The Imaging System

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.

Ray Tracing

Missing Image: A rendering of SMEI's optical paths

This shows a particular set of rays as they travel through the SMEI optics to the CCD.

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).