[Paper Review] Enhanced high-dispersion coronagraphy with KPIC phase II: design, assembly and status of sub-modules

Nemanja Jovanović, Benjamin Calvin+18 more|arXiv (Cornell University)|Dec 11, 2020

Adaptive optics and wavefront sensing19 references4 citations

TL;DR

This paper presents the design, assembly, and laboratory testing of upgraded sub-modules for KPIC Phase II, a high-dispersion coronagraphy instrument at the Keck Telescope. It enhances exoplanet spectroscopy by integrating a 1000-element deformable mirror, lossless beam shaping, a pupil apodizer, a vortex mask, and atmospheric dispersion compensation to maximize planet light throughput and minimize stellar leakage, reducing integration time for high signal-to-noise spectra of directly imaged exoplanets.

ABSTRACT

The Keck Planet Imager and Characterizer (KPIC) is a purpose-built instrument for high-dispersion coronagraphy in the K and L bands on Keck. This instrument will provide the first high resolution (R$>$30,000) spectra of known directly imaged exoplanets and low-mass brown dwarf companions visible in the northern hemisphere. KPIC is developed in phases. Phase I is currently at Keck in the early operations stage, and the phase II upgrade will deploy in late 2021. The goal of phase II is to maximize the throughput for planet light and minimize the stellar leakage, hence reducing the exposure time needed to acquire spectra with a given signal-to-noise ratio. To achieve this, KPIC phase II exploits several innovative technologies that have not been combined this way before. These include a 1000-element deformable mirror for wavefront correction and speckle control, a set of lossless beam shaping optics to maximize coupling into the fiber, a pupil apodizer to suppress unwanted starlight, a pupil plane vortex mask to enable the acquisition of spectra at and within the diffraction limit, and an atmospheric dispersion compensator. These modules, when combined with the active fiber injection unit present in phase I, will make for a highly efficient exoplanet characterization platform. In this paper, we will present the final design of the optics and opto-mechanics and highlight some innovative solutions we implemented to facilitate all the new capabilities. We will provide an overview of the assembly and laboratory testing of the sub-modules and some of the results. Finally, we will outline the deployment timeline.

Motivation & Objective

• To reduce integration time for high-resolution spectroscopy of directly imaged exoplanets by maximizing planet light throughput and minimizing stellar leakage.
• To develop and test a suite of innovative optical sub-modules that enable high-contrast, high-throughput fiber injection for exoplanet characterization.
• To ensure precise, repeatable beam steering and alignment across multiple optical configurations using flexure-compensated mechanisms.
• To integrate advanced wavefront correction and beam control technologies into a compact, space-constrained environment within Keck AO.
• To validate all sub-modules in the lab prior to deployment, ensuring reliable performance for late 2021 commissioning.

Proposed method

• The system uses a double off-axis parabolic (OAP) relay to maintain optimal F/# and provide focal/pupil planes for wavefront sensing and beam control.
• A 1000-element deformable mirror is employed for wavefront correction and speckle suppression in high-contrast imaging.
• Lossless beam shaping optics are integrated to maximize coupling efficiency into the single-mode fiber.
• A pupil apodizer and a pupil-plane vortex mask are used to suppress stellar halo and enable high-contrast spectroscopy at and within the diffraction limit.
• An atmospheric dispersion compensator is included to correct for chromatic effects across the K and L bands.
• Mechanisms for the PyWFS pickoff and tracking camera pickoff use precision flexure stages with hysteresis compensation to ensure repeatable beam alignment across multiple mirror positions.

Experimental results

Research questions

• RQ1 How can planet light throughput be maximized while minimizing stellar leakage in high-dispersion coronagraphy?
• RQ2 What opto-mechanical solutions enable stable, repeatable beam switching and alignment across multiple optical configurations in a compact instrument?
• RQ3 How can wavefront correction and beam shaping be combined with pupil-plane optics to achieve high-contrast spectroscopy at R > 30,000?
• RQ4 What performance metrics are achieved by the new sub-modules in laboratory testing, particularly in beam centroid stability and alignment repeatability?
• RQ5 How do flexure and hysteresis effects in moving mirror mechanisms impact beam alignment, and can they be compensated for?

Key findings

• The PyWFS pickoff mechanism achieved sub-50-mas centroid stability across all mirror positions, with a 70% reduction in scatter when hysteresis compensation was applied.
• The tracking camera pickoff mechanism demonstrated a centroid RMS spread of less than 1 pixel per mirror, with pointing offsets of only ~5λ/D (~40 mas), well within the camera’s central region.
• Laboratory testing confirmed that the flexure mechanism had sufficient range to co-align all four beams, and hysteresis compensation enabled deterministic, repeatable alignment.
• The system met all alignment specifications even without hysteresis compensation, with the blue dashed circle in Figure 7 indicating the maximum allowable misalignment.
• All sub-modules were successfully assembled and tested, with integration into the Keck system scheduled for early 2021 and deployment planned for late 2021.
• The final opto-mechanical design enables efficient, high-contrast fiber injection with minimal loss, supporting high-resolution spectroscopy of exoplanets at R > 30,000.