Difference between revisions of "Polarization Mixing Correction (Old)"

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(Explanation of Polarization Mixing)
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= Explanation of Polarization Mixing =
 
= Explanation of Polarization Mixing =
The newer 2.1-m antennas [Ants 1-8 and 12] have AzEl (azimuth-elevation) mounts, which means that the 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 the 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:
+
The newer 2.1-m antennas [Ants 1-8 and 12] have AzEl (azimuth-elevation) mounts (also referred to as AltAz, where 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:
  
 
<center><math>\chi = \arctan(\cos\lambda \sin A, \sin\lambda \cos E - \cos\lambda \sin E \cos A)</math>,</center>
 
<center><math>\chi = \arctan(\cos\lambda \sin A, \sin\lambda \cos E - \cos\lambda \sin E \cos A)</math>,</center>
  
where <math>\lambda</math> is the site latitude, <math>A</math> is the Azimuth angle [0 north], and <math>E</math> is the Elevation angle [0 on horizon].
+
where <math>\lambda</math> is the site latitude, <math>A</math> is the Azimuth angle [0 north], and <math>E</math> 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.
  
 
= Status of tests =
 
= Status of tests =

Revision as of 15:25, 20 October 2016

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, where 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:

,

where is the site latitude, is the Azimuth angle [0 north], and 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.

Status of tests