Exoplanet direct imaging using adaptive optics (AO) is often limited by non-common path aberrations (NCPAs) and aberrations that are invisible to traditional pupil-plane wavefront sensors (WFSs). This can be remedied by focal-plane (FP) WFSs that characterize aberrations directly from a final science image. Photonic lanterns (PLs) can act as low-order FPWFSs with the ability to direct some light to downstream science instruments. Using a PL on the SEAL (Santa Cruz Extreme AO Laboratory) high-contrast imaging testbed, we demonstrate (1) linear ranges and (2) closed-loop control. Additionally, we simulate the use of the PL in a multi-wavefront sensor AO system, in which multiple WFSs feed back to the same common-path deformable mirror. Building on previous multi-WFS AO demonstrations on SEAL, we simulate a modulated pyramid WFS to sense aberrations of high spatial order and large amplitude, and the PL to sense low order aberrations including NCPAs. We assess adaptive optics performance in this setting using three different PL wavefront reconstruction algorithms. We also provide a new method to experimentally identify the propagation matrix of a PL, making advanced model-based algorithms practical. This work demonstrates the role of photonic technologies and multi-stage wavefront sensing in the context of extreme AO and high contrast imaging.
Starlight suppression techniques for High-Contrast Imaging (HCI) are crucial to achieving the demanding contrast ratios and inner working angles required for detecting and characterizing exoplanets with a wide range of masses and separations. The advent of photonic technologies provides new opportunities to control the amplitude and phase characteristics of light, with the potential to enhance and control starlight suppression. Here, we present a focal plane optical-fiber-based nulling interferometer working with commercially available components for amplitude and phase modulation. The instrument implements single-mode fiber-coupled elements: a MEMS variable optical attenuator (VOA) matches the on-axis and off-axis starlight amplitude, while a piezoelectric-driven fiber stretcher modifies the optical path difference between the channels to achieve the π phase shift condition for destructive interference. We show preliminary lab results using a narrowband light source working at 632nm and discuss future opportunities for testing on-sky with the Astrophotonics Advancement Platform at Lick Observatory (APALO) at the Shane 3-m Telescope.
Since the start of science operations in 1993, the twin 10-meter W. M. Keck Observatory (WMKO) telescopes have continued to maximize their scientific impact to produce transformative discoveries that keep the U.S. observing community on the frontiers of astronomical research. Upgraded capabilities and new instrumentation are provided though collaborative partnerships primarily with the Caltech and University of California instrument development teams and through additional collaborations with the University of Notre Dame, the University of Hawaii, Swinburne University of Technology, industry, and other organizations. This paper summarizes the status and performance of observatory infrastructure projects, technology upgrades, and new additions to the suite of observatory instrumentation. We also provide a status of instrumentation projects in early and advanced stages of development that will achieve the goals and objectives summarized in the 2023 Keck Observatory strategic plan. Developed in collaboration with the WMKO science community, the Keck strategic plan sets our sites on 2035 and meets goals identified in the Astro2020 Decadal Survey.
Astrophotonics, with its potential for creating low-cost, mass-producible devices, offers a path to dramatically reduce the cost of future astronomical spectrographs. However, coupling the light from large astronomical telescopes into small, photonic chip-based instruments remains a challenge. Photonic lanterns offer a potential solution. Photonic lanterns predictably decompose the inherently multimode light from a ground-based telescope into a series of single-mode outputs, thus eliminating the need for exotic optical elements or extreme AO to achieve high efficiency. We have built a custom assembly for the AO system at Lick Observatory’s 3m Shane Telescope to test photonic lantern behavior on-sky. Here we report on multiple nights of observations over the past year using a lantern with a design wavelength of 1550 nm. Our data reveals the lantern’s basic performance over a 605–1000 nm band and its time domain response to turbulent PSFs with AO correction residuals. These measurements are important for determining the efficacy of future efforts to preferentially select or combine output modes in “real-world” scenarios across scientifically useful bandwidths.
