Showing papers on "Wavefront sensor published in 2018"

High Contrast Imaging for Python (HCIPy): an open-source adaptive optics and coronagraph simulator

Abstract: HCIPy is a package written in Python for simulating the interplay between wavefront control and coronagraphic systems. By defining an element which merges values/coefficients with its sampling grid/modal basis into a single object called Field, this minimizes errors in writing the code and makes it clearer to read. HCIPy provides a monochromatic Wavefront and defines a Propagator that acts as the transformation between two wavefronts. In this way a Propagator acts as any physical part of the optical system, be it a piece of free space, a thin complex apodizer or a microlens array. HCIPy contains Fraunhofer and Fresnel propagators through free space. It includes an implementation of a thin complex apodizer, which can modify the phase and/or amplitude of a wavefront, and forms the basis for more complicated optical elements. Included in HCIPy are wavefront errors (modal, power spectra), complex apertures (VLT, Keck or Subaru pupil), coronagraphs (Lyot, vortex or apodizing phase plate coronagraph), deformable mirrors, wavefront sensors (Shack-Hartmann, Pyramid, Zernike or phase-diversity wavefront sensor) and multi-layer atmospheric models including scintillation). HCIPy aims to provide an easy-to-use, modular framework for wavefront control and coronagraphy on current and future telescopes, enabling rapid prototyping of the full high-contrast imaging system. Adaptive optics and coronagraphic systems can be easily extended to include more realistic physics. The package includes a complete documentation of all classes and functions, and is available as open-source software.

Generalized Hartmann-Shack array of dielectric metalens sub-arrays for polarimetric beam profiling.

Abstract: To define and characterize optical systems, obtaining the amplitude, phase, and polarization profile of optical beams is of utmost importance. Traditional polarimetry is well established to characterize the polarization state. Recently, metasurfaces have successfully been introduced as compact optical components. Here, we take the metasurface concept to the system level by realizing arrays of metalenses, allowing the determination of the polarization profile of an optical beam. We use silicon-based metalenses with a numerical aperture of 0.32 and a mean measured focusing efficiency in transmission mode of 28% at a wavelength of 1550 nm. Our system is extremely compact and allows for real-time beam diagnostics by inspecting the foci amplitudes. By further analyzing the foci displacements in the spirit of a Hartmann-Shack wavefront sensor, we can simultaneously detect phase-gradient profiles. As application examples, we diagnose the profiles of a radially polarized beam, an azimuthally polarized beam, and of a vortex beam. Obtaining information on the amplitude, phase and polarization profile of optical beams is of huge interest. Here, the authors create a generalized Hartmann-Shack array with metalenses which measures phase and phase-gradient profiles of optical beams but also measures spatial polarization profiles at the same time.

Generalized Hartmann-Shack array of dielectric metalens sub-arrays for polarimetric beam profiling

Abstract: To define and characterize optical systems, obtaining information on the amplitude, phase, and polarization profile of optical beams is of utmost importance. Polarimetry using bulk optics is well established to characterize the polarization state. Recently, metasurfaces and metalenses have successfully been introduced as compact optical components. Here, we take the metasurface concept to the system level by realizing arrays of metalens 2*3 sub-arrays, allowing to determine the polarization profile of an optical beam. We use silicon-based metalenses with a numerical aperture of 0.32 and a mean measured diffraction efficiency in transmission mode of 28% at 1550 nm wavelength. Together with a standard camera recording the array foci, our system is extremely compact and allows for real-time beam diagnostics by inspecting the foci amplitudes. By further analyzing the foci displacements in the spirit of a Hartmann-Shack wavefront sensor, we can simultaneously detect phase-gradient profiles. As application examples, we diagnose the polarization profiles of a radially polarized beam, an azimuthally polarized beam, and of a vortex beam.

