Design Of The Heliospheric Imager For The Stereo Mission

The Heliospheric Imager (HI) is part of the SECCHI suite of instruments on-board the two STEREO spacecrafts to be launched in 2005. The two I-H instruments will provide stereographic image pairs of solar coronal plasma and address the observational problem of very faint coronal mass ejections (CME) over a wide field of view (similar to90degrees) ranging from 13 to 330 R-0. The key element of the instrument design is to reject the solar disk light, with straylight attenuation of the order of 10(-13) to 10(-15) in the camera systems. This attenuation is accomplished by a specific design of straylight baffling system, and two separate observing cameras with complimentary FOV's cover the wide field of view. A multi-vane diffractive system has been theoretically optimized to achieve the lower requirement (10-13 for HI-1) and is combined with a secondary baffling system to reach the 10-15 rejection performance in the second camera system (HI-2).This paper presents the design concept of the HI optics and baffles, and the preparation of verification tests that will demonstrate the instrument straylight performances. The baffle design has been optimized according to accommodation constrains on the spacecraft, and the optics were studied to provide adequate light gathering power and image quality. Straylight has been studied in the complete configuration, including the lens barrels and the focal plane assemblies. A specific testing facility is currently being studied to characterise the effective straylight rejection of the HI baffling. An overview of the developments for those tests is presented.