Polarimetric Calibration of Large-Aperture Telescopes II: The   sub-aperture method
  A new method for absolute polarimetric calibration of large telescopes is presented. The proposed method is highly accurate and is based on the calibration of a small sub-aperture, which is then extended to the full system by means of actual observations of an astronomical source. The calibration procedure is described in detail along with numerical simulations that explore its robustness and accuracy. The advantages and disadvantages of this technique with respect to other possible alternatives are discussed. 
H. Socas-Navarro