VLA/VLBA Polarization Calibration Page
Steve Myers & Greg Taylor
Last update: 2 Sep 2010
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.
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.
Questions about this page should be directed to either analysts or smyers at nrao.edu.