High-accuracy wavefront sensing for x-ray free electron lasers

Abstract: Systematic understanding and real-time feedback capability for x-ray free electron laser (FEL) accelerator and optical components are critical for scientific experiments and instrument performance. Single-shot wavefront sensing enables characterization of the intensity and local electric field distribution at the sample plane, something that is important for understanding scientific experiments such as nonlinear studies. It can also provide feedback for alignment and tuning of the FEL beam and instrumentation optics, leading to optimal instrument performance and greater operational efficiency. A robust, sensitive, and accurate single-shot wavefront sensor for x-ray FEL beams using single grating Talbot interferometry has been developed. Experiments performed at the Linac Coherent Light Source (LCLS) demonstrate 3σ sensitivity and accuracy, both better than λ/100, and retrieval of hard x-ray (λ=0.13 nm, E=9.5 keV) wavefronts in 3D. Exhibiting high performance from both unfocused and focused beams, the same setup can be used to systematically study the wavefront from the FEL output, beam transport optics, and endstation focusing optics. This technique can be extended for use with softer and harder x rays with modified grating configurations.

Adaptive optics with an infrared Pyramid wavefront sensor

Abstract: Wavefront sensing in the infrared is highly desirable for the study of M-type stars and cool red objects, as they are sufficiently bright in the infrared to be used as the adaptive optics guide star This aids in high contrast imaging, particularly for low mass stars where the star-to-planet brightness ratio is reduced Here we discuss the combination of infrared detector technology with the highly sensitive Pyramid wavefront sensor (WFS) for a new generation of systems Such sensors can extend the capabilities of current telescopes and meet the requirements for future instruments, such as those proposed for the giant segmented mirror telescopes Here we introduce the infrared Pyramid WFS and discuss the advantages and challenges of this sensor We present a new infrared Pyramid WFS for Keck, a key sub-system of the Keck Planet Imager and Characterizer (KPIC) The design, integration and testing is reported on, with a focus on the characterization of the SAPHIRA detector used to provide the H-band wavefront sensing Initial results demonstrate a required effective read noise <1e– at high gain

Centroid computation for Shack-Hartmann wavefront sensor in extreme situations based on artificial neural networks.

Abstract: This paper proposes a method used to calculate centroid for Shack-Hartmann wavefront sensor (SHWFS) in adaptive optics (AO) systems that suffer from strong environmental light and noise pollutions. In these extreme situations, traditional centroid calculation methods are invalid. The proposed method is based on the artificial neural networks that are designed for SHWFS, which is named SHWFS-Neural Network (SHNN). By transforming spot detection problem into a classification problem, SHNNs first find out the spot center, and then calculate centroid. In extreme low signal-noise ratio (SNR) situations with peak SNR (SNRp) of 3, False Rate of SHNN-50 (SHNN with 50 hidden layer neurons) is 6%, and that of SHNN-900 (SHNN with 900 hidden layer neurons) is 0%, while traditional methods’ best result is 26 percent. With the increase of environmental light interference’s power, the False Rate of SHNN-900 remains around 0%, while traditional methods’ performance decreases dramatically. In addition, experiment results of the wavefront reconstruction are presented. The proposed SHNNs achieve significantly improved performance, compared with the traditional method, the Root Mean Square (RMS) of residual decreases from 0.5349 um to 0.0383 um. This method can improve SHWFS’s robustness.

Optical and mechanical design of the extreme AO coronagraphic instrument MagAO-X

Abstract: Here we review the current optical mechanical design of MagAO-X. The project is post-PDR and has finished the design phase. The design presented here is the baseline to which all the optics and mechanics have been fabricated. The optical/mechanical performance of this novel extreme AO design will be presented here for the first time. Some highlights of the design are: 1) a floating, but height stabilized, optical table; 2) a Woofer tweeter (2040 actuator BMC MEMS DM) design where the Woofer can be the current f/16 MagAO ASM or, more likely, fed by the facility f/11 static secondary to an ALPAO DM97 woofer; 3) 22 very compact optical mounts that have a novel locking clamp for additional thermal and vibrational stability; 4) A series of four pairs of super-polished off-axis parabolic (OAP) mirrors with a relatively wide FOV by matched OAP clocking; 5) an advanced very broadband (0.5-1.7μm) ADC design; 6) A Pyramid (PWFS), and post-coronagraphic LOWFS NCP wavefront sensor; 7) a vAPP coronagraph for starlight suppression. Currently all the OAPs have just been delivered, and all the rest of the optics are in the lab. Most of the major mechanical parts are in the lab or instrument, and alignment of the optics has occurred for some of the optics (like the PWFS) and most of the mounts. First light should be in early 2019.

