Wednesday, May 2, 2012

Crab nebula and polarized light

      Besides wavelength (color), photons have other properties too: if seen as a wave their electric field oscillates in a plane which can remain fixed or rotate in time and by convention we describe this polarization state by specifying the orientation of the electric field over one oscillation.

      Polarimetry is a very useful tool for determining what kind of physical processes are at work in the light source (especially for synchrotron radiation) and the distribution of magnetic fields in it: there is much more to light than just color!

     Polarimetry color information superimposed on a "normal" photograph of Crab nebula creates the unusual picture below. The red color in the nebula comes from an arbitrary initial polarisation angle (we weren't able to find out yet the true polarizing angle), the green shows 45 degrees more and the blue is with the filter rotated by 90 degrees compared to the original position.

The image has been made by me (color and luminance data) and David Muelheims (polarized data), in Bonn, Germany.

        And although "popular wisdom" might tell you that the human eye cannot distinguish between polarization states, it's not really true, as Haidinger found out many, many years ago :)


  1. I am interested in knowing if I may reach the same results with an amateur telescope, let's say of 200mm diameter cassegrain.
    Do I simply need a polarizing filter in front of the CCD sensor? If this is the case, which one would you suggest me?

    1. Yes, you may be able to achieve similar results with a smaller telescope. I used a 0.5m meter telescope, but I live in one of the most light polluted regions of Europe, in Bonn. Just place somehow the polarizing filter in front of the ccd sensor and then be careful to rotate it at 45 and 90 degree for subsequent exposures. Use a linearly polarizing filter. In a few nights of data gathering from a good site and a lot of post-processing, you might get even better results :) (our telescope also sits on top of a heated building so the seeing is terrible most of the times (~2.5-5")).