Kepler is NASA's first space mission dedicated to the study of exoplanets. The primary scientific goal is statistical-to estimate the frequency of planetary systems associated with sun-like stars, especially the detection of earth-size planets in the habitable zones. Kepler was launched into an Earth-trailing heliocentric "drift-away" orbit (period ¼ 372 days) in March 2009. The instrument detects the faint photometric signals of transits of planets across the stellar disks of those systems

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... orbital planes fortuitously oriented in our line of sight. Since the probability of such alignments is small, Kepler must observe a large number of stars. In fact, Kepler is monitoring approximately 150,000 stars with a 30-min cadence. The scientific goals led to the choice of a classical Schmidt telescope, and requirements on field-of-view, throughput, spectral bandpass, image quality, scattered light, thermal and opto-mechanical stability, and in-flight adjustment authority. We review the measurement requirements, telescope design, prelaunch integration, alignment, and test program, and we describe the in-flight commissioning that optimized the performance. The stability of the flight system has enabled increasing recognition of small effects and sophistication in data processing algorithms. Astrophysical noise arising from intrinsic stellar variability is now the dominant term in the photometric error budget. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.