Spectrally resolved single-shot wavefront sensing of broadband high-harmonic sources.

Abstract: Wavefront sensors are an important tool to characterize coherent beams of extreme ultraviolet radiation. However, conventional Hartmann-type sensors do not allow for independent wavefront characterization of different spectral components that may be present in a beam, which limits their applicability for intrinsically broadband high-harmonic generation (HHG) sources. Here we introduce a wavefront sensor that measures the wavefronts of all the harmonics in a HHG beam in a single camera exposure. By replacing the mask apertures with transmission gratings at different orientations, we simultaneously detect harmonic wavefronts and spectra, and obtain sensitivity to spatiotemporal structure such as pulse front tilt as well. We demonstrate the capabilities of the sensor through a parallel measurement of the wavefronts of 9 harmonics in a wavelength range between 25 and 49 nm, with up to λ/32 precision.

Hartmann wavefront sensor characterization of a high charge vortex beam in the extreme ultraviolet spectral range.

Abstract: We demonstrate for the first time, to the best of our knowledge, the ability of extreme ultraviolet (XUV) Hartmann wavefront sensors to characterize high charge vortex beams produced by high-order harmonic generation up to the order of 25. We also show that phase matched absorption limited high harmonic generation is able to maintain the high charge vortex structure of the XUV beam even in a rather long (1 cm) generation medium.

Fabrication of an infrared Shack-Hartmann sensor by combining high-speed single-point diamond milling and precision compression molding processes.

Abstract: A novel fabrication method by combining high-speed single-point diamond milling and precision compression molding processes for fabrication of discontinuous freeform microlens arrays was proposed. Compared with slow tool servo diamond broaching, high-speed single-point diamond milling was selected for its flexibility in the fabrication of true 3D optical surfaces with discontinuous features. The advantage of single-point diamond milling is that the surface features can be constructed sequentially by spacing the axes of a virtual spindle at arbitrary positions based on the combination of rotational and translational motions of both the high-speed spindle and linear slides. By employing this method, each micro-lenslet was regarded as a microstructure cell by passing the axis of the virtual spindle through the vertex of each cell. An optimization arithmetic based on minimum-area fabrication was introduced to the machining process to further increase the machining efficiency. After the mold insert was machined, it was employed to replicate the microlens array onto chalcogenide glass. In the ensuing optical measurement, the self-built Shack-Hartmann wavefront sensor was proven to be accurate in detecting an infrared wavefront by both experiments and numerical simulation. The combined results showed that precision compression molding of chalcogenide glasses could be an economic and precision optical fabrication technology for high-volume production of infrared optics.

Efficient, nonlinear phase estimation with the nonmodulated pyramid wavefront sensor

Abstract: The sensitivity of the pyramid wavefront sensor (PyWFS) has made it a popular choice for astronomical adaptive optics (AAO) systems. The PyWFS is at its most sensitive when it is used without modulation of the input beam. In nonmodulated mode, the device is highly nonlinear. Hence, all PyWFS implementations on current AAO systems employ modulation to make the device more linear. The upcoming era of 30-m class telescopes and the demand for ultra-precise wavefront control stemming from science objectives that include direct imaging of exoplanets make using the PyWFS without modulation desirable. This article argues that nonlinear estimation based on Newton’s method for nonlinear optimization can be useful for mitigating the effects of nonlinearity in the nonmodulated PyWFS. The proposed approach requires all optical modeling to be pre-computed, which has the advantage of avoiding real-time simulations of beam propagation. Further, the required real-time calculations are amenable to massively parallel computation. Numerical experiments simulate a PyWFS with faces sloped 3.7° to the horizontal, operating at a wavelength of 0.85 μm, and with an index of refraction of 1.45. A singular value analysis shows that the common practice of calculating two “slope” images from the four PyWFS pupil images discards critical information and is unsuitable for the nonmodulated PyWFS simulated here. Instead, this article advocates estimators that use the raw pixel values not only from the four geometrical images of the pupil, but from surrounding pixels as well. The simulations indicate that nonlinear estimation can be effective when the Strehl ratio of the input beam is greater than 0.3, and the improvement relative to linear estimation tends to increase at larger Strehl ratios. At Strehl ratios less than about 0.5, the performances of both the nonlinear and linear estimators are relatively insensitive to noise since they are dominated by nonlinearity error.

