Design of a circular slit spectrometer

Despite images of the solar corona in selected bands of the electromagnetic spectrum have been regularly acquired for years, several key questions on its physics and chemistry are unanswered. In order to properly undertake a comprehensive physical study of the solar corona, it is mandatory to investigate its emission lines. Several spectrometer have been investigating the solar corona over the solar disk. The only UV coronagraph/spectrometer flown so far, that has continuously observed the extended solar corona out of the solar disk (from 1.5 R☉ to 5 R☉), is UVCS (UltraViolet Coronagraph Spectrometer), operative from 1995 to 2009 aboard the SOHO (Solar and Heliospheric Observatory) mission. It was characterized by a 40 arcmin linear slit tangent to the solar limb, moved at different heliocentric heights by means of a tiltable mirror. Different polar angles were explored by rolling the whole instrument. Despite the milestone results it successfully provided, UVCS was not suited to follow the solar corona dynamics: coronal phenomena may evolve on times that range from minutes to hours, while a complete coronal spectrocopic map at full resolution took about 1 day for UVCS to be acquired. A circular slit with variable radius coupled with a grating having circular concentric grooves would reduce by at least 1 order of magnitude the time range needed for mapping the whole solar corona spectrum and would therefore be ideal for performing spectroscopical studies of the UV/EUV dynamics of the solar corona. CISS (CIrcular Slit Spectrometer), funded by the Italian National Institute for Astrophysics (INAF), is a project dedicated to the development of a prototype of a spectrometer with two innovative key features: a circular slit with variable radius and a grating with concentric circular grooves. A certain slit radius identifies a fixed heliocentric height: a radial spectrum of the whole corona at that height is then imaged onto the detector. By changing the slit radius, different heliocentric heights can be explored. This work presents the prototype design and the status of the project.