The SPHERE XAO system SAXO: integration, test, and laboratory performance

The next generation of giant ground and space telescopes will have the light-collecting power to detect and characterize potentially habitable terrestrial exoplanets using high-contrast imaging for the first time. This will only be achievable if the performance of the Giant Segment Mirror Telescopes (GSMTs) extreme adaptive optics (ExAO) systems are optimized to their full potential. A key component of an ExAO system is the wavefront sensor (WFS), which measures aberrations from atmospheric turbulence. A common choice in current and next-generation instruments is the pyramid wavefront sensor (PWFS). ExAO systems require high spatial and temporal sampling of wavefronts to optimize performance and, as a result, require large detectors for the WFS. We present a closed-loop testbed demonstration of a three-sided pyramid wavefront sensor (3PWFS) as an alternative to the conventional four-sided pyramid wavefront (4PWFS) sensor for GSMT-ExAO applications on the innovative comprehensive adaptive optics and coronagraph test instrument (CACTI). The 3PWFS is less sensitive to read noise than the 4PWFS because it uses fewer detector pixels. The 3PWFS has further benefits: a high-quality three-sided pyramid optic is easier to manufacture than a four-sided pyramid. We describe the design of the two components of the CACTI system, the adaptive optics simulator and the PWFS testbed that includes both a 3PWFS and 4PWFS. We detail the error budget of the CACTI system, review its operation and calibration procedures, and discuss its current status. A preliminary experiment was performed on CACTI to study the performance of the 3PWFS to the 4PWFS in varying strengths of turbulence using both the raw intensity and slopes map signal processing methods. This experiment was repeated for a modulation radius of 1.6 and 3.25 λ / D. We found that the performance of the two wavefront sensors is comparable if modal loop gains are tuned.

Context.The direct imaging of potentially habitable exoplanets is one prime science case for the next generation of high contrast imaging instruments on ground-based, extremely large telescopes. To reach this demanding science goal, the instruments are equipped with eXtreme Adaptive Optics (XAO) systems which will control thousands of actuators at a framerate of kilohertz to several kilohertz. Most of the habitable exoplanets are located at small angular separations from their host stars, where the current control laws of XAO systems leave strong residuals.Aims.Current AO control strategies such as static matrix-based wavefront reconstruction and integrator control suffer from a temporal delay error and are sensitive to mis-registration, that is, to dynamic variations of the control system geometry. We aim to produce control methods that cope with these limitations, provide a significantly improved AO correction, and, therefore, reduce the residual flux in the coronagraphic point spread function (PSF).Methods.We extend previous work in reinforcement learning for AO. The improved method, called the Policy Optimization for Adaptive Optics (PO4AO), learns a dynamics model and optimizes a control neural network, called a policy. We introduce the method and study it through numerical simulations of XAO with Pyramid wavefront sensor (PWFS) for the 8-m and 40-m telescope aperture cases. We further implemented PO4AO and carried out experiments in a laboratory environment using Magellan Adaptive Optics eXtreme system (MagAO-X) at the Steward laboratory.Results.PO4AO provides the desired performance by improving the coronagraphic contrast in numerical simulations by factors of 3–5 within the control region of deformable mirror and PWFS, both in simulation and in the laboratory. The presented method is also quick to train, that is, on timescales of typically 5–10 s, and the inference time is sufficiently small (<ms) to be used in real-time control for XAO with currently available hardware even for extremely large telescopes.

Although extreme adaptive optics (ExAO) systems can greatly reduce the effects of atmospheric turbulence and deliver diffraction-limited images, our ability to observe faint objects such as extrasolar planets or debris disks at small angular separations is greatly limited by the presence of a speckle halo caused by imperfect wavefront corrections. These speckles change with a variety of timescales, from milliseconds to many hours, and various techniques have been developed to mitigate them during observations and during data reduction. Detection limits improve with increased speckle reduction, so an understanding of how speckles evolve (particularly at near-infrared wavelengths, which is where most adaptive optics science instruments operate) is of distinct interest. We used a SAPHIRA detector behind Subaru Telescope’s SCExAO instrument to collect H-band images of the ExAO-corrected point-spread function (PSF) at a frame rate of 1.68 kHz. We analyzed these images using two techniques to measure the timescales over which the speckles evolved. In the first technique, we analyzed the images in a manner applicable to predicting performance of real-time speckle-nulling loops. We repeated this analysis using data from several nights to account for varying weather and AO conditions. In our second analysis, which follows the techniques employed by Milli et al. () but using data with three orders of magnitude better temporal resolution, we identified a new regime of speckle behavior that occurs at timescales of milliseconds. It is not purely an instrument effect and likely is an atmospheric timescale filtered by the ExAO response. We also observed an exponential decay in the Pearson’s correlation coefficients (which we employed to quantify the change in speckles) on timescales of seconds and a linear decay on timescales of minutes, which is in agreement with the behavior observed by Milli et al. For both of our analyses, we also collected similar data sets using SCExAO’s internal light source to separate atmospheric effects from instrumental effects.

RISTRETTO is a visible high-resolution spectrograph fed by an extreme adaptive optics (AO) system, to be proposed as a visitor instrument on ESO VLT. The main science goal of RISTRETTO is the detection and atmospheric characterisation of exoplanets in reflected light, in particular the temperate rocky planet Proxima b. RISTRETTO will be able to measure albedos and detect atmospheric features in a number of exoplanets orbiting nearby stars for the first time. It will do so by combining a high-contrast AO system working at the diffraction limit of the telescope to a high-resolution spectrograph, via a 7-spaxel integral-field unit (IFU) feeding single-mode fibers. Further science cases for RISTRETTO include the study of accreting protoplanets such as PDS70b/c through spectrally-resolved H-alpha emission, and spatially-resolved studies of Solar System objects such as icy moons and the ice giants Uranus and Neptune. The project is in an advanced design phase for the spectrograph and IFU/fiber-link sub-systems, and a preliminary design phase for the AO front-end. Construction of the spectrograph and IFU/fiber-link will start early 2022. RISTRETTO is a pathfinder instrument in view of similar developments at the ELT, in particular the SCAO-IFU mode of ELT-HIRES and the future ELT-PCS instrument.

SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch) is the second-generation VLT instrument devoted primarily to direct imaging and characterization of exoplanets, and allowing a large number of promising observation modes. In this framework, an IDL-based end-to-end numerical tool has been developed within the problem-solving environment CAOS (or CAOS system): the Software Package SPHERE, dedicated to the complete and detailed simulation of the whole instrument. It hence includes detailed instrumental modeling of the extreme adaptive optics system SAXO, the dual-band imaging camera IRDIS, the Integral Field Spectrograph (IFS), and the Zurich IMaging POLarimeter (ZIMPOL). The status of the whole package (which has now reached version 3.0) is exposed. An example of application is also detailed: a sub-system study aspect concerning the near-infrared apodized Lyot coronagraph.

The Integrated Optical System (IOS) is an extreme adaptive optics system designed for NASA’s Laser Com- munication Relay Demonstration mission. There is a great deal of overlap between the requirements for laser communication AO and high-contrast exoplanet imaging AO systems. Both require very high Strehl ratios with narrow fields of view. This overlap allows the IOS to serve as a testbed and technology demonstrator for astronomical extreme adaptive optics systems