arXiv subjects

Sam Ragland

Publications and source records attributed to Sam Ragland.

At least 19 recordsLinked to original sources

arXiv2026

On-sky capabilities and performance of the Keck All Sky Precision Adaptive Optics system

The Keck All Sky Precision Adaptive optics (KAPA) project upgrades the Keck I adaptive optics system to enable laser tomography using a four laser guide star (LGS) asterism. KAPA is now in operation in both narrow field and wide field modes to optimize correction on-axis or over the science field of view of the camera. The use of four LGSs, in conjunction with a tomographic reconstructor and pseudo open-loop control, leads to a significant reduction in wavefront error. We describe the overall architecture, development of the tomographic algorithm, real-time implementation and preliminary on-sky results here. By comparing the on-sky image quality with that obtained using a single LGS (sLGS) we clearly demonstrate the benefits of laser tomography, a technology which is crucial to the success of the next generation of extremely large telescopes.

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arXiv2026

DM/WFS mis-registration tracking: Implementation and on-sky validation of SPRINT at LBT

The advent of telescopes with an integrated deformable mirror (DM) presents new challenges for adaptive optics (AO) systems. The alignment between the DM and wavefront sensor (WFS) is expected to regularly evolve during operations due to their large separation. Without tracking and correction, these mis-registrations between the DM and WFS lead to loop instability, preventing diffraction limited performance from being realised. SPRINT\cite{heritier2021} provides an approach to track these mis-registrations during observations. Rotation, shift, and magnification mis-registrations can all be recovered. The Large Binocular Telescope (LBT) currently lacks an operational solution for tracking these mis-registrations, while SPRINT has been selected as the baseline approach for several instruments on the forthcoming Extremely Large Telescope (ELT). We report on the implementation of SPRINT into the LBT real time computer and present experimental results from both daytime and on-sky testing to validate the method.

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arXiv2025

The Simultaneous Operation of a Controllable Segmented Primary Mirror and Single Conjugate Adaptive Optics System part 2 -- Simulated Operation

There are scientific and technological needs to improve the co-phasing of the primary mirrors of segmented telescopes. We have developed a methodology for using the wavefront sensor of an adaptive optics (AO) system to disentangle the phase of a Controllable Segmented Primary mirror (CSP) from the residual phase aberrations to be corrected by the rest of the AO system. We show simulations of the Keck-II AO system where we simultaneously control the CSP surface and a single-conjugate AO system using a pyramid wavefront sensor in a low-bandwidth closed loop, resulting in significantly decreased aberrations on the primary and improvements in the performance of the AO system. Analysis of on-sky telemetry data from the Keck-II AO system validates these simulations and shows that accurate measurements of the primary phase can be made in the presence of real atmospheric turbulence. Ultimately, this work suggests that it is feasible to simultaneously monitor and control the CSP with an AO wavefront sensor while in closed-loop operation and that doing so may significantly improve the performance of the AO system.

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arXiv2024

AI-Powered Low-Order Focal Plane Wavefront Sensing in Infrared

Adaptive optics (AO) systems are crucial for high-resolution astronomical observations by compensating for atmospheric turbulence. While laser guide stars (LGS) address high-order wavefront aberrations, natural guide stars (NGS) remain vital for low-order wavefront sensing (LOWFS). Conventional NGS-based methods like Shack-Hartmann sensors have limitations in field of view, sensitivity, and complexity. Focal plane wavefront sensing (FPWFS) offers advantages, including a wider field of view and enhanced signal-to-noise ratio, but accurately estimating low-order modes from distorted point spread functions (PSFs) remains challenging. We propose an AI-powered FPWFS method specifically for low-order mode estimation in infrared wavelengths. Our approach is trained on simulated data and validated on on-telescope data collected from the Keck I adaptive optic (K1AO) bench calibration source in K-band. By leveraging the enhanced signal-to-noise ratio in the infrared and the power of AI, our method overcomes the limitations of traditional LOWFS techniques. This study demonstrates the effectiveness of AI-based FPWFS for low-order wavefront sensing, paving the way for more compact, efficient, and high-performing AO systems for astronomical observations.

