Mem. S.A.It. Vol. 74, 528 c  SAIt 2003 Memorie della L. Zangrilli, S. Fineschi, D. Loreggia, D. Gardiol, E. Antonucci, and A. Cora INAF, Osservatorio Astronomico di Torino, Pino Torinese, Italy Abstract. We describe the project of an imaging Stokes polarimeter based on liquid crystals, for the study of the K-solar corona. Liquid crystals retarders are electrically variable waveplates. The change of the retardance, induced by a vari- able applied voltage, is a function of the wavelength. As observations of the visible coronal continuum are usually made over the band 450-600 nm, we are interested in studying the properties of these retarders as a function of the wavelength. This polarimeter is thought to be implemented on ground-based and space-borne coronagraphs. Key words. instrumentation: polarimeters – Sun: corona 1. Introduction The visible light continuum coronal emis- sion arises from Thomson scattering of the solar disc radiation by free electrons in the coronal medium (K-corona), and from the radiation scattered by coronal dust parti- cles (F-corona). Measurements of the K- corona intensity provide information on the coronal medium electron density. The K- corona emission turns out to be linearly po- larized, as light scattered by coronal elec- trons is incident from a particular direc- tion: unpolarized light is emitted isotrop- ically from the solar disc and electrons scatter it; for a sufficient distant position of the scattering electrons the Sun can be considered as a point source and an Send offprint requests to : L. Zangrilli Correspondence to : INAF, Osservatorio Astro- nomico di Torino, Strada dell’Osservatorio 20, I-10025 Pino Torinese, Italy observer will see radiation with the elec- tric field vibrations in a plane parallel to the solar limb. In the following we de- scribe the project for an imaging Stokes polarimeter based on liquid crystals, for the study of the K-solar corona, to be as- sembled in the Optics Laboratory of the Astronomical Observatory of Torino. This instrument is thought to be implemented on ground-based telescopes for solar eclipse observations, and on space-borne coron- agraphs, such as the HERSCHEL/UVCI (He Resonant Scattering in Corona and Heliosphere / UV and Visible-light Coro- nagraphic Imager) sounding rocket project. 2. Polarimeter concept In most astronomical applications the de- gree of polarization, represented by the nor- malized Stokes parameters Q/I , U/I (lin- ear polarization), or V/I (circular polariza-