS runfiles are available for B-config (May 2005), with info in these files to change for different configs and calibrators. These are: CALBC, CALBX, CALBK, CALBQ for each of CXKQ bands.

In the AIPS versions 31DEC01 and later, FILLM will by default apply gain curves and approximate opacity corrections, and thus you should not use manual gain curve corrections unless you defeat these (check BPARMS). You can check your CL 1 table to see if you have done this. If so, and you want to change this, you can delete CL 1 and remake it with INDXR to start over with either unity gains [BPARM(2)=-1] or default gains [BPARM(2)=0]. The default gain curves can be found in your AIPS installation in the TEXT/IONS/VLA.GAINS file. I recommend using these, so by default set BPARM=-1,0 in FILLM, where BPARM(1)=-1 defeats the WX-based opacity correction (see below).

If you want to use TIP curve derived opacities in CLCOR, set BPARM(1)=-1 to turn off the automatic application of WX-based opacities when using the 31DEC01 or later AIPS. Because the WX opacities are not very accurate, and are time variable over the run, I do not recommend using these at this time if better TIP data is available. I will investigate this further.

If you have scheduled tipping scans in order to determine atmospheric optical depths, you can find the results on Bryan Butler's Tip Scan Archive page.

Steve Myers & Greg Taylor


  Contact Info:

Questions about this page should be directed to either analysts or smyers at nrao.edu.

Modified on Thursday, 02-Sep-2010 17:00:35 MDT