Polarization Mixing Correction (Old)

Fig. 1: Parallactic angle for a range of hour angle and declination. There is a large deviation for sources whose Dec = latitude (37 degrees), when they pass directly overhead.

Explanation of Polarization Mixing

The newer 2.1-m antennas [Ants 1-8 and 12] have AzEl (azimuth-elevation) mounts (also referred to as AltAz; the terms Altitude and Elevation are used synonymously), which means that their crossed linear feeds have a constant angle relative to the horizon (the axis of rotation being at the zenith). The older 2.1-m antennas [Ants 9-11 and 13], and the 27-m antenna [Ant 14], have Equatorial mounts, which means that their crossed linear feeds have a constant angle with respect to the celestial equator, the axis of rotation being at the north celestial pole. Thus, the celestial coordinate system is tilted by the local co-latitude (complement of the latitude). This tilt results in a relative feed rotation between the 27-m antenna and the AzEl mounts, but not between the 27-m and the older equatorial mounts. This angle is called the "parallactic angle," and is given by:

$\chi =\arctan(\cos \lambda \sin A,\sin \lambda \cos E-\cos \lambda \sin E\cos A)$,

where $\lambda$ is the site latitude, $A$ is the Azimuth angle [0 north], and $E$ is the Elevation angle [0 on horizon]. This function obviously changes with position on the sky, and as we follow a celestial source (e.g. the Sun) across the sky this rotation angle is continuously changing in a surprisingly complex manner. Note that $\chi =0$ at zero hour angle for declinations less than the local latitude (37.233 degrees at OVRO), but is $\pm \pi$ at higher declinations.

Fig. 2: Illustration of 2.1-m feed package, and rotation of feed orientation by parallactic angle $\chi$. Note that the feeds are oriented at 45-degrees from the horizontal at 0 hour angle, with X (= H) shown in yellow, and Y (=V) shown in blue. The 27-m horn feeds

The crossed linear dipole feeds on all antennas