The Exoplanet Transmission Spectroscopy Imager (ETSI), a new instrument for rapid characterization of exoplanet atmospheres

The Exoplanet Transmission Spectroscopy Imager (ETSI) amalgamates a low resolution slitless prism spectrometer with custom multiband filters to simultaneously image 15 spectral bandpasses between 430 nm and 975 nm with an average spectral resolution of R = lambda/Delta lambda similar to 20. This enables a new technique, common-path multi-band imaging (CMI), used to observe transmission spectra of exoplanets transiting bright (V<14th mag.) 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, data reduction pipeline, and upcoming science observations. We discuss ETSI's science capabilities and the results of our initial April 2022 commissioning run. ETSI requires only moderate telescope apertures (similar to 2 m) and is capable of characterizing the atmospheres of dozens of exoplanets per year, enabling selection of the most interesting targets for further characterization with other ground and space-based observatories.