Broadband stability of the Habitable Zone Planet Finder Fabry-P\'erot etalon calibration system: evidence for chromatic variation
Abstract
The comb-like spectrum of a white light-illuminated Fabry-P\'erot etalon can serve as a cost-effective and stable reference for precise Doppler measurements. Understanding the stability of these devices across their broad (100's of nm) spectral bandwidths is essential to realize their full potential as Doppler calibrators. However, published descriptions remain limited to small bandwidths or short timespans. We present a 6 month broadband stability monitoring campaign of the Fabry-P\'erot etalon system deployed with the near-infrared Habitable Zone Planet Finder spectrograph (HPF). We monitor the wavelengths of each of 3500 resonant modes measured in HPF spectra of this Fabry-P\'erot etalon (free spectral range = 30 GHz, bandwidth = 820 - 1280 nanometers), leveraging the accuracy and precision of an electro-optic frequency comb reference. These results reveal chromatic structure in the Fabry-P\'erot mode locations and in their evolution with time. We measure an average drift on the order of 2 cm s -1 d-1, with local departures up to 5 cm s -1 d-1. We discuss these behaviors in the context of the Fabry-P\'erot 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 at the 30 cm s -1 level over 10 d. 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.
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