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arXiv2024

A Survey of Protoplanetary Disks Using the Keck/NIRC2 Vortex Coronagraph

Recent Atacama Large Millimeter/submillimeter Array (ALMA) observations of protoplanetary disks in the millimeter continuum have shown a variety of radial gaps, cavities, and spiral features. These substructures may be signposts for ongoing planet formation, and therefore these systems are promising targets for direct imaging planet searches in the near-infrared. To this end, we present results from a deep imaging survey in the $L'$-band (3.8 $\mu$m) with the Keck/NIRC2 vortex coronagraph to search for young planets in 43 disks with resolved features in the millimeter continuum or evidence for gaps/central cavities from their spectral energy distributions. Although we do not detect any new point sources, using the vortex coronagraph allows for high sensitivity to faint sources at small angular separations (down to ${\sim}$0$^{\prime\prime}$.1), allowing us to place strong upper limits on the masses of potential gas giant planets. We compare our mass sensitivities to the masses of planets derived using ALMA observations, and while we are sensitive to $\sim$1 M$_{Jup}$ planets in the gaps in some of our systems, we are generally not sensitive to planets of the masses expected from the ALMA observations. In addition to placing upper limits on the masses of gas giant planets that could be interacting with the dust in the disks to form the observed millimeter substructures, we are also able to map the micron-sized dust as seen in scattered light for 8 of these systems. Our large sample of systems also allows us to investigate limits on planetary accretion rates and disk viscosities.

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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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arXiv2024

LBT SHARK-VIS Observes a Major Resurfacing Event on Io

Since volcanic activity was first discovered on Io from Voyager images in 1979, changes on Io's surface have been monitored from both spacecraft and ground-based telescopes. Here, we present the highest spatial resolution images of Io ever obtained from a ground-based telescope. These images, acquired by the SHARK-VIS instrument on the Large Binocular Telescope, show evidence of a major resurfacing event on Io's trailing hemisphere. When compared to the most recent spacecraft images, the SHARK-VIS images show that a plume deposit from a powerful eruption at Pillan Patera has covered part of the long-lived Pele plume deposit. Although this type of resurfacing event may be common on Io, few have been detected due to the rarity of spacecraft visits and the previously low spatial resolution available from Earth-based telescopes. The SHARK-VIS instrument ushers in a new era of high resolution imaging of Io's surface using adaptive optics at visible wavelengths.

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arXiv2023

Keck/KPIC Emission Spectroscopy of WASP-33b

We present Keck/KPIC high-resolution ($R\sim35,000$) $K$-band thermal emission spectroscopy of the ultra-hot Jupiter WASP-33b. The use of KPIC's single-mode fibers greatly improves both blaze and line-spread stabilities relative to slit spectrographs, enhancing the cross-correlation detection strength. We retrieve the dayside emission spectrum with a nested sampling pipeline which fits for orbital parameters, the atmospheric pressure-temperature profile, and molecular abundances.We strongly detect the thermally-inverted dayside and measure mass-mixing ratios for CO ($\log\rm CO_{MMR} = -1.1^{+0.4}_{-0.6}$), H$_2$O ($\log\rm H_2O_{MMR} = -4.1^{+0.7}_{-0.9}$) and OH ($\log\rm OH_{MMR} = -2.1^{+0.5}_{-1.1}$), suggesting near-complete dayside photodissociation of H$_2$O. The retrieved abundances suggest a carbon- and possibly metal-enriched atmosphere, with a gas-phase C/O ratio of $0.8^{+0.1}_{-0.2}$, consistent with the accretion of high-metallicity gas near the CO$_2$ snow line and post-disk migration or with accretion between the soot and H$_2$O snow lines. We also find tentative evidence for $\rm ^{12}CO/^{13}CO \sim 50$, consistent with values expected in protoplanetary disks, as well as tentative evidence for a metal-enriched atmosphere (2--15$\times$ solar). These observations demonstrate KPIC's ability to characterize close-in planets and the utility of KPIC's improved instrumental stability for cross-correlation techniques.

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arXiv2022

Phase II of the Keck Planet Imager and Characterizer: system-level laboratory characterization and preliminary on-sky commissioning

