UVCI for HERSCHEL: instrument description and activity status report

The Ultraviolet and Visible light Coronagraphic Imager (UVCI) is an imaging coronagraph that will take pictures of the solar corona from 1.4 up to 3.5 solar radii at three different wavelengths, HI Ly-alpha 121.6 nm, HeII Ly-alpha 30.4 nm, and broadband visible polarized light. It is part of the HERSCHEL experiment (HElium Resonant Scattering in the Corona and HELiosphere) and it will fly on a sounding rocket. The instrument optical design consists of two twin off-axis Gregorian externally occulted telescopes, with multilayer-coated optics optimised respectively for the HI and HeII lines. We describe the instrument structure design and the associated optical tolerances analysis. The structure is conceived to attain high stiffness with the lowest pos-sible weight. Tolerances on the positioning of the optical elements, for alignment purpose, have been evaluated through a geometrical approach and ray-tracing method.