Ultraviolet and visible-light coronagraphic imager (UVCI) for HERSCHEL (Helium Resonance Scattering in Corona & HELiosphere)

The HERSCHEL (HElium Resonance Scattering in the Corona and HELiosphere) Sun-Earth Sub-Orbital Program is a proposed sounding-rocket payload designed to investigate helium coronal abundance and solar wind acceleration from a range of solar source structures by obtaining simultaneous observations of the electron, proton and helium solar coronae. HERSCHEL will provide the first measurements of the coronal helium abundance in source regions of the solar wind, thus bringing key elements to our understanding of the Sun-Earth connections. The HERSCHEL instrument package consists of the Extreme Ultraviolet Imaging Telescope (EIT) for on-disk coronal observations and the Ultraviolet and Visible-light Coronagraphic Imager (UVCI) for off-limb observations of the corona. The UVCI is an externally occulted, reflecting coronagraph with an off-axis Gregorian telescope. UVCI will be able to take coronal images at heliocentric heights comprised between 1.2 to 3.5 solar radii of a) K-corona polarized brightness (pB); b) H I Lyman-?, 121.6 nm, line-emission; c) He II Lyman-?, 30.4 nm, line. The key element in the UVCI instrument concept is that the mirrors with multilayer coatings optimized for 30.4 nm still have good reflectivity at 121.6 nm and in the visible. The optical design concept for the UVCI instrument will be discussed, together with its expected optical and throughput performances.