The second element in the SMEI optical design is the imaging system, which is located beyond the instrument pupil, aperture Z0. The SMEI optics use two mirrors to focus the sky onto the CCD. The resulting illumination lies within a circular arc on the CCD which is about 100 pixels wide and subtends just over 60◦ on the sky. This then covers essentially the full 1242-pixel available width of the CCD.
The ﬁve important elements in this part of the system are shown to the right. Light enters through aperture Z0 and is then focused by the primary mirror M1 towards conical mirror M2. This mirror serves to ﬂatten the image onto the planar CCD surface. The fourth element is a vane (V) which performs some of the function of a Lyot stop. This intercepts unwanted light that reﬂected from M1 but originated outside of the ﬁeld of view. This includes sky objects beyond the ﬁeld of view, but also stray light which ﬁnds its way through the bafﬂe. This latter light mostly scattered last from the edge or rear of aperture Z3 (Bufﬁngton, Jackson, and Hick, 2002), but may also be stray light scattering inside the optics enclosure which entered through Z0 at a large angle. The ﬁfth optical element is the CCD itself.
Mirror M1 was made from an aluminum ring diamond-turned to the correct ﬁgure and separated into quadrants of the correct size. The quadrants were separated prior to the ﬁnal machining to avoid possible ﬁgure change due to stress relief upon separation. M2 was made similarly, but in semicircular segments. Diamond turning was selected for manufacturing these mirrors to reduce scattering, by a few degrees, to below 10-5 without the need for "post polishing". In addition the process was carefully controlled to minimize smaller angle scattering from the regular turning grooves. For further detail, see Eyles et al. (2003) and Buffington et al. (SPIE 2009).