HARPS Polarimeter

The HARPS instrument at La Silla, Chile is famous for its planet detections through the radial velocity method. The ultrastable spectrograph system will be upgraded to a precise spectropolarimeter by replacing the obsolete Iodine cell in the Cassegrain adapter with a dual-beam polarimetry unit. This compact unit encompasses two separate modulators, one for circular polarimetry and one for linear polarimetry. Each modulator consists of a superachromatic wave-plate that doesn’t introduce (polarized) spectral fringes and a Foster prism that acts as a polarizing beam splitter. The two beams out of the Foster prism are imaged onto the two fibers of the HARPS fiber head. The new instrument mode in combination with the ultrastable spectrograph enables unprecedented observations of stellar magnetism, e.g. through Zeeman-Doppler imaging.