The Keck Planet Imager and Characterizer (KPIC) is a series of upgrades for the Keck II Adaptive Optics (AO) system and the NIRSPEC spectrograph to enable diffraction-limited, high-resolution ($R>30,000$) spectroscopy of exoplanets and low-mass companions in the K and L bands. Phase I consisted of single-mode fiber injection/extraction units (FIU/FEU) used in conjunction with an H-band pyramid wavefront sensor. Phase II, deployed and commissioned in 2022, adds a 1000-actuator deformable mirror, beam-shaping optics, a vortex coronagraph, and other upgrades to the FIU/FEU. The use of single-mode fibers provides a gain in stellar rejection, a substantial reduction in sky background, and an extremely stable line-spread function on the spectrograph. In this paper we present the results of extensive system-level laboratory testing and characterization showing the instrument's Phase II throughput, stability, repeatability, and other key performance metrics prior to delivery and during installation at Keck. We also demonstrate the capabilities of the various observing modes enabled by the new system modules using internal test light sources. Finally, we show preliminary results of on-sky tests performed in the first few months of Phase II commissioning along with the next steps for the instrument. Once commissioning of Phase II is complete, KPIC will continue to characterize exoplanets at an unprecedented spectral resolution, thereby growing its already successful track record of 23 detected exoplanets and brown dwarfs from Phase I. Using the new vortex fiber nulling (VFN) mode, Phase II will also be able to search for exoplanets at small angular separations less than 45 milliarcseconds which conventional coronagraphs cannot reach.

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arXiv2022

Retrieving C and O Abundance of HR 8799 c by Combining High- and Low-Resolution Data

The formation and evolution pathway for the directly-imaged multi-planetary system HR 8799 remains mysterious. Accurate constraints on the chemical composition of the planetary atmosphere(s) are key to solving the mystery. We perform a detailed atmospheric retrieval on HR 8799~c to infer the chemical abundances and abundance ratios using a combination of photometric data along with low- and high-resolution spectroscopic data (R$\sim$20-35,000). We specifically retrieve [C/H], [O/H], and C/O and find them to be 0.55$^{+0.36}_{-0.39}$, 0.47$^{+0.31}_{-0.32}$, and 0.67$^{+0.12}_{-0.15}$ at 68\% confidence. The super-stellar C and O abundances, yet a stellar C/O ratio, reveal a potential formation pathway for HR 8799~c. Planet c, and likely the other gas giant planets in the system, formed early on (likely within $\sim$1 Myr), followed by further atmospheric enrichment in C and O through the accretion of solids beyond the CO iceline. The enrichment either preceded or took place during the early phase of the inward migration to the planet current locations.

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arXiv2022

AIROPA II: Modeling Instrumental Aberrations for Off-Axis Point Spread Functions in Adaptive Optics

Images obtained with single-conjugate adaptive optics (AO) show spatial variation of the point spread function (PSF) due to both atmospheric anisoplanatism and instrumental aberrations. The poor knowledge of the PSF across the field of view strongly impacts the ability to take full advantage of AO capabilities. The AIROPA project aims to model these PSF variations for the NIRC2 imager at the Keck Observatory. Here, we present the characterization of the instrumental phase aberrations over the entire NIRC2 field of view and we present a new metric for quantifying the quality of the calibration, the fraction of variance unexplained (FVU). We used phase diversity measurements obtained on an artificial light source to characterize the variation of the aberrations across the field of view and their evolution with time. We find that there is a daily variation of the wavefront error (RMS of the residuals is 94~nm) common to the whole detector, but the differential aberrations across the field of view are very stable (RMS of the residuals between different epochs is 59~nm). This means that instrumental calibrations need to be monitored often only at the center of the detector, and the much more time-consuming variations across the field of view can be characterized less frequently (most likely when hardware upgrades happen). Furthermore, we tested AIROPA's instrumental model through real data of the fiber images on the detector. We find that modeling the PSF variations across the field of view improves the FVU metric by 60\% and reduces the detection of fake sources by 70\%.

