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Design and analysis of the composite spider structure within the Kepler Schmidt Telescope

NASA's planned Kepler mission uses a space-born Schmidt telescope to search for Earth-size and smaller planets around distant stars using differential photometry. This paper reports the successful design, analysis and implementation of suspending a large actively cooled (-90C) focal plane array with associated electronics inside the warm (0C) Kepler photometer. Since a Schmidt Telescope requires the focal plane to be in the middle of the telescope, it must be suspended while obscuring only a small portion of the incoming light. The Kepler focal plane is comprised of 21 individual science CCD modules and 4 guidance sensor modules covering an area that is roughly 1200 square centimeters in a telescope with a 0.95m aperture. The Kepler system requires the detector data to be digitized near the focal plane, so a detector electronics box is also suspended behind the CCD array. A total of 65 kilograms is supported by the spider structure inside the telescope and must remain stable through environments and during on-orbit operations. Key to the performance of the system is a stiff, light-weight composite structure that supports the focal plane and electronics above the primary mirror. This spider structure is used to align the focal plane with respect to the primary mirror in the system, and is intentionally over-constrained after alignment. Techniques used to align the focal plane to the optical system are discussed and predicted alignment performance and stability are reported.