Differential high precision photometry with narrowband filters using HIRAX: instrument overview

Transmission spectroscopy is a powerful observing technique to probe the atmospheric spectrum of an exoplanet that transits its host star. Low resolution transmission spectroscopy can probe both spectral features and the continuum of a planet's atmosphere, but is difficult to do from the ground due to Earth's atmosphere. Here we present HIRAX, a ground-based instrument capable of imaging a system simultaneously in multiple narrowband filters to perform transmission spectroscopy in three 3Å wide bands. HIRAX uses self-referenced bandpasses and a simple imaging design to reduce systematic effects related to Earth's atmosphere and probe an exoplanet transmission spectrum at a few stable wavelengths. HIRAX has been designed for characterizing the sodium doublet (5889.9Å and 5895.9Å) in hot Jupiter atmospheres using the Hale telescope at Palomar Observatory. Here we present the motivation for HIRAX, detail its optical and mechanical design, and present several requirements we imposed on HIRAX in order for it to achieve a photometric precision near the photon limit. We also detail the status of HIRAX and future observing plans.