The CARMA 1.3 mm polarization system consists of dual-polarization receivers that are sensitive to right- ( R ) and left-circular ( L ) polarization , and a spectral-line correlator that measures all four cross polarizations ( RR , LL , LR , RL ) on each of the 105 baselines connecting the 15 telescopes .
Each receiver comprises a single feed horn , a waveguide circular polarizer , an orthomode transducer ( OMT ) , two heterodyne mixers , and two low-noise amplifiers ( LNAs ) , all mounted in a cryogenically cooled dewar .
Here we review the basics of polarization observations , describe the construction and performance of key receiver components ( circular polarizer , OMT , and mixers—but not the correlator ) , and discuss in detail the calibration of the system , particularly the calibration of the R – L phase offsets and the polarization leakage corrections .
The absolute accuracy of polarization position angle measurements was checked by mapping the radial polarization pattern across the disk of Mars .
Transferring the Mars calibration to the well known polarization calibrator 3C286 , we find a polarization position angle of \chi = 39.2 \pm 1 \degree for 3C286 at 225 GHz , consistent with other observations at millimeter wavelengths .
Finally , we consider what limitations in accuracy are expected due to the signal-to-noise ratio , dynamic range , and primary beam polarization .