arXiv subjects

Jonathan Crass

Publications and source records attributed to Jonathan Crass.

At least 19 recordsLinked to original sources

arXiv2026

The Calibration System of the iLocater Spectrograph

iLocater is a new near-infrared, extreme precision radial velocity (EPRV) instrument delivered to the Large Binocular Telescope in the summer of 2026. The instrument utilizes single-mode fibers (SMFs) for light transmission, including its calibration systems. We present an overview of the iLocater calibration system, detailing the specific sources and the hardware required to inject calibration light into the SMFs. Furthermore, we discuss the hardware that efficiently switches between these sources and distributes the light to all necessary instrument locations, entirely eliminating the need for free-space propagation.

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arXiv2026

Commissioning and on-sky performance verification of iLocater

iLocater is a high-resolution near-infrared extreme precision radial velocity (EPRV) spectrograph that was deployed to the Large Binocular Telescope (LBT) in June 2026. iLocater operates over $\lambda=966-1312$ nm with a median resolving power of $R=205,000$ as measured in the laboratory. We present the commissioning and initial on-sky verification program using solar and night-time observations at the LBT. First light was achieved on 27 June 2026, and nearly 150 on-sky spectra have now been recorded. Observations include single stars ranging in spectral type from B5 to M6. iLocater uses the LBT AO system, and we have demonstrated its ability to obtain spatially resolved spectra of close ($\theta < 1''$) binary stars.

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arXiv2026

Laboratory characterization of iLocater

iLocater is a diffraction-limited, fiber-fed spectrograph designed for the Large Binocular Telescope (LBT), which targets high-precision radial velocity measurements in the near-infrared. Prior to deployment, comprehensive laboratory characterization was essential to validate instrument performance and inform alignment strategies. This paper presents results from four key areas of lab characterization: (1) adjustment and optimization of detector orientation to optimize spectrum alignment with the detector pixel grid across the focal plane; (2) the design and installation of a LED illumination source to enable high-fidelity flat-fields; (3) a model-based focusing methodology using OpticStudio image simulations to optimize the optical alignment of the spectrograph; and (4) assessment of instrument mechanical and optical stability under laboratory conditions. Together, these efforts established baseline performance metrics and demonstrated instrument readiness for delivery and on-sky commissioning.

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arXiv2026

Long-term performance of the MLR-TT sensor: two-photon polymerization validation in the telescope environment

We report on the long-term optical performance of a two-photon polmerized microlens ring tip-tilt sensor. The study is backed by repeated measurements over the last six years, since the sensor was first tested in the sky at the Large Binocular Telescope. The goal of the study is to assess the feasibility of the underlying technology of two-photon polymerization for future instrumentation in the realistic environments experienced at astronomical telescopes.

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arXiv2024

Little iLocater: paving the way for iLocater

Diffraction-limited radial velocity instruments offer a pathway towards improved precision and stability, and the exploration of new parameter spaces at high spatial and spectral resolution. However, achieving the necessary performance requires careful instrument design and considerable on-sky testing. We describe the design and construction of ``Little iLocater'' (Lili), a compact spectrograph that has been used to validate the performance of the front-end fibre-injection system of the iLocater spectrograph. We present the design, assembly, and performance using on-sky data obtained at the Large Binocular Telescope (LBT), including extraction of spectra from standard stars, testing of the atmospheric dispersion corrector to elevations of 40 degrees, and spatially resolved spectra from close companion systems. These results show the front-end fibre-injection system is performing as expected and is indicative of iLocater's capabilities once installed at the LBT.