Hartmann-Shack wavefront sensing without a lenslet array using a digital micromirror device.

Abstract: The common Hartmann–Shack wavefront sensor makes use of a lenslet array to sample in-parallel optical wavefronts. Here, we introduce a Hartmann–Shack wavefront sensor that employs a digital micromirror device in combination with a single lens for serial sampling by scanning. Sensing is analyzed numerically and validated experimentally using a deformable mirror operated in closed-loop adaptive optics with a conventional Hartmann–Shack wavefront sensor, as well as with a set of ophthalmic trial lenses, to generate controllable amounts of monochromatic aberrations. The new sensor is free of crosstalk and can potentially operate at kilohertz speed. It offers a reconfigurable aperture that can exclude unwanted parts of the wavefront.

WFIRST low order wavefront sensing and control dynamic testbed performance under the flight like photon flux

Abstract: To maintain the required performance for the WFIRST Coronagraph Instrument (CGI) in a realistic space environment, a Low Order Wavefront Sensing and Control (LOWFS/C) subsystem is necessary. The WFIRST CGI LOWFS/C subsystem will use the Zernike wavefront sensor, which has a phase-shifting disk combined with the coronagraph’s focal plane mask, to sense the low-order wavefront drift and line-of-sight (LoS) error using the rejected starlight. The dynamic tests on JPL’s Occulting Mask Coronagraph (OMC) Testbed have demonstrated that LOWFS/C can maintain coronagraph contrast to better than 10-8 in presence of WFIRST-like line of sight and low order wavefront disturbances in both Shaped Pupil Coronagraph (SPC) and Hybrid Lyot Coronagraph (HLC) modes. However, the previous dynamic tests have been done using a bright source with photon flux equivalent to stellar magnitude of MV = -3.5. The LOWFS/C technology development on the OMC testbed has since then concentrated in evaluating and improving the LOWFS/C performance under the realistic photon flux that is equivalent to WFIRST Coronagraph target stars. Our recent testbed tests have demonstrated that the LOWFS/C can work cohesively with the stellar light suppression wavefront control, which brings broad band coronagraph contrast from ~1x10-6 to 6x10-9, while LOWF/C is simultaneously suppressing the WFIRST like LoS and low order wavefront drift disturbances on a source that photon flux is equivalent to a MV = 2 star. This lab demonstration mimics the CGI initial dark hole establish process on a bright reference star. We have also demonstrated on the testbed that LOWFS/C can maintain the coronagraph contrast by suppressing the WFIRST like line-of-sight disturbances on a fainter MV = 5 star. This mimics scenario of CGI science target observations. In this paper we will present the recent dynamic testbed performance results of LOWFS/C LoS loops and low order wavefront error correction loop on the flight like photon flux.

Fast Coherent Differential Imaging on Ground-based Telescopes Using the Self-coherent Camera

Abstract: Direct imaging and spectral characterization of exoplanets using extreme adaptive optics (ExAO) is a key science goal of future extremely large telescopes and space observatories. However, quasi-static wavefront errors will limit the sensitivity of this endeavor. Additional limitations for ground-based telescopes arise from residual AO-corrected atmospheric wavefront errors, generating millisecond-lifetime speckles that average into a halo over a long exposure. A solution to both of these problems is to use the science camera of an ExAO system as a wavefront sensor to perform a fast measurement and correction method to minimize these aberrations as soon as they are detected. We develop the framework for one such method based on the self-coherent camera (SCC) to be applied to ground-based telescopes, called the Fast Atmospheric SCC Technique (FAST). We show that with the use of a specially designed coronagraph and coherent differential imaging algorithm, recording images every few milliseconds allows for a subtraction of atmospheric and static speckles while maintaining an algorithmic exoplanet throughput close to unity. Detailed simulations reach a contrast close to the photon noise limit after 30 s for a 1% bandpass in the H band on both zeroth and fifth magnitude stars. For the latter case, this is about 110 times better in raw contrast than what is currently achieved from ExAO instruments if we extrapolate for an hour of observing time, illustrating that the improvement in sensitivity from this method could play an important role in the future detection and characterization of lower mass exoplanets.

