Astrophysical spectro-polarimetry: New developments

   Jan Stenflo 
   Institute of Astronomy 
   ETH Zurich


   With a new class of imaging polarimeters it is now possible to 
   completely eliminate the previous main limiting factors of 
   seeing and gain-table noise in the polarization images. This 
   has allowed us to record solar spectra with high spectral 
   resolution and a noise in the fractional polarization as low 
   as 5 x 10^{-6}. At such levels of sensitivity the whole solar 
   spectrum is polarized, also in the absence of magnetic fields. The 
   polarized spectrum has a surprising structural richness displaying 
   a multitude of new effects, like quantum interferences and optical 
   pumping, which have been unfamiliar in an astrophysical context. 
   The high sensitivity has also opened the door to a new class of 
   magnetic-field diagnostic techniques based on the Hanle effect, 
   which represents the influence of magnetic fields on the 
   polarization that is produced by coherent scattering processes. 
   While we are exploring this new world of polarization physics 
   for the Sun we are in the process of developing and implementing 
   a Stokes imaging polarimetry system based on this technology but to 
   be used for night-time astronomy. Besides presenting an overview 
   of the solar applications we report on first exploratory results 
   with night-time imaging polarimetry and discuss its future 
   possibilities and limitations