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arXiv2024

Resolving the Young 2 Cygni Run-away Star into a Binary using iLocater

Precision radial velocity (RV) spectrographs that use adaptive optics (AO) show promise to advance telescope observing capabilities beyond those of seeing-limited designs. We are building a spectrograph for the Large Binocular Telescope (LBT) named iLocater that uses AO to inject starlight directly into single mode fibers (SMF). iLocater's first acquisition camera system (the `SX' camera), which receives light from one of the 8.4m diameter primary mirrors of the LBT, was initially installed in summer 2019 and has since been used for several commissioning runs. We present results from first-light observations that include on-sky measurements as part of commissioning activities. Imaging measurements of the bright B3IV star 2 Cygni ($V=4.98$) resulted in the direct detection of a candidate companion star at an angular separation of only $\theta = 70$ mas. Follow-up AO measurements using Keck/NIRC2 recover the candidate companion in multiple filters. An $R\approx1500$ miniature spectrograph recently installed at the LBT named ``Lili'' provides spatially resolved spectra of each binary component, indicating similar spectral types and strengthening the case for companionship. Studying the multiplicity of young runaway star systems like 2 Cygni ($36.6 \pm 0.5$ Myr) can help to understand formation mechanisms for stars that exhibit anomalous velocities through the galaxy. This on-sky demonstration illustrates the spatial resolution of the iLocater SX acquisition camera working in tandem with the LBT AO system; it further derisks a number of technical hurdles involved in combining AO with Doppler spectroscopy.

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arXiv2024

The instrumentation program at the Large Binocular Telescope Observatory in 2024

The Large Binocular Telescope, with its expansive collecting area, angular resolving power, and advanced optical design, provides a robust platform for development and operation of advanced instrumentation for astronomical research. The LBT currently hosts a mature suite of instruments for spectroscopy and imaging at optical through mid-infrared wavelengths, supported by sophisticated adaptive optics systems. This contribution summarizes the current state of instrumentation, including upgrades to existing instruments and commissioning of second generation instruments now in progress. The LBT is soliciting proposals for next generation instrument concepts, with participation open to consortium members and others interested in participation in the Observatory.

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arXiv2023

Assessing Phase Reconstruction Accuracy for Different Nonlinear Curvature Wavefront Sensor Configurations

The nonlinear curvature wavefront sensor (nlCWFS) offers improved sensitivity for adaptive optics (AO) systems compared to existing wavefront sensors, such as the Shack-Hartmann. The nominal nlCWFS design uses a series of imaging planes offset from the pupil along the optical propagation axis as inputs to a numerically-iterative reconstruction algorithm. Research into the nlCWFS has assumed that the device uses four measurement planes configured symmetrically around the optical system pupil. This assumption is not strictly required. In this paper, we perform the first systematic exploration of the location, number, and spatial sampling of measurement planes for the nlCWFS. Our numerical simulations show that the original, symmetric four-plane configuration produces the most consistently accurate results in the shortest time over a broad range of seeing conditions. We find that the inner measurement planes should be situated past the Talbot distance corresponding to a spatial period of $r_0$. The outer planes should be large enough to fully capture field intensity and be situated beyond a distance corresponding to a Fresnel-number-scaled equivalent of $Z\approx50$ km for a $D=0.5$ m pupil with $\lambda=532$ nm. The minimum spatial sampling required for diffraction-limited performance is 4-5 pixels per $r_0$ as defined in the pupil plane. We find that neither three-plane nor five-plane configurations offer significant improvements compared to the original design. These results can impact future implementations of the nlCWFS by informing sensor design.

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arXiv2022

The final design of the iLocater spectrograph: An optimized architecture for diffraction-limited EPRV instruments

iLocater is a near-infrared, extremely precise radial velocity (EPRV) spectrograph under construction for the dual 8.4 m diameter Large Binocular Telescope (LBT). The instrument will undertake precision radial velocity studies of Earth-like planets orbiting low-mass stars. Operating in the diffraction-limited regime, iLocater uses adaptive optics to efficiently inject starlight directly into single-mode fibers that illuminate a high spectral resolution (R=190,500 median), cryogenic, diffraction-limited spectrograph. To maximize performance, the spectrograph uses a new design strategy for EPRV instruments, combining intrinsically stable materials for its optomechanical fabrication with precision optical fabrication. This novel combination will enable unique EPRV capabilities for exoplanet and astrophysics studies of the solar neighborhood. We present the final optical and mechanical designs of the spectrograph system. Ensuring the as-built spectrograph achieves its designed spectral resolution and diffraction-limited performance has required careful control of the end-to-end system wavefront error (WFE) budget. We discuss the efforts undertaken to achieve this goal including minimizing residual WFE in the optical design, assessing diffraction grating WFE performance, optimizing material choices, and requiring precision optical design and fabrication. Our goal is to deliver diffraction-limited performance across the full spectral format, which, combined with intrinsic thermal stability requirements for EPRV science, has driven the selection of silicon optics and Invar optomechanics. The system performance is further optimized using precision (sub-mK) thermal control. This set of design features will allow iLocater to achieve sub-m/s radial velocity precision in the near-infrared, and to serve as the first optimized diffraction-limited spectrograph for EPRV science.

