2025PASP..137h4402L

We present a new instrumentation technique for high-precision, ground-based spectrophotometric measurements ideal for low-resolution (R ∼ 20-60) exoplanet transmission spectroscopy. This technique employs novel thin-film coating technology for wide-band, simultaneous multi-band imaging, enabling high-precision differential photometry through self-referencing and comparison stars. Theoretical calculations and on-sky results show that this method effectively reduces 96% of amplitude scintillation noise and other systematics that typically limit ground-based spectrophotometric precision for bright stars. On-sky results are demonstrated using the custom-built Exoplanet Transmission Spectroscopy Imager (ETSI), deployed at the McDonald Observatory 2.1 m telescope in 2022. For a V = 9.7 mag star, ETSI achieves a spectrophotometric precision of 50 ppm in a 40 nm band over 1.0 hr, below the atmospheric scintillation noise floor. Using ETSI, we present a measured transmission spectrum of the hot Saturn HAT-P-44Bb to demonstrate the technique's application. We perform a basic spectral retrieval to verify the measurement's consistency with the planet's known or expected physical parameters. Future deployment of this technique in instruments on mid to large-class observatories could enable the ability to characterize hundreds of exoplanet atmospheres in the visible with low resolution transmission spectroscopy.