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).