Wavefront control architecture and expected performance for the TMT Planetary Systems Imager

Abstract: The Planetary Systems Imager (PSI) is a modular instrument optimized for direct imaging and characterization of exoplanet and disks with the Thirty Meter Telescope (TMT). PSI will operate across a wide wavelength range (≈0.6 - 5μm) to image exoplanets and circumstellar disks in both reflected light and thermal emission. Thanks to the TMT’s large collecting area, PSI will have the sensitivity to directly image and spectrally characterize large gaseous planets with unprecedented sensitivity. PSI will also be capable of imaging rocky planets in the habitable zones of the nearest M-type stars in reflected light and search for biomarkers in their atmospheres. Imaging habitable planets in reflected light is PSI’s most challenging goal, requiring high contrast imaging (HCI) capabilities well beyond what current instruments achieve. This science goal drives PSI’s wavefront sensing and control requirements and defines the corresponding architecture discussed in this paper. We show that PSI must deliver 1e-5 image contrast ≈15 mas separation at λ ≈ 1μm-1.5μm, and that a conventional extreme-AO architecture relying on a single high speed wavefront sensor (WFS) is not sufficient to meet this requirement. We propose a wavefront control architecture relying on both visible light (λ 1.1 μm) sensors to address wavefront chromaticity terms and provide high contrast imaging capability. We show that this combination will enable speckle halo suppression at the < 1e-5 raw contrast level in near-IR, allowing detection and spectroscopic characterization of potentially habitable exoplanets orbiting nearby M-type stars.

Wavefront Sensing in Space: Flight Demonstration II of the PICTURE Sounding Rocket Payload

Abstract: A NASA sounding rocket for high-contrast imaging with a visible nulling coronagraph, the Planet Imaging Concept Testbed Using a Rocket Experiment (PICTURE) payload, has made two suborbital attempts to observe the warm dust disk inferred around Epsilon Eridani. The first flight in 2011 demonstrated a 5 mas fine pointing system in space. The reduced flight data from the second launch, on November 25, 2015, presented herein, demonstrate active sensing of wavefront phase in space. Despite several anomalies in flight, postfacto reduction phase stepping interferometer data provide insight into the wavefront sensing precision and the system stability for a portion of the pupil. These measurements show the actuation of a 32 × 32-actuator microelectromechanical system deformable mirror. The wavefront sensor reached a median precision of 1.4 nm per pixel, with 95% of samples between 0.8 and 12.0 nm per pixel. The median system stability, including telescope and coronagraph wavefront errors other than tip, tilt, and piston, was 3.6 nm per pixel, with 95% of samples between 1.2 and 23.7 nm per pixel.

MEC: the MKID exoplanet camera for high contrast astronomy at Subaru (Conference Presentation)

