Abstract
Direct exoplanet spectroscopy aims to measure the spectrum of an exoplanet while simultaneously minimizing the light collected from its host star. Isolating the planet light from the starlight improves the signal-to-noise ratio (S/N) per spectral channel when noise due to the star dominates, which may enable new studies of the exoplanet atmosphere with unprecedented detail at high spectral resolution (>30,000). However, the optimal instrument design depends on the flux level from the planet and star compared to the noise due to other sources, such as detector noise and thermal background. Here we present the design, fabrication, and laboratory demonstration of specially-designed optics to improve the S/N in two potential regimes in direct exoplanet spectroscopy with adaptive optics instruments. The first is a pair of beam-shaping lenses that increase the planet signal by improving the coupling efficiency into a single-mode fiber at the known position of the planet. The second is a grayscale apodizer that reduces the diffracted starlight for planets at small angular separations from their host star. The former especially increases S/N when dominated by detector noise or thermal background, while the latter helps reduce stellar noise. We show good agreement between the theoretical and experimental point spread functions in each case and predict the exposure time reduction (∼33%) that each set of optics provides in simulated observations of 51 Eridani b using the Keck Planet Imager and Characterizer instrument at W. M. Keck Observatory.
Highlights
With over 4000 exoplanets confirmed to date, detection has given way to the era of characterization, critical to understanding the properties of these systems
We present the development of two technologies that can help boost the performance of High dispersion Coronagraphy (HDC) on Keck Planet Imager and Characterizer (KPIC) and similar instruments, namely: Phase Induced Amplitude Apodization (PIAA) optics for re-shaping the beam and boosting coupling and a grayscale microdot apodizer (MDA) for suppressing diffraction features at small angular separations
The PIAA and MDA optics that we developed for KPIC are optimized to maximize the coupling efficiency for planet light, ηp, and minimize the fraction of the starlight that is coupled into the single-mode fiber (SMF), ηs
Summary
With over 4000 exoplanets confirmed to date, detection has given way to the era of characterization, critical to understanding the properties of these systems In this vein, high contrast imagers (HCI), which isolate the light from the planet, offer numerous advantages over indirect techniques that rely on the signal from the host star alone. HCI use advanced wave front control techniques combined with coronagraphs to extinguish the starlight and minimize contamination to the planet signal (Macintosh et al 2014; Jovanovic et al 2015; Males et al 2018; Beuzit et al 2019) These systems often exploit low to medium resolving power (R ∼ 10–1000), integral field spectrographs to characterize the targets (e.g., the CHARIS instrument Groff et al 2016).
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