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1. Keck Planet Finder: design updates

   https://doi.org/10.1117/12.2561783  

   Gibson, Steven R.; Howard, Andrew W.; Rider, Kodi; Roy, Arpita; Edelstein, Jerry; Kassis, Marc; Grillo, Jason; Halverson, Samuel P.; Sirk, Martin M.; Smith, Christopher; et al (December 2020, Keck Planet Finder: Design Updates)

   Evans, Christopher J.; Bryant, Julia J.; Motohara, Kentaro (Ed.)

   The Keck Planet Finder (KPF) is a fiber-fed, high-resolution, high-stability spectrometer in development at the UC Berkeley Space Sciences Laboratory for the W.M. Keck Observatory. KPF is designed to characterize exoplanets via Doppler spectroscopy with a goal of a single measurement precision of 0.3 m s-1 or better, however its resolution and stability will enable a wide variety of astrophysical pursuits. Here we provide post-preliminary design review design updates for several subsystems, including: the main spectrometer, the fabrication of the Zerodur optical bench; the data reduction pipeline; fiber agitator; fiber cable design; fiber scrambler; VPH testing results and the exposure meter. 

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2. A UV double pass spectrograph for monitoring stellar activity for the Keck Planet Finder

   https://doi.org/10.1117/12.2628149  

   Baker, Ashley; Gibson, Steven; Grillo, Jason; Ishikawa, Yuzo; Howard, Andrew W.; Halverson, Samuel; Rider, Kodi; Jelinsky, Sharon; Deich, Will; Sirk, Martin; et al (January 2022, Proceedings of the SPIE)

   Evans, Christopher J.; Bryant, Julia J.; Motohara, Kentaro (Ed.)

   We present a compact, double-pass cross-dispersed echelle spectrograph that is tailored specifically to cover the 383 nm to 403 nm spectral range and record R∼16,000 spectra of the stellar chromospheric Ca II H and K lines. This `H and K' spectrometer was developed as a subsystem of the Keck Planet Finder (KPF), which is an extremely precise optical (440 - 870 nm) radial velocity spectrograph for Keck I, scheduled for commissioning Fall 2022, with the science objective of measuring precise masses of exoplanets. The H and K spectrometer will observe simultaneously with KPF to independently track the chromospheric activity of the host stars that KPF observes, which is expected to dominate the KPF measurement floor over long timescales. The H and K Spectrometer is fiber fed from the KPF fiber injection unit with total throughput of 4-7% (top of telescope to CCD) over its operating spectral range. Here we detail the optical design trade offs, mechanical design, and first results from alignment and integration testing. 

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3. A fiber injection unit for the Keck Planet Finder: opto-mechanical design

   https://doi.org/10.1117/12.2628818  

   Lilley, Scott .; Rider, Kodi; Thorne, Jim; Kassis, Marc; Gibson, Steve; Howard, Andrew W.; Lanclos, Kyle; Walawender, Josh (August 2022, Proceedings of the SPIE)

   Evans, Christopher J.; Bryant, Julia J.; Motohara, Kentaro (Ed.)

   As part of the Keck Planet Finder (KPF) project, a Fiber Injection Unit (FIU) was implemented and will be deployed on the Keck Ⅰ telescope, with the aim of providing dispersion compensated and tip/tilt corrected light to the KPF instrument and accompanying H&K spectrometer. The goal of KPF is to characterize exoplanets via the radial velocity technique, with a single measurement precision of 30cm/s or better. To accomplish this, the FIU must provide a stable F-number and chief ray angle to the Science and Calcium H&K fibers. Our design approach was use a planar optical layout with atmospheric dispersion compensation for both the Science and Calcium H&K arms. A SWIR guider camera and piezo tip/tilt mirror are used to keep the target centered on the fibers. 