Abstract: Direct Imaging of exoplanets is one of the most technically difficult techniques used to study exoplanets, but holds immense promise for not just detecting but characterizing planets around the nearest stars. Ambitious instruments at the world’s largest telescopes have been built to carry out this science: the Gemini Planet Imager (GPI), SPHERE at VLT, SCExAO at Subaru, and the P1640 and Stellar Double Coronagraph (SDC) at Palomar. These instruments share a common archetype consisting of an extreme AO system feeding a coronagraph for on-axis stellar light rejection followed by a focal plane Integral Field Spectrograph (IFS). They are currently limited by uncontrolled scattered and diffracted light which produces a coherent speckle halo in the image plane. A number of differential imaging schemes exist to mitigate these issues resulting in star-planet contrast ratios as deep as ~10^-6 at low angular separations. Surpassing this contrast limit requires high speed active speckle nullification from a focal plane wavefront sensor (FPWS) and new processing techniques. MEC, the Microwave Kinetic Inductance Detector (MKID) Exoplanet Camera, is a J-band IFS module behind Subaru Telescope’s SCExAO system. MEC is capable of producing an image cube several thousand times a second without the read noise that dominates conventional high speed IFUs. This enables it to integrate with SCExAO as an extremely fast FPWS while eliminating non-common path aberrations by doubling as a science camera. Key science objectives can be further explored if longer wavelengths (H and K band) are simultaneously sent to CHARIS for high resolution spectroscopy. MEC, to be commissioned at Subaru in early 2018, is the second MKID IFS for high contrast imaging following DARKNESS’ debut at Palomar in July 2016. MEC will follow up on young planets and debris disks discovered in the SEEDS survey or by Project 1640 as well as discover self-luminous massive planets. The increased sensitivity, combined with the advanced coronagraphs in SCExAO which have inner working angles (IWAs) as small as 0.03” at 1.2 μm, allows young Jupiter-sized objects to be imaged as close as 4 AU from their host star. If the wavefront control enabled by MEC is fully realized, it may begin to probe the reflected light of giant planets around some nearby stars, opening a new parameter space for direct imaging targeting older stars. While direct imaging of reflected light exoplanets is the most challenging of the scientific goals, it is a promising long-term path towards characterization of habitable planets around nearby stars using Extremely Large Telescopes (ELTs). With diameters of about 30-m, an ELT can resolve the habitable zones of nearby M-type stars, for which an Earth-sized planet would be at ~10^-7 contrast at 1 μm. This will complement future space-based high contrast optical imaging targeting the wider habitable zones of sun-like stars for ~10^-10 contrast earth analogs. We will present lessons learned from the first few months of MEC’s operation including initial lab and on-sky (weather permitting) results. We already have preliminary data from Palomar testing a new statistical speckle discrimination post-processing technique using the photon arrival time measured with MKIDs. Residual stellar light in the form of a speckle masquerading as a planetary companion is pulled from a modified Rician distribution and can be statistically discerned from a true off-axis Poisson point source. Additionally, the progress of active focal plane wavefront control will be briefly discussed.

A near-infrared pyramid wavefront sensor for Keck adaptive optics: real-time controller

Abstract: A new real-time control system will be implemented within the Keck II adaptive optics system to support the new near-infrared pyramid wavefront sensor. The new real-time computer has to interface with an existing, very productive adaptive optics system. We discuss our solution to install it in an operational environment without impacting science. This solution is based on an independent SCExAO-based pyramid wavefront sensor realtime processor solution using the hardware interfaces provided by the existing Keck II real-time controller. We introduce the new pyramid real-time controller system design, its expected performance, and the modification of the operational real-time controller to support the pyramid system including interfacing with the existing deformable and tip-tilt mirrors. We describe the integration of the Saphira detector-based camera and the Boston Micromachines kilo-DM in this new architecture. We explain the software architecture and philosophy, the shared memory concept and how the real-time computer uses the power of GPUs for adaptive optics control. We discuss the strengths and weaknesses of this architecture and how it can benefit other projects. The motion control of the devices deployed on the Keck II adaptive optics bench to support the alignment of the light on the sensors is also described. The interfaces, developed to deal with the rest of the Keck telescope systems in the observatory distributed system, are reviewed. Based on this experience, we present which design ideas could have helped us integrate the new system with the previous one and the resultant performance gains.

A modal approach to optical gain compensation for the pyramid wavefront sensor

Abstract: Extremely Large Telescopes are making Pyramid Wavefront Sensors (PWFS) the preferred engineering choice for Adaptive Optics designs, such as the MICADO camera SCAO subsystem currently developed at LESIA A major PWFS issue is the so-called Optical Gain (OG) effect: PWFSs experience a nonlinearity-induced sensitivity reduction – of 60% or worse at the fitting error on standard atmospheric conditions – which degrades as the turbulence residual increases OG affects system performance, jeopardizes loop stability and prevents efficient non-common path aberration compensation We investigate a modal approach to OG impact mitigation, and investigate its impact on nonlinearity error depending on the AO control basis We evidence that scalar gain compensation of the OG is insufficient on high order systems, as the high spatial frequency range spanned covers high OG value discrepancies over the controlled basis We quantify the performance improvements obtained with OG modal compensation by end-to-end numerical simulations Finally, we propose a modelization of OG modal compensation coefficients, in order to allow their computation on-the-fly provided telemetry of the immediate turbulence conditions is available