The Affordable Multiple Aperture Spectroscopy Explorer (AMASE) is a planned high-resolution spatially-resolved spectroscopy survey of the interstellar medium in the Milky Way and nearby galaxies. The prototype telescope and instrument system, AMASE-P, is under development. We provide an update on the instrument design and report the status of this project. A major design change from the previous report is the use of fused silica etched grating in place of VPH grating. The new gratings would provide a significant improvement in the high-efficiency bandwidth of the spectrograph. For fiber positioning on the pseudo-slit, we adopted a new design for the fiber slit blocks to reduce the risk of damaging the fibers during the fiber insertion process and to strengthen the mechanical property of the blocks. We have also chosen octagonal fibers for more uniform near-field and far-field light outputs to yield more stable line spread function. We report the progress of the project and challenges we encountered.
Astrophotonic technologies, specifically mass-produced “spectrometers-on-a-chip,” offer an exciting path toward dramatically reducing the cost-per-spectrum of astronomical spectrographs. This technology could one day enable significant multiplexing upgrades to fiber-based instruments and inspire new facilities capable of collecting 100,000 simultaneous spectra in both single-fiber and IFU formats. Here, we report on a new astrophotonics platform at Lick Observatory for on-sky testing of such technologies. Our initial focus is on the problem of efficiently coupling telescope light into photonic devices, which are typically optimized to work with a single mode, i.e., with diffraction-limited light. While photonic lanterns can deliver multiple single-mode outputs given multi-modal input, here we introduce the concept of Adaptive Mode Extraction (AME), which uses a second, reference lantern to select the brightest instantaneous mode or modes for injection into photonic devices. Analogous to “speckle spectroscopy,” this technique has the potential to increase the signal-to-noise ratio for faint sources by spatially filtering out the sky background. We have deployed our testing platform behind the AO system at the Shane Telescope and demonstrate that it meets requirements for our planned on-sky tests of AME, namely the ability to couple AO-corrected light from two nearby stars (within 2′′) into two dynamically-positioned lanterns, with adequate throughput (<40%) and image quality (0.15′′).
The Fibre-Optic Broadband Spectrograph (FOBOS) is a facility-class multi-object spectrograph currently being designed, and to be deployed to the Keck II telescope. FOBOS is able to simultaneously observe 1729-objects across a 20′ field of view, with 30% instrument throughput from 0.31-1.0 µm and a spectral resolution of R<3500 delivered by three, bench-mounted 4-channel spectrographs. The FOBOS focal plane will be configured using 1729 ‘Starbug’ robots, which are vacuum-adhered piezo actuators that ‘walk’ across the field plate to position fibres. Using Starbugs to position fibres allows fast configuration and flexibility in payloads, with a mixture of Single Fibre, IFUs, and Imaging Bundles (used for guiding) capable of being rapidly positioned across the field. The FOBOS team have recently passed their conceptual design review. The FOBOS positioner design builds on the experience gained from the TAIPAN instrument, which used 150 Starbugs and demonstrated their viability as a science instrument. In this paper we detail the conceptual design of the FOBOS focal positioner system. This includes details of the Starbug design, optomechanics, and optical designs that allow the focal plane positioner to operate. The FOBOS focal positioner design builds on the experience gained from TAIPAN, a prototype instrument built to demonstrate the Starbugs technology
Since the start of science operations in 1993, the twin 10-meter W. M. Keck Observatory (WMKO) telescopes have continued to maximize their scientific impact and to produce transformative discoveries that keep the observing community on the frontiers of astronomical research. Upgraded capabilities and new instrumentation are provided though collaborative partnerships with Caltech, the University of California, and the University of Hawaii instrument development teams, as well as industry and other organizations. This paper summarizes the performance of recently commissioned infrastructure projects, technology upgrades, and new additions to the suite of observatory instrumentation. We also provide a status of projects currently in design or development phases and, since we keep our eye on the future, summarize projects in exploratory phases that originate from our 2022 strategic plan developed in collaboration with our science community to adapt and respond to evolving science needs.
We describe the current plans for developing an adaptive secondary mirror-based (ASM) adaptive optics (AO) system for WMKO. An ASM allows for the integration of AO into the telescope itself, broadening use of AO to include wide-field enhanced seeing, high contrast observations, and enabling future multi-conjugate upgrades. Such a system has the potential for enhancing a range of science objectives, improving the performance of both existing and future instrumentation at Keck. We describe a system level ASM-AO concept based on hybrid variable reluctance actuators, developed by TNO that simplifies the implementation of ASM’s.