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arXiv2022

A Clear View of a Cloudy Brown Dwarf Companion from High-Resolution Spectroscopy

Direct imaging studies have mainly used low-resolution spectroscopy ($R\sim20-100$) to study the atmospheres of giant exoplanets and brown dwarf companions, but the presence of clouds has often led to degeneracies in the retrieved atmospheric abundances (e.g. C/O, metallicity). This precludes clear insights into the formation mechanisms of these companions. The Keck Planet Imager and Characterizer (KPIC) uses adaptive optics and single-mode fibers to transport light into NIRSPEC ($R\sim35,000$ in $K$ band), and aims to address these challenges with high-resolution spectroscopy. Using an atmospheric retrieval framework based on petitRADTRANS, we analyze KPIC high-resolution spectrum ($2.29-2.49~μ$m) and archival low-resolution spectrum ($1-2.2~μ$m) of the benchmark brown dwarf HD 4747 B ($m=67.2\pm1.8~M_{\rm{Jup}}$, $a=10.0\pm0.2$ au, $T_{\rm eff}\approx1400$ K). We find that our measured C/O and metallicity for the companion from the KPIC high-resolution spectrum agree with that of its host star within $1-2σ$. The retrieved parameters from the $K$ band high-resolution spectrum are also independent of our choice of cloud model. In contrast, the retrieved parameters from the low-resolution spectrum are highly sensitive to our chosen cloud model. Finally, we detect CO, H$_2$O, and CH$_4$ (volume mixing ratio of log(CH$_4$)=$-4.82\pm0.23$) in this L/T transition companion with the KPIC data. The relative molecular abundances allow us to constrain the degree of chemical disequilibrium in the atmosphere of HD 4747 B, and infer a vertical diffusion coefficient that is at the upper limit predicted from mixing length theory.

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arXiv2022

Daytime calibration and testing of the Keck All sky Precision Adaptive Optics Tomography System

The development of the Keck All sky Precision Adaptive optics (KAPA) project was initiated in September 2018 to upgrade the Keck I adaptive optics (AO) system to enable laser tomography adaptive optics (LTAO) with a four laser guide star (LGS) asterism. The project includes the replacement of the existing LMCT laser with a Toptica laser, the implementation of a new real-time controller (RTC) and wavefront sensor optics and camera, and a new daytime calibration and test platform to provide the required infrastructure for laser tomography. The work presented here describes the new daytime calibration infrastructure to test the performance for the KAPA tomographic algorithms. This paper outlines the hardware infrastructure for daytime calibration and performance assessment of tomographic algorithms. This includes the implementation of an asterism simulator having fiber-coupled light sources simulating four Laser Guide Stars (LGS) and two Natural Guide Stars (NGS) at the AO bench focus, as well as the upgrade of the existing TelSim on the AO bench to simulate focal anisoplanatism and wind driven atmospheric turbulence. A phase screen, that can be adjusted in effective altitude, is used to simulate wind speeds up to 10 m/s for a duration of upto 3 s.

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arXiv2022

AIROPA III: Testing Simulated and On-Sky Data

Adaptive optics images from the W. M. Keck Observatory have delivered numerous influential scientific results, including detection of multi-system asteroids, the supermassive black hole at the center of the Milky Way, and directly imaged exoplanets. Specifically, the precise and accurate astrometry these images yield was used to measure the mass of the supermassive black hole using orbits of the surrounding star cluster. Despite these successes, one of the major obstacles to improved astrometric measurements is the spatial and temporal variability of the point-spread function delivered by the instruments. AIROPA is a software package for the astrometric and photometric analysis of adaptive optics images using point-spread function fitting together with the technique of point-spread function reconstruction. In adaptive optics point-spread function reconstruction, the knowledge of the instrument performance and of the atmospheric turbulence is used to predict the long-exposure point-spread function of an observation. In this paper we present the results of our tests using AIROPA on both simulated and on-sky images of the Galactic Center. We find that our method is very reliable in accounting for the static aberrations internal to the instrument, but it does not improve significantly the accuracy on sky, possibly due to uncalibrated telescope aberrations.

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arXiv2022

Predictive wavefront control on Keck II adaptive optics bench: on-sky coronagraphic results

The behavior of an adaptive optics (AO) system for ground-based high contrast imaging (HCI) dictates the achievable contrast of the instrument. In conditions where the coherence time of the atmosphere is short compared to the speed of the AO system, the servo-lag error can become the dominant error term of the AO system. While the AO system measures the wavefront error and subsequently applies a correction (typically taking a total of one or a few milliseconds), the atmospheric turbulence above the telescope has changed resulting in the servo-lag error. In addition to reducing the Strehl ratio, the servo-lag error causes a build-up of speckles along the direction of the dominant wind vector in the coronagraphic image, severely limiting the contrast at small angular separations. One strategy to mitigate this problem is to predict the evolution of the turbulence over the delay time. Our predictive wavefront control algorithm minimizes, in a mean square sense, the wavefront error over the delay and has been implemented on the Keck II AO bench. In this paper, we report on the latest results of our algorithm and discuss updates to the algorithm itself. We explore how to tune various filter parameters based on both daytime laboratory tests and on-sky tests. We show a reduction in the residual-mean-square wavefront error for the predictor compared to the leaky integrator (the standard controller for Keck) implemented on Keck for three separate nights. Finally, we present contrast improvements for daytime and on-sky tests for the first time. Using the L-band vortex coronagraph for Keck's NIRC2 instrument, we find a contrast gain of up to 2 at a separation of 3 lambda/D and up to 3 for larger separations (3-7 lambda/D).

