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The CfA Digital Speedometers were spectrographs of a design developed in the 1980s as a Center for Astrophysics (CfA) project, for the purpose of radial velocity (RV) measurements, to pin down the RV component of proper motions of stars for detecting and measuring spectroscopic binaries. They were also eventually used in attempts to detect extra-solar planets. They were digital in that they used a photon counter to collect digital data rather than photographic plates. At least three units were built and installed on research telescopes. They detected radial velocities with a resolution on the order of a kilometer per second. Achieving this required considerable post-processing, evaluating matches between a spectrum observation versus calculated spectra based upon a range of radial velocities.