The Baffle

A SMEI Baffle

Missing Image: SMEI Rendering

An image of a SMEI baffle before blackening.

The design and laboratory measurements of the SMEI baffles are described in detail by Buffington, Jackson, and Hick (2002). A labyrinthine baffle controls stray light by combining a hierarchical sequence of aperture openings, each positioned to block a view of its previous neighbor, with blackened surfaces that absorb rather than scatter most of the unwanted light. Briefly, the design exploits SMEI’s asymmetrical field of view to provide three baffle stages in one projection, and two stages in the other. The image to the right illustrates the baffle’s arrangement of apertures and connecting septums. The three stages are defined respectively by apertures Z0 to Z3, Z3–Z6, and finally Z6–Z8. Secondary apertures placed between these further improve performance by reducing single-scattering paths through each stage (Buffington, Jackson, and Hick, 2002). Some of these secondary apertures (Z4 and Z5) are effective only in the narrow dimension, while Z1 and Z7 only partially cover their respective bottom primary aperture faces. Vane opening Z0 defines the SMEI entrance pupil.

To meet the SMEI specification, a critical element is achieving a sufficiently low reflectivity that four scatters plus the aperture geometry reduces the stray light below 10−10. The internal surfaces of the baffle sections were blackened to a nominal reflectivity of 0.005. Measurements on the SMEI flight baffles confirmed this (Buffington, Jackson, and Hick, 2002) and also demonstrate the expected 10-10 light reduction through Z0 relative to direct sunlight. Analysis of in-flight data confirms the expected performance over most of the sky (Buffington, Jackson, Hick, 2005).