Advanced wavefront reconstruction methods for segmented Extremely Large Telescope pupils using pyramid sensors

Abstract: The generation of Extremely Large Telescopes (ELTs) with mirror diameters up to 40 m has thick secondary mirror support structures (also known as spider legs), which cause difficulties in the wavefront reconstruction process. These spider legs create areas where the information of the phase is disconnected on the wavefront sensor detector, leading to pupil fragmentation and a loss of data on selected subapertures. The effects on wavefront reconstruction are differential pistons between segmented areas, leading to poor wavefront reconstruction. The resulting errors make the majority of existing control algorithms unfeasible for telescope systems having spider legs incorporated. A solution, named the split approach, is presented, which suggests to separate reconstruction of segment piston modes from the rest of the wavefront. Further, two methods are introduced for the direct reconstruction of the segment pistons. Due to the separate handling of the piston offsets on the segments, the split approach makes any of the existing phase reconstruction algorithms developed for nonsegmented pupils suitable for wavefront control in the presence of telescope spiders. We present end-to-end simulation results showing accurate, stable, and extremely fast wavefront reconstruction for the first light instrument mid-infrared ELT imager and spectograph of the ELT that is currently under construction.

Combined hardware and computational optical wavefront correction.

Abstract: In many optical imaging applications, it is necessary to overcome aberrations to obtain high-resolution images. Aberration correction can be performed by either physically modifying the optical wavefront using hardware components, or by modifying the wavefront during image reconstruction using computational imaging. Here we address a longstanding issue in computational imaging: photons that are not collected cannot be corrected. This severely restricts the applications of computational wavefront correction. Additionally, performance limitations of hardware wavefront correction leave many aberrations uncorrected. We combine hardware and computational correction to address the shortcomings of each method. Coherent optical backscattering data is collected using high-speed optical coherence tomography, with aberrations corrected at the time of acquisition using a wavefront sensor and deformable mirror to maximize photon collection. Remaining aberrations are corrected by digitally modifying the coherently-measured wavefront during imaging reconstruction. This strategy obtains high-resolution images with improved signal-to-noise ratio of in vivo human photoreceptor cells with more complete correction of ocular aberrations, and increased flexibility to image at multiple retinal depths, field locations, and time points. While our approach is not restricted to retinal imaging, this application is one of the most challenging for computational imaging due to the large aberrations of the dilated pupil, time-varying aberrations, and unavoidable eye motion. In contrast with previous computational imaging work, we have imaged single photoreceptors and their waveguide modes in fully dilated eyes with a single acquisition. Combined hardware and computational wavefront correction improves the image sharpness of existing adaptive optics systems, and broadens the potential applications of computational imaging methods.

On-sky compensation of non-common path aberrations with the ZELDA wavefront sensor in VLT/SPHERE

Abstract: Circumstellar environments are now routinely observed by dedicated high-contrast imagers on large, ground-based observatories. These facilities combine extreme adaptive optics and coronagraphy to achieve unprecedented sensitivities for exoplanet detection and spectral characterization. However, non-common path aberrations (NCPA) in these coronagraphic systems represent a critical limitation for the detection of giant planets with a contrast lower than a few $10^{-6}$ at very small separations ($<$0.3$^{\prime\prime}$) from their host star. In 2013 we proposed ZELDA, a Zernike wavefront sensor to measure these residual quasi-static phase aberrations and a prototype was installed in SPHERE, the exoplanet imager for the VLT. In 2016, we demonstrated the ability of our sensor to provide a nanometric calibration and compensation for these aberrations on an internal source in the instrument, resulting in a contrast gain of 10 at 0.2$^{\prime\prime}$ in coronagraphic images. However, initial on-sky tests in 2017 did not show a tangible gain in contrast when calibrating the NCPA internally and then applying the correction on sky. In this communication, we present recent on-sky measurements to demonstrate the potential of our sensor for the NCPA compensation during observations and quantify the contrast gain in coronagraphic data.

