HECOR: a HElium CORonagraphy aboard the Herschel sounding rocket

Abstract

HECOR (HElium CORonagraph) is a coronagraph designed to observe the solar corona at 30.4 nm between 1.2 and 4 solar radii. The instrument is part of the Herschel sounding rocket payload to be flown from White Sands Missile Range in December 2007. Much like for neutral hydrogen, the residual singly ionized helium present in the corona can be detected because it resonantly scatters the intense underlying chromospheric radiation. Combined with the simultaneous measurements of the neutral hydrogen corona made by SCORE, the other coronagraph of the Herschel payload, the HECOR observations will provide novel diagnostics of the solar wind outflow. HECOR is an externally occulted coronagraph of very simple design. It uses a triple-disc external occulting system, a single off axis multilayer coated mirror and a CCD camera. We present measurements of the EUV mirror roughness and reflectivity, tests of the image quality, and measurements of the stray light rejection performance. The mirror uses a novel multilayer design with three components that give HECOR a high throughput.