The Exoplanet Transmission Spectroscopy Imager (ETSI) amalgamates a low resolution slitless prism spectrometer with custom multi-band filters to simultaneously image 15 spectral bandpasses between 430 nm and 975 nm with an average spectral resolution of $R = λ/δλ\sim 20$. ETSI requires only moderate telescope apertures ($\sim2$ m) and is capable of characterizing an exoplanet atmosphere in as little as a single transit, enabling selection of the most interesting targets for further characterization with other ground and space-based observatories and is also well suited to multi-band observations of other variable and transient objects. This enables a new technique, common-path multi-band imaging (CMI), used to observe transmission spectra of exoplanets transiting bright (V$<$14 magnitude) stars. ETSI is capable of near photon-limited observations, with a systematic noise floor on par with the Hubble Space Telescope and below the Earth's atmospheric amplitude scintillation noise limit. We report the as-built instrument optical and optomechanical design, detectors, control system, telescope hardware and software interfaces, and data reduction pipeline. A summary of ETSI's science capabilities and initial results are also included.

33 pages, 17 figures, published in JATIS