Scheduling EVN polarization observations
There are two aspects to proper calibration of polarization-sensitive VLBI observations: (1) calibration and removal of the instrumental polarizations, or D-terms, for each antenna and (2) calibration of the absolute orientation of the polarization position angles (PPAs).
D-term calibration
Determination of the instrumental polarizations is relatively straightforward if observations of a source that is either unpolarized or is known to have a simple polarization structure are made. In case of a polarized source it is necessary to observe it over a wide range of parallactic angles. Usually, five or six scans of duration several minutes spread over a range of parallactic angles exceeding about 90 degrees should be adequate. In some cases, a program source may be suitable for use as a polarization calibrator, if it is known to have a simple polarization structure. If the program sources are expected to have relatively complex polarization structures, it is best to observe a different source specifically for determination of the D-terms.
In case of a strong unpolarized source a single observation scan of 8-10 minutes is sufficient.
Some good polarization calibrations at frequencies up to 15-GHz are 3C84 (lots of structure but unpolarized), OQ208 (also a fair bit of structure but unpolarized), and DA193 (weakly polarized with very compact structure). For suggestions about other sources that may be suitable for D-term calibration, contact Zsolt Paragi or someone else you know who has experience with polarization VLBI observations.
Position angle calibration
The most common way to calibrate the orientation of the polarization position angles is to use simultaneous or nearly simultaneously integrated and VLBI observations of a source with compact polarization (in which essentially all the integrated polarization is detected on short VLBI baselines). By comparing the orientation of the PPAs for the total VLBI-scale polarization with their known orientation in the integrated measurements, it is possible to determine the necessary rotation to calibrate the VLBI polarization angles, i.e., to give them their true observed values. For this purpose, it is necessary to have VLBI and integrated observations of a source with compact polarization. In some cases, program sources may be suitable for this. If you have doubts about whether your program sources have sufficiently compact polarization, it is better to observe another source specifically for this purpose - since a source used for PPA calibration should have a relatively simple structure, five or six several-minute scans spread out over the time it is visible by most of the antennas in the array should be sufficient. For advice about sources that should be good for PPA calibration, get in touch with Zsolt Paragi or someone else you know who has experience with polarization VLBI observations.
There are currently no known sources with constant polarization position angles on VLBI scales; thus, it is not feasible to simply observe such a source and rotate the VLBI-scale PPAs to agree with the known constant value. This is why it is necessary to also obtain integrated polarization measurements for the PPA calibration. If the phased VLA or phased WSRT is included as part of the VLB array, integrated measurements can be derived from the data for these instruments. Otherwise, other arrangements for the aquisition of integrated polarization measurements must be made. Some possible source for integrated measurements is the University of Michigan Radio Astronomy Observatory database, the VLA/VLBA Polarization Calibration Page, and the Master EVLA POLCAL Database. However, in this case, there is no guarantee that there will be observations of your PPA calibrator sources near your VLBI observations. It is important to try to have the integrated measurements as close to the VLBI observations as possible, since the polarization of compact sources can vary on timescales of days or weeks.
Note that it may be possible to use observations of a compact source with a simple polarization structure for both D-term calibration and PPA calibration, making it possible to spend less total time on the polarization calibration. Here, it is necessary to find a source that is both relatively highly polarized and has a simple polarization structure.
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