Calculations for, and laboratory measurements of a multistage labyrinthine baffle for “SMEI”

The spaceborne Solar Mass Ejection Imager (SMEI) is scheduled for launch into near-earth orbit (>800 km) in early 2003. Three SMEI CCD cameras on the zenith-oriented CORIOLIS spacecraft cover most of the sky each 100-minute orbit. Data from this instrument will provide precision visible-light photometric maps. Once starlight and other constant or slowly varying backgrounds are subtracted, the residue is mostly sunlight that has Thomson-scattered from heliospheric electrons. These maps will enable 3-dimensional tomographic reconstruction of heliospheric density and velocity. The SMEI design provides three cameras, one of which views to within 18 degrees of the solar disk with a field of view 60° long by 3° wide. Placed end-to-end, three fields of view then cover a nearly 180° long strip that sweeps out the sky over each orbit. The 3-dimensional tomographic analysis requires 0.1% photometry and background-light reduction below one S10 (the brightness equivalent of a 10th magnitude star per square degree). Thus 10 of surfacebrightness reduction is required relative to the solar disk. The SMEI labyrinthine baffle provides roughly 10 of this reduction; the subsequent optics provides the remainder. We describe the baffle design and present laboratory measurements of prototypes that confirm performance at this level.