Diffraction-limited spectrographs are key instruments for upcoming large telescopes thanks to their compact sizes that do not scale with telescope diameter and stable point spread functions (PSFs) that are conducive to precise radial velocity (RVs) measurements. A challenge to achieving sub meter-per-second RVs with diffraction-limited spectrographs that does not show up in the RV error budgets of their seeing-limited counterparts, is “differential limb coupling”, which we define as a preferential coupling of one side of a star’s limb over the other. This effect can result in a more blue- or red-shifted stellar spectrum being coupled into a single-mode fiber due imperfect centroiding of the star on the fiber that differs between each RV measurement. This effect is exacerbated when the star is faster rotating and more resolved by an optical system, and can become significant when coupling to single-mode fibers due to their sensitivity to source position on the fiber. In this paper we quantify the RV amplitude of stellar differential limb coupling, and show that the effect can have a significant impact on next generation adaptive-optics (AO) fed spectrographs on large aperture telescopes if uncorrected. We apply these calculations to HISPEC on Keck and MODHIS on TMT and estimate that for 1 mas pointing errors, the RV error term for a star 0.5 mas in diameter rotating at 2 kms−1 would be 1 ms−1and 7.7 ms−1, respectively. We present several design strategies that we expect would reduce the differential limb coupling effect to varying degrees.
HISPEC (High-resolution Infrared Spectrograph for Exoplanet Characterization) is an infrared (0.98 to 2.46 microns) cross-dispersed, R=100,000 single-mode fiber-fed diffraction-limited echellette spectrograph for the Keck II telescope’s adaptive optics (AO) system. MODHIS (Multi-Objective Diffraction-limited High-resolution Infrared Spectrograph) shares similar specifications as HISPEC while being optimized for TMT’s first-light AO system NFIRAOS. Keck-HISPEC, currently in full-scale development and slated for first light in 2026, and TMTMODHIS, currently in conceptual design phase, will provide increasingly compelling science capabilities from exoplanet atmosphere characterization through both transit and direct high-contrast spectroscopy, to detection and mass measurements through infrared precision radial velocity (RV). The science cases include the precise RV measurements of stars orbiting the Galactic Center, Solar System studies, and the chemodynamical history of nearby dwarf galaxies and the galactic halo.
The High-Resolution Infrared Spectrograph for Exoplanet Characterization (HISPEC) is a new instrument for the W. M. Keck Observatory that enables R∼100,000 spectroscopy simultaneously across the y, J, H, and K astronomical bands (0.98-2.5 μm). The front-end instrument steers the adaptive optics corrected beam delivered by Keck to single-mode fibers used to route the light to the spectrographs. The basic architecture of the front-end instrument leverages from the design from the Keck Planet Imager and Characterizer where a tracking camera is used to monitor the location of the target and send commands to a tip/tilt mirror mounted in a pupil plane, which aligns the beam with the fiber in the downstream focal plane. The system will have an atmospheric dispersion corrector to minimize chromatic smearing of the PSF, phase induced amplitude apodization optics to mitigate coupling limitations imposed by the pupil geometry, and vortex masks to enable vortex fiber nulling. The front-end instrument will utilize a Teledyne H2RG for tracking allowing for the ability to guide on targets as faint as 15th magnitude and for tip/tilt control up to 500 Hz on brighter targets. In this paper we provide an overview of the detailed design of the front-end instrument and elucidate the design choices driven by de-risking exercises. We will describe our plan to utilize the J-H gap for tracking which will allow for uninterrupted science for a large population of targets. We present how the front-end instrument will be integrated into the Keck adaptive optics bench to allow for easy removal and cable management. Finally, we provide an update on the project status and the timeline for the sub-system.
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