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

Abstract

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. BACKGROUNDThe planned Kepler mission will find out how common Earth-like planets are, and whether or not the structure of our solar system is unique. This is achieved by surveying a large sample of stars to: 1) Determine the percentage of terrestrial and larger planets there are in or near the habitable zone of a wide variety of stars; 2) Determine the distribution of sizes and shapes of the orbits of these planets; 3) Estimate how many planets there are in multiple-star systems; 4) Determine the variety of orbit sizes and planet reflectivities, sizes, masses and densities of shortperiod giant planets; 5) Identify additional members of each discovered planetary system using other techniques; and 6) Determine the properties of those stars that harbor planetary systems.Kepler will search for Earth-size and smaller planets in the habitable zone around distant stars using the transit method. A transit is detected when the light from a star dims slightly due to a planet passing in front of the star. When a planet crosses in front of its star as viewed by an observer, the event is called a transit. Transits produce a small change in a star's brightness of about 1/10,000 or 100 parts per million (ppm) for terrestrial planets and lasting for 2 to 16 hours. This change must be absolutely periodic if it is caused by a planet. In addition, all transits produced by the same planet must be of the same change in brightness and last the same amount of time, thus providing a highly repeatable signal and robust detection method.Once detected, the planet's orbit...