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4. Staring at the Sun with the Keck Planet Finder: An Autonomous Solar Calibrator for High Signal-to-noise Sun-as-a-star Spectra

   https://doi.org/10.1088/1538-3873/ad0b30  

   Rubenzahl, Ryan A; Halverson, Samuel; Walawender, Josh; Hill, Grant M; Howard, Andrew W; Brown, Matthew; Ida, Evan; Tehero, Jerez; Fulton, Benjamin J; Gibson, Steven R; et al (December 2023, Publications of the Astronomical Society of the Pacific)

   Abstract Extreme precision radial velocity (EPRV) measurements contend with internal noise (instrumental systematics) and external noise (intrinsic stellar variability) on the road to 10 cm s⁻¹“exo-Earth” sensitivity. Both of these noise sources are well-probed using “Sun-as-a-star” RVs and cross-instrument comparisons. We built the Solar Calibrator (SoCal), an autonomous system that feeds stable, disk-integrated sunlight to the recently commissioned Keck Planet Finder (KPF) at the W. M. Keck Observatory. With SoCal, KPF acquires signal-to-noise ratio (S/N) ∼ 1200,R= 98,000 optical (445–870 nm) spectra of the Sun in 5 s exposures at unprecedented cadence for an EPRV facility using KPF’s fast readout mode (<16 s between exposures). Daily autonomous operation is achieved by defining an operations loop using state machine logic. Data affected by clouds are automatically flagged using a reliable quality control metric derived from simultaneous irradiance measurements. Comparing solar data across the growing global network of EPRV spectrographs with solar feeds will allow EPRV teams to disentangle internal and external noise sources and benchmark spectrograph performance. To facilitate this, all SoCal data products are immediately available to the public on the Keck Observatory Archive. We compared SoCal RVs to contemporaneous RVs from NEID, the only other immediately public EPRV solar data set. We find agreement at the 30–40 cm s⁻¹level on timescales of several hours, which is comparable to the combined photon-limited precision. Data from SoCal were also used to assess a detector problem and wavelength calibration inaccuracies associated with KPF during early operations. Long-term SoCal operations will collect upwards of 1000 solar spectra per six-hour day using KPF’s fast readout mode, enabling stellar activity studies at high S/N on our nearest solar-type star. 

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5. A Monolithic Spatial Heterodyne Raman Spectrometer: Initial Tests

   https://doi.org/10.1177/0003702820936643  

   Waldron, Abigail; Allen, Ashley; Colón, Arelis; Carter, J_Chance; Angel, S_Michael (August 2020, Applied Spectroscopy)

   A monolithic spatial heterodyne Raman spectrometer (mSHRS) is described, where the optical components of the spectrometer are bonded to make a small, stable, one-piece structure. This builds on previous work, where we described bench top spatial heterodyne Raman spectrometers (SHRS), developed for planetary spacecraft and rovers. The SHRS is based on a fixed grating spatial heterodyne spectrometer (SHS) that offers high spectral resolution and high light throughput in a small footprint. The resolution of the SHS is not dependent on a slit, and high resolution can be realized without using long focal length dispersing optics since it is not a dispersive device. Thus, the SHS can be used as a component in a compact Raman spectrometer with high spectral resolution and a large spectral range using a standard 1024 element charge-coupled device. Since the resolution of the SHRS is not dependent on a long optical path, it is amenable to the use of monolithic construction techniques to make a compact and robust device. In this paper, we describe the use of two different monolithic SHSs (mSHSs), with Littrow wavelengths of 531.6 nm and 541.05 nm, each about 3.5 × 3.5 × 2.5 cm in size and weighing about 80 g, in a Raman spectrometer that provides ∼3500 cm⁻¹spectral range with 4–5 cm⁻¹and 8–9 cm⁻¹resolution, for 600 grooves/mm and 150 grooves/mm grating-based mSHS devices, respectively. In this proof of concept paper, the stability, spectral resolution, spectral range, and signal-to-noise ratio of the mSHRS spectrometers are compared to our bench top SHRS that uses free-standing optics, and signal to noise comparisons are also made to a Kaiser Holospec f/1.8 Raman spectrometer. 

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