A polarimetric unit for HARPS-North at the Telescopio Nazionale Galileo: HANPO

Usually observational astronomy is based on direction and intensity of radiation considered as a function of wavelength and time. Despite the polarisation degree of radiation provides information about asymmetry, anisotropy and magnetic fields within the radiative source or in the medium along the line of sight, it is commonly ignored. Because of the importance of high resolution spectropolarimetry to study a large series of phenomena related to the interaction of radiation with matter, as in stellar atmospheres or more generally stellar envelopes, we designed and built a dual beam polarimeter for HARPS-N that is in operation at the Telescopio Nazionale Galileo. Since the polarisation degree is measured from the combination of a series of measurements and accuracy is limited by the instrumental stability, just the great stability (0.6 m/s) and spectral resolution (R=115000) of the HARPS-N spectrograph should result in an accuracy in the measurements of Stokes parameters as small as 0.01%. Here we report on the design, realization, assembling, aligning and testing of the polarimetric unit whose first light is planned in August 2014.