The HARPS polarimeter

The very successful HARPS spectrograph, attached to ESO’s 3.6-metre telescope at La Silla, Chile, has been enhanced with a new capability: the most precise polarimeter in the world.  Polarisation describes the preferential plane in which light vibrates upon propagation and is measured with a polarimeter. The first observations with the HARPS polarimeter show that this new instrument already exceeds expectations. It can detect the polarisation of light, at a level as small as 1 part in 100 000, without any disturbances from the Earth’s atmosphere and the instrument itself. Together with the unprecedented stability of the spectrograph, this renders the HARPS polarimeter the most precise of its kind. Furthermore, it is the only polarimeter of its kind in the southern hemisphere.