Broadband Stability of the Habitable Zone Planet Finder Fabry-Pérot Etalon Calibration System: Evidence for Chromatic Variation

The comblike spectrum of a white light-illuminated Fabry-Pérot etalon can serve as a cost-effective and stable reference for precise Doppler measurements. Understanding the stability of these devices across their broad (hundreds of nanometers) spectral bandwidths is essential to realizing their full potential as Doppler calibrators. However, published descriptions remain limited to small bandwidths or short time spans. We present an ∼6 month broadband stability monitoring campaign of the Fabry-Pérot etalon system deployed with the near-infrared Habitable Zone Planet Finder (HPF) spectrograph. We monitor the wavelengths of each of ∼3500 resonant modes measured in HPF spectra of this Fabry-Pérot etalon (free spectral range = 30 GHz, bandwidth = 820-1280 nm), leveraging the accuracy and precision of an electro-optic frequency comb reference. These results reveal chromatic structure in the Fabry-Pérot mode locations and their evolution with time. We measure an average drift on the order of 2 cm s-1 day-1, with local departures up to ±5 cm s-1 day-1. We discuss these behaviors in the context of the Fabry-Pérot etalon mirror dispersion and other optical properties of the system and the implications for the use of similar systems for precise Doppler measurements. Our results show that this system supports the wavelength calibration of HPF at the ≲10 cm s-1 level over a night and the ≲30 cm s-1 level over ∼10 days. Our results also highlight the need for long-term and spectrally resolved study of similar systems that will be deployed to support Doppler measurement precision approaching ∼10 cm s-1.