Ingot Laser Guide Stars Wavefront Sensing

Abstract: We revisit one class of z-invariant WaveFront sensor where the LGS is fired aside of the telescope aperture. In this way there is a spatial dependence on the focal plane with respect to the height where the resonant scattering occurs. We revise the basic parameters involving the geometry and we propose various merit functions to define how much improvement can be attained by a z-invariant approach. We show that refractive approaches are not viable and we discuss several solutions involving reflective ones in what has been nicknamed "ingot wavefront sensor" discussing the degrees of freedom required to keep tracking and the basic recipe for the optical design.

Compensation for the orbital angular momentum of a vortex beam in turbulent atmosphere by adaptive optics

Abstract: A method which can be used to compensate for a distorted orbital angular momentum and wavefront of a beam in atmospheric turbulence, simultaneously, has been proposed. To confirm the validity of the method, an experimental setup for up-link propagation of a vortex beam in a turbulent atmosphere has been simulated. Simulation results show that both of the distorted orbital angular momentum and the distorted wavefront of a beam due to turbulence can be compensated by an adaptive optics system with the help of a cooperative beacon at satellite. However, when the number of the lenslet of wavefront sensor (WFS) and the actuators of the deform mirror (DM) is small, satisfactory results cannot be obtained.

Single Conjugate Adaptive Optics for METIS

Abstract: METIS is the Mid-infrared Extremely large Telescope Imager and Spectrograph, one of the first generation instruments of ESO’s 39m ELT. All scientific observing modes of METIS require adaptive optics (AO) correction close to the diffraction limit. Demanding constraints are introduced by the foreseen coronagraphy modes, which require highest angular resolution and PSF stability. Further design drivers for METIS and its AO system are imposed by the wavelength regime: observations in the thermal infrared require an elaborate thermal, baffling and masking concept. METIS will be equipped with a Single-Conjugate Adaptive Optics (SCAO) system. An integral part of the instrument is the SCAO module. It will host a pyramid type wavefront sensor, operating in the near-IR and located inside the cryogenic environment of the METIS instrument. The wavefront control loop as well as secondary control tasks will be realized within the AO Control System, as part of the instrument. Its main actuators will be the adaptive quaternary mirror and the field stabilization mirror of the ELT. In this paper we report on the phase B design work for the METIS SCAO system; the opto-mechanical design of the SCAO module as well as the control loop concepts and analyses. Simulations were carried out to address a number of important aspects, such as the impact of the fragmented pupil of the ELT on wavefront reconstruction. The trade-off that led to the decision for a pyramid wavefront sensor will be explained, as well as the additional control tasks such as pupil stabilization and compensation of non-common path aberrations.

Development of Components for Adaptive Optics Systems for Solar Telescopes

Abstract: Wavefront aberrations at the entrance aperture of the Large Solar Vacuum Telescope were measured with a wavefront sensor of the adaptive optics system by a sunspot. To calculate the image shifts, a correlation algorithm with quadratic interpolation of the correlation function maximum position is used. The quality of astronomical vision, characterized by the Fried length, was estimated from the same experimental data as the statistical characteristics of the fluctuations of the coefficients of expansion of wavefront aberrations in Zernicke polynomials. The results were obtained at a Fried length of 51.6 mm in a sample 43 s long with a sampling frequency of 70 Hz. The means and standard deviations of the expansion coefficients are calculated. The analysis of the given spectra shows that the wavefront aberrations should be compensated in the frequency band 0–20 Hz for the effective correction of the images formed.

The deformable mirror demonstration mission (DeMi) CubeSat: optomechanical design validation and laboratory calibration

Abstract: Coronagraphs on future space telescopes will require precise wavefront correction to detect Earth-like exoplanets near their host stars. High-actuator count microelectromechanical system (MEMS) deformable mirrors provide wavefront control with low size, weight, and power. The Deformable Mirror Demonstration Mission (DeMi) payload will demonstrate a 140 actuator MEMS Deformable Mirror (DM) with 5:5 μm maximum stroke. We present the flight optomechanical design, lab tests of the flight wavefront sensor and wavefront reconstructor, and simulations of closed-loop control of wavefront aberrations. We also present the compact flight DM controller, capable of driving up to 192 actuator channels at 0-250V with 14-bit resolution. Two embedded Raspberry Pi 3 compute modules are used for task management and wavefront reconstruction. The spacecraft is a 6U CubeSat (30 cm x 20 cm x 10 cm) and launch is planned for 2019.