I is the total intensity of the image and Q and U are the two linear parameters. V is the fourth parameter, but refers to circular polarization, which we do not examine. It makes sense that the best value of intensity would be the average of the two estimates of intensity. It is important to remember that IDL works in radians, so the resultant angle is in radians. In the 50's, the Crab Nebula was determined to contain a pulsar emitting synchrotron radiation.
Synchrotron radiation is emitted from electrons gyrating in a magnetic field.
The electrons are moving in a circle, always being accelerated inwards. The polarized radiation is therefore perpendicular to the magnetic field. The electrons are also moving at relatavistic speeds, so the radiation that was initially emitted through all directions in the frame of the electron is actually thrown into a small cone which shifts the directions of polarization away from the perpendicular to the magnetic field. Therefore, synchrotron radiation cannot be entirely polarized. The four polarized images after subtraction and alignment can be seen here .