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arXiv2022

The iLocater cryostat and thermal control system: enabling extremely precise radial velocity measurements for diffraction-limited spectrographs

Extremely precise radial velocity (EPRV) measurements are critical for characterizing nearby terrestrial worlds. EPRV instrument precisions of $σ_{\mathrm{RV}} = 1-10\,\mathrm{cm/s}$ are required to study Earth-analog systems, imposing stringent, sub-mK, thermo-mechanical stability requirements on Doppler spectrograph designs. iLocater is a new, high-resolution ($R=190,500$ median) near infrared (NIR) EPRV spectrograph under construction for the dual 8.4 m diameter Large Binocular Telescope (LBT). The instrument is one of the first to operate in the diffraction-limited regime enabled by the use of adaptive optics and single-mode fibers. This facilitates affordable optomechanical fabrication of the spectrograph using intrinsically stable materials. We present the final design and performance of the iLocater cryostat and thermal control system which houses the instrument spectrograph. The spectrograph is situated inside an actively temperature-controlled radiation shield mounted inside a multi-layer-insulation (MLI) lined vacuum chamber. The radiation shield provides sub-mK thermal stability, building on the existing heritage of the Habitable-zone Planet Finder (HPF) and NEID instruments. The instrument operating temperature ($T=80-100\,\mathrm{K}$) is driven by the requirement to minimize detector background and instantaneous coefficient of thermal expansion (CTE) of the materials used for spectrograph fabrication. This combination allows for a reduced thermomechanical impact on measurement precision, improving the scientific capabilities of the instrument.

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arXiv2022

Spatial frequency response and sensitivity of the nonlinear curvature wavefront sensor

The nonlinear curvature wavefront sensor (nlCWFS) has been shown to be a promising alternative to existing wavefront sensor designs. Theoretical studies indicate that the inherent sensitivity of this device could offer up to a factor of 10 times improvement compared to the widely-used Shack-Hartmann wavefront sensor (SHWFS). The nominal nlCWFS design assumes the use of four detector measurement planes in a symmetric configuration centered around an optical system pupil plane. However, the exact arrangement of these planes can potentially be optimized to improve aberration sensitivity, and minimize the number of iterations involved in the wavefront reconstruction process, and therefore reduce latency. We present a systematic exploration of the parameter space for optimizing the nlCWFS design. Using a suite of simulation tools, we study the effects of measurement plane position on the performance of the nlCWFS and detector pixel sampling. A variety of seeing conditions are explored, assuming Kolmogorov turbulence. Results are presented in terms of residual wavefront error following reconstruction as well as the number of iterations required for solution convergence. Alternative designs to the symmetric four-plane design are studied, including three-plane and five-plane configurations. Finally, we perform a preliminary investigation of the effects of broadband illumination on sensor performance relevant to astronomy and other applications.

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arXiv2022

Characterizing and Mitigating the Impact of Telluric Absorption in Precise Radial Velocities

Precise radial velocity (PRV) surveys are important for the search of Earth analogs around nearby bright stars. Such planets induce a small stellar reflex motion with RV amplitude of $\sim$10 cm/s. Detecting such a small RV signal poses important challenges to instrumentation, data analysis, and the precision of astrophysical models to mitigate stellar jitter. In this work, we investigate an important component in the PRV error budget - the spectral contamination from the Earth's atmosphere (tellurics). We characterize the effects of telluric absorption on the RV precision and quantify its contribution to the RV budget over time and across a wavelength range of 350 nm - 2.5$μ$m. We investigate the effectiveness in mitigating tellurics using simulated spectra of a solar twin star with telluric contamination over a year's worth of observations, and we extracted the RVs using two commonly adopted algorithms: dividing out a telluric model before performing cross-correlation or Forward Modeling the observed spectrum incorporating a telluric model. We assume various degrees of cleanness in removing the tellurics, including mimicking the lack of accurate knowledge of the telluric lines by using a mismatched line profile to model the "observed" tellurics. We conclude that the RV errors caused by telluric absorption can be suppressed to close to or even below the photon-limited precision in the optical region, especially in the blue, around 1-10 cm/s. At red through near-infrared wavelengths, however, the residuals of tellurics can induce an RV error on the m/s level even under the most favorable assumptions for telluric removal, leading to significant systematic noise in the RV time series and periodograms. If the red-optical or near-infrared becomes critical in the mitigation of stellar activity, systematic errors from tellurics can be eliminated with a space mission such as EarthFinder.

