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We present the design of a novel instrument tuned to detect transiting exoplanet atmospheres. The instrument, which we call the exoplanet transmission spectroscopy imager (ETSI), makes use of a new technique called common-path multi-band imaging (CMI). ETSI uses a prism and multi-band lter to simultaneously image 15 spectral bandpasses on two detectors from 430 􀀀 975nm (with a average spectral resolution of R = =
 = 23) during exoplanet transits of a bright star. A prototype of the instrument achieved photon-noise limited results which were below the atmospheric amplitude scintillation noise limit. ETSI can detect the presence and composition of an exoplanet atmosphere in a relatively short time on a modest-size telescope. We show the optical design of the instrument. Further, we discuss design trades of the prism and multi-band lter which are driven by the science of the ETSI instrument. We describe the upcoming survey with ETSI that will measure dozens of exoplanet atmosphere spectra in  2 years on a two meter telescope. Finally, we will discuss how ETSI will be a powerful means for follow up on all gas giant exoplanets that transit bright stars, including a multitude of recently identi ed TESS (NASA's Transiting Exoplanet Survey Satellite) exoplanets.