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arXiv2022

On-sky reconstruction of Keck Primary Mirror Piston Offsets using a Zernike Wavefront Sensor

The next generation of large ground- and space-based optical telescopes will have segmented primary mirrors. Co-phasing the segments requires a sensitive wavefront sensor capable of measuring phase discontinuities. The Zernike wavefront sensor (ZWFS) is a passive wavefront sensor that has been demonstrated to sense segmented-mirror piston, tip, and tilt with picometer precision in laboratory settings. We present the first on-sky results of an adaptive optics fed ZWFS on a segmented aperture telescope, W.M. Keck Observatory's Keck II. Within the Keck Planet Imager and Characterizer (KPIC) light path, the ZWFS mask operates in the H-band using an InGaAs detector (CRED2). We piston segments of the primary mirror by a known amount and measure the mirror's shape using both the ZWFS and a phase retrieval method on data acquired with the facility infrared imager, NIRC2. In the latter case, we employ slightly defocused NIRC2 images and a modified Gerchberg-Saxton phase retrieval algorithm to estimate the applied wavefront error. We find good agreement when comparing the phase retrieval and ZWFS reconstructions, with average measurements of 408 +/- 23 nm and 394 +/- 46 nm, respectively, for three segments pistoned by 400 nm of optical path difference (OPD). Applying various OPDs, we are limited to 100 nm OPD of applied piston due to our observations' insufficient averaging of adaptive optics residuals. We also present simulations of the ZWFS that help explain the systematic offset observed in the ZWFS reconstructed data.

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arXiv2022

Retrieving the C and O Abundances of HR 7672~AB: a Solar-Type Primary Star with a Benchmark Brown Dwarf

A benchmark brown dwarf (BD) is a BD whose properties (e.g., mass and chemical composition) are precisely and independently measured. Benchmark BDs are valuable in testing theoretical evolutionary tracks, spectral synthesis, and atmospheric retrievals for sub-stellar objects. Here, we report results of atmospheric retrieval on a synthetic spectrum and a benchmark BD -- HR 7672~B -- with \petit. First, we test the retrieval framework on a synthetic PHOENIX BT-Settl spectrum with a solar composition. We show that the retrieved C and O abundances are consistent with solar values, but the retrieved C/O is overestimated by 0.13-0.18, which is $\sim$4 times higher than the formal error bar. Second, we perform retrieval on HR 7672~B using high spectral resolution data (R=35,000) from the Keck Planet Imager and Characterizer (KPIC) and near infrared photometry. We retrieve [C/H], [O/H], and C/O to be $-0.24\pm0.05$, $-0.19\pm0.04$, and $0.52\pm0.02$. These values are consistent with those of HR 7672~A within 1.5-$σ$. As such, HR 7672~B is among only a few benchmark BDs (along with Gl 570~D and HD 3651~B) that have been demonstrated to have consistent elemental abundances with their primary stars. Our work provides a practical procedure of testing and performing atmospheric retrieval, and sheds light on potential systematics of future retrievals using high- and low-resolution data.

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arXiv2021

An Imaging Search for Post-Main-Sequence Planets of Sirius B

We present deep imaging of Sirius B, the closest and brightest white dwarf, to constrain post-main-sequence planetary evolution in the Sirius system. We use Keck/NIRC2 in L'-band (3.776 $μ$m) across three epochs in 2020 using the technique of angular differential imaging. Our observations are speckle-limited out to 1 AU and background-limited beyond. The 5$σ$ detection limits from our best performing epoch are 17 to 20.4 L' absolute magnitude. We consider multiple planetary formation pathways in the context of Sirius B's evolution to derive mass sensitivity limits, and achieve sub-Jupiter sensitivities at sub-AU separations, reaching 1.6 $\mathrm{M_J}$ to 2.4 $\mathrm{M_J}$ at 0.5 AU down to a sensitivity of 0.7 $\mathrm{M_J}$ to 1.2 $\mathrm{M_J}$ at >1 AU. Consistent with previous results, we do not detect any companions around Sirius B. Our strong detection limits demonstrate the potential of using high-contrast imaging to characterize nearby white dwarfs.

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