Straylight tests for the Heliospheric imagers of STEREO

The design of the Heliospheric Imager (HI) of the NASA Solar TErrestrial Relations Observatory (STEREO) is based on an optical baffle system. It will reject the solar disk light with attenuation of the order of 10-13 and 10-15, and let two separate camera systems (HI-1 and HI-2) measure the extremely faint solar coronal mass ejections. A multi-vane diffractive system has been 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). The theoretical performances of the baffling systems will be experimentally verified during the instrument development phase. A specific straylight test facility is being studied at CSL and preliminary tests have been conducted to prepare the HI instrument testing. The test set-up requirements and design considerations are discussed in this paper. The very high rejection performance requires to perform those tests under vacuum to avoid ambient air perturbations. Several light trapping systems have been developed for this application. A first breadboard has been built and is currently under testing. Preliminary results of those tests are presented