The Fiber Optic Broad-band Optical Spectrometer (FOBOS) is a high-priority spectroscopic facility concept for the W. M. Keck Observatory. Here, we provide an update on the FOBOS conceptual design. FOBOS will deploy 1800 fibers across the 20-arcminute field-of-view of the Keck II Telescope. Starbugs fiber positioners will be used to deploy individual fibers as well as fiber-bundle arrays (integral field units, IFUs). Different combinations of active single fibers or IFUs can be selected to carry light to one of three mounted spectrographs, each with a 600-fiber pseudoslit. Each spectrograph has four wavelength channels, enabling end-to-end instrument sensitivity greater than 30% from 0.31-1.0 µm at a spectral resolution of R ~ 3500. With its high fiber density on a large telescope and modest field-of-view, FOBOS is optimized to obtain deep spectroscopy for large samples. In single- fiber mode, it will deliver premier spectroscopic reference sets for maximizing the information (e.g., photometric redshifts) that can be extracted from panoramic imaging surveys obtained from the forthcoming Rubin and Roman Observatories. Its IFUs will map emission from the circumgalactic interface between forming galaxies and the intergalactic medium at z ~ 2–3, and lay the path for multiplexed resolved spectroscopy of high-z galaxies aided by ground-layer and multi-object adaptive optics. In the nearby universe, its high sampling density and combination of single-fiber and IFU modes will revolutionize our understanding of the M31 disk and bulge via stellar populations and kinematics. Finally, with a robust and intelligent target and program allocation system, FOBOS will be a premier facility for follow-up of rare, faint, and transient sources that can be interleaved into its suite of observing programs. With a commitment to delivering science-ready data products, FOBOS will enable unique and powerful combinations of focused, PI-led programs and community-driven observing campaigns that promise major advances in cosmology, galaxy formation, time-domain astronomy, and stellar evolution.
Starbugs are robotic devices that have the capability to simultaneously position many optical fibers, over the telescope’s focal plane to carry-out efficient spectroscopic surveys. The conceptual design of FOBOS, the Fiber-Optic Broadband Optical Spectrograph, deploys Starbugs at the Keck II focal plane to enable high-multiplex, deep spectroscopic follow-up of upcoming deep-imaging surveys. FOBOS requires configured fields of many-hundreds of targets (significantly more than TAIPAN and MANIFEST instruments) in a few minutes, consistent with typical detector readout times. FOBOS also requires the inclusion of different optical payloads, like integral field-units, calibration bundles, coherent imaging bundles and perhaps wavefront sensors. Therefore, with these new challenges, it is important to optimize the target allocation and routing algorithms for Starbugs that yield the best configuration times and science outcomes for FOBOS. We provide a description of the Starbug parameters required by the FOBOS conceptual design, perform relevant allocation simulations, and discuss their performance.
We present the design of the prototype telescope and spectrograph system for the Affordable Multiple Aperture Spectroscopy Explorer (AMASE) project. AMASE is a planned project that will pair 100 identical multi-fiber spectrographs with a large array of telephoto lenses to achieve a large area integral field spectroscopy survey of the sky at the spatial resolution of half an arcminute and a spectral resolution of R=15,000, covering important emission lines in the optical for studying the ionized gas in the Milky Way and beyond. The project will be enabled by a significant reduction in the cost of each spectrograph unit, which is achieved by reducing the beam width and the use of small-pixel CMOS detectors, 50µm-core optical fibers, and commercial photographic lenses in the spectrograph. Although constrained by the challenging high spectral resolution requirement, we realize a 40% reduction in cost per fiber at constant etendue relative to, e.g., DESI. As the reduction of cost is much more significant than the reduction in the amount of light received per fiber, replicating such a system many times is more cost effective than building a single large spectrograph that achieves the same survey speed. We present the design of the prototype telescope and instrument system and the study of its cost effectiveness.
An Adaptive secondary mirror (ASM) allows for the integration of adaptive optics (AO) into the telescope itself. Adaptive secondary mirrors, based on hybrid variable reluctance (HVR) actuator technology, developed by TNO, provide a promising path to telescope-integrated AO. HVR actuators have the advantage of allowing mirrors that are sti↵er, more power ecient, and potentially less complex than similar, voice-coil based ASM’s. We are exploring the application of this technology via a laboratory testbed that will validate the technical approach. In parallel, we are developing conceptual designs for ASMs at several telescopes including the Automated Planet Finder Telescope (APF) and for Keck Observatory. An ASM for APF has the potential to double the light through the slit for radial velocity measurements, and dramatically improved the image stability. An ASM for WMKO enables ground layer AO correction and lower background infrared AO observations, and provides for more flexible deployment of instruments via the ability to adjust the location of the Cassegrain focus.
Spectrographs are integral in panoramic surveys. An optimized spectrograph design can facilitate the observation of faint objects. One such optimization lies in its bundle of optical fibers and their numerical apertures (NA). Low NA fibers are less commonly used and studied, but can be advantageous in terms of cost and precision. Here, we describe the properties of 0.12 NA and 0.22 NA fibers with different input beam f-ratio, quantities of wraps, and bend radii.
We investigate an image slicer module for an optical multiobject spectrograph, wide-field optical spectrograph (WFOS), which is one of the first-light instruments of the Thirty Meter Telescope (TMT). The image slicer divides the target image into three slices, thus providing a one-third narrower slit width. By positioning a suite of such modules at the telescope focal surface, multiobject spectroscopy with high spectral resolution can be achieved. Three optical designs are developed: a two-mirror design, a four-mirror design, and a flat-mirror design. Comparing them, the flat-mirror design is found to be the most preferable for WFOS. From a tolerance analysis, the tolerances of manufacturing and assembling appear challenging but not insurmountable. We describe how the steep field curvature of TMT requires at least nine module variants, tuned to reduce defocus in specific focal surface zones. Finally, we introduce a viable mechanical packaging concept.
Ground-layer adaptive optics (GLAO) systems offer the possibility of improving the ”seeing” of large ground-based telescopes and increasing the efficiency and sensitivity of observations over a wide field-of-view. We explore the utility and feasibility of deploying a GLAO system at the W. M. Keck Observatory in order to feed existing and future multi-object spectrographs and wide-field imagers. We also briefly summarize science cases spanning exoplanets to high-redshift galaxy evolution that would benefit from a Keck GLAO system. Initial simulations indicate that a Keck GLAO system would deliver a 1.5x and 2x improvement in FWHM at optical (500 nm) and infrared (1.5
μm), respectively. The infrared instrument, MOSFIRE, is ideally suited for a Keck GLAO feed in that it has excellent image quality and is on the telescope’s optical axis. However, it lacks an atmospheric dispersion compensator, which would limit the minimum usable slit size for long-exposure science cases. Similarly, while LRIS and DEIMOS may be able to accept a GLAO feed based on their internal image quality, they lack either an atmospheric dispersion compensator (DEIMOS) or flexure compensation (LRIS) to utilize narrower slits matched to the GLAO image quality. However, some science cases needing shorter exposures may still benefit from Keck GLAO and we will investigate the possibility of installing an ADC.
The Wide Field Optical Spectrometer (WFOS) is a seeing limited, multi-object spectrograph and first light instrument for the Thirty Meter Telescope (TMT) scheduled for first observations in 2027. The spectrograph will deliver a minimum resolution of R~5,000 over a simultaneous wavelength range of 310 nm to 1,000 nm with a multiplexing goal of between 20 and 700 targets. The WFOS team consisting of partners in China, India, Japan, and the United States has completed a trade study of two competing concepts intended to meet the design requirements derived from the WFOS detailed science case. The first of these design concepts is a traditional slit mask instrument capable of delivering R~1,000 for up to 100 simultaneous targets using 1 x 7 arc second slits, and a novel focal plane slicing method for R~5,000 on up to 20 simultaneous targets can be achieved by reformatting the 1 arc-second wide slits into three 0.3 arc-second slits projected next to each other in the spatial direction. The second concept under consideration is a highly multiplexed fiber based system utilizing a robotic fiber positioning system at the focal plane containing 700 individual collectors, and a cluster of up to 12 replicated spectrographs with a minimum resolution of R~5,000 over the full pass band. Each collecting element will contain a bundle of 19 fibers coupled to micro-lens arrays that allow for contiguous coverage of targets and adaptation of the f/15 telescope beam to f/3.2 for feeding the fiber system. This report describes the baseline WFOS design, provides an overview of the two trade study concepts, and the process used to down-select between the two options. Also included is a risk assessment regarding the known technical challenges in the selected design concept.
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