MMA Memo #116

The Effect of Beam Offsets on Polarization Measurements

A.R. Thompson [1]

May 17, 1994

Keywords:

In this memorandum an attempt is made to understand how the offsets of the beams with opposite circular polarization could affect the performance of the VLA. A number of the points mentioned result from discussions with B.G. Clark, P.J. Napier, E. Raimond, and L.R. D'Addario. At the time of the writing the cause fo the beam offsets is not definitely known, but the most probable one is surmissed to be the offset feed geometry, since a similar offset for prime-focus fed paraboloids has been described by Chu and Turrin (1973).

As a beginning it may be helpful to recall the response of an interferometer to the four Stokes' parameters with various arrangements of linearly and circularly polarized antennas. These are given in Table 1 and a derivation of the expression for the general case will be found in Morris, Radhakrishnan, and Seielstad (1994). The linear polarization parameters, Q and U, are usually 1-10% of I, and to measure them it is therefore advantageous to use crossed polarization arrangements that do not respond to I. The use of opposite circular feeds dates from Conway and Kronberg (1969) and is more convenient than the use of crossed linears. With the latter it is necessary to maintain accurate orthogonality between the feeds of different antennas and also to be able to rotate the feeds through 45 degree angles. The circular polarization parameter V is usually very small for observations beyond the solar system, and it is often assumed to be zero to simplify measurement of linear polarization. To separate V from I there is some advantage in using crossed linear feeds.

Last modified: 09 December, 1999