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arXiv2021

Extreme Precision Radial Velocity Working Group Final Report

Precise mass measurements of exoplanets discovered by the direct imaging or transit technique are required to determine planet bulk properties and potential habitability. Furthermore, it is generally acknowledged that, for the foreseeable future, the Extreme Precision Radial Velocity (EPRV) measurement technique is the only method potentially capable of detecting and measuring the masses and orbits of habitable-zone Earths orbiting nearby F, G, and K spectral-type stars from the ground. In particular, EPRV measurements with a precision of better than approximately 10 cm/s (with a few cm/s stability over many years) are required. Unfortunately, for nearly a decade, PRV instruments and surveys have been unable to routinely reach RV accuracies of less than roughly 1 m/s. Making EPRV science and technology development a critical component of both NASA and NSF program plans is crucial for reaching the goal of detecting potentially habitable Earthlike planets and supporting potential future exoplanet direct imaging missions such as the Habitable Exoplanet Observatory (HabEx) or the Large Ultraviolet Optical Infrared Surveyor (LUVOIR). In recognition of these facts, the 2018 National Academy of Sciences (NAS) Exoplanet Science Strategy (ESS) report recommended the development of EPRV measurements as a critical step toward the detection and characterization of habitable, Earth-analog planets. In response to the NAS-ESS recommendation, NASA and NSF commissioned the EPRV Working Group to recommend a ground-based program architecture and implementation plan to achieve the goal intended by the NAS. This report documents the activities, findings, and recommendations of the EPRV Working Group.

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arXiv2021

Studying the Impact of Optical Aberrations on Diffraction-Limited Radial Velocity Instruments

Spectrographs nominally contain a degree of quasi-static optical aberrations resulting from the quality of manufactured component surfaces, imperfect alignment, design residuals, thermal effects, and other other associated phenomena involved in the design and construction process. Aberrations that change over time can mimic the line centroid motion of a Doppler shift, introducing radial velocity (RV) uncertainty that increases time-series variability. Even when instrument drifts are tracked using a precise wavelength calibration source, barycentric motion of the Earth leads to a wavelength shift of stellar light causing a translation of the spectrum across the focal plane array by many pixels. The wavelength shift allows absorption lines to experience different optical propagation paths and aberrations over observing epochs. We use physical optics propagation simulations to study the impact of aberrations on precise Doppler measurements made by diffraction-limited, high-resolution spectrographs. We quantify the uncertainties that cross-correlation techniques introduce in the presence of aberrations and barycentric RV shifts. We find that aberrations which shift the PSF photo-center in the dispersion direction, in particular primary horizontal coma and trefoil, are the most concerning. To maintain aberration-induced RV errors less than 10 cm/s, phase errors for these particular aberrations must be held well below 0.05 waves at the instrument operating wavelength. Our simulations further show that wavelength calibration only partially compensates for instrumental drifts, owing to a behavioral difference between how cross-correlation techniques handle aberrations between starlight versus calibration light. Identifying subtle physical effects that influence RV errors will help ensure that diffraction-limited planet-finding spectrographs are able to reach their full scientific potential.

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arXiv2021

On-sky results for the novel integrated micro-lens ring tip-tilt sensor

We present the first on-sky results of the micro-lens ring tip-tilt (MLR-TT) sensor. This sensor utilizes a 3D printed micro-lens ring feeding six multi-mode fibers to sense misaligned light, allowing centroid reconstruction. A tip-tilt mirror allows the beam to be corrected, increasing the amount of light coupled into a centrally positioned single-mode (science) fiber. The sensor was tested with the iLocater acquisition camera at the Large Binocular Telescope in November 2019. The limit on the maximum achieved root mean square reconstruction accuracy was found to be 0.19 $λ$/D in both tip and tilt, of which approximately 50% of the power originates at frequencies below 10 Hz. We show the reconstruction accuracy is highly dependent on the estimated Strehl ratio and simulations support the assumption that residual adaptive optics aberrations are the main limit to the reconstruction accuracy. We conclude that this sensor is ideally suited to remove post-adaptive optics non-common path tip tilt residuals. We discuss the next steps for the concept development, including optimizations of the lens and fiber, tuning of the correction algorithm and selection of optimal science cases.

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arXiv2020

Final Design and On-Sky Testing of the iLocater SX Acquisition Camera: Broadband Single-Mode Fiber Coupling

Enabling efficient injection of light into single-mode fibers (SMFs) is a key requirement in realizing diffraction-limited astronomical spectroscopy on ground-based telescopes. SMF-fed spectrographs, facilitated by the use of adaptive optics (AO), offer distinct advantages over comparable seeing-limited designs, including higher spectral resolution within a compact and stable instrument volume, and a telescope independent spectrograph design. iLocater is an extremely precise radial velocity (EPRV) spectrograph being built for the Large Binocular Telescope (LBT). We have designed and built the front-end fiber injection system, or acquisition camera, for the SX (left) primary mirror of the LBT. The instrument was installed in 2019 and underwent on-sky commissioning and performance assessment. In this paper, we present the instrument requirements, acquisition camera design, as well as results from first-light measurements. Broadband single-mode fiber coupling in excess of 35% (absolute) in the near-infrared (0.97-1.31μm) was achieved across a range of target magnitudes, spectral types, and observing conditions. Successful demonstration of on-sky performance represents both a major milestone in the development of iLocater and in making efficient ground-based SMF-fed astronomical instruments a reality.

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arXiv2020

EarthFinder Probe Mission Concept Study: Characterizing nearby stellar exoplanet systems with Earth-mass analogs for future direct imaging

EarthFinder is a NASA Astrophysics Probe mission concept selected for study as input to the 2020 Astrophysics National Academies Decadal Survey. The EarthFinder concept is based on a dramatic shift in our understanding of how PRV measurements should be made. We propose a new paradigm which brings the high precision, high cadence domain of transit photometry as demonstrated by Kepler and TESS to the challenges of PRV measurements at the cm/s level. This new paradigm takes advantage of: 1) broad wavelength coverage from the UV to NIR which is only possible from space to minimize the effects of stellar activity; 2) extremely compact, highly stable, highly efficient spectrometers (R>150,000) which require the diffraction-limited imaging possible only from space over a broad wavelength range; 3) the revolution in laser-based wavelength standards to ensure cm/s precision over many years; 4) a high cadence observing program which minimizes sampling-induced period aliases; 5) exploiting the absolute flux stability from space for continuum normalization for unprecedented line-by-line analysis not possible from the ground; and 6) focusing on the bright stars which will be the targets of future imaging missions so that EarthFinder can use a ~1.5 m telescope.

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arXiv2020

Characterization of Single-Mode Fiber Coupling at the Large Binocular Telescope

Optimizing on-sky single-mode fiber (SMF) injection is an essential part of developing precise Doppler spectrometers and new astrophotonics technologies. We installed and tested a prototype SMF injection system at the Large Binocular Telescope (LBT) in April 2016. The fiber injection unit was built as part of the de-risking process for a new instrument named iLocater that will use adaptive optics (AO) to feed a high resolution, near-infrared spectrograph. In this paper we report Y-band SMF coupling measurements for bright, M-type stars. We compare theoretical expectations for delivered Strehl ratio and SMF coupling to experimental results, and evaluate fundamental effects that limit injection efficiency. We find the pupil geometry of the telescope itself limits fiber coupling to a maximum efficiency of rho_tel=0.78. Further analysis shows the individual impact of AO correction, tip-tilt residuals, and static (non-common-path) aberrations contribute coupling coefficients of rho_Strehl=0.33, rho_tip/tilt=0.84, and rho_ncpa=0.8 respectively. Combined, these effects resulted in an average Y-band SMF efficiency of 0.18 for all observations. Finally, we investigate the impact of fiber coupling on radial velocity (RV) precision as a function of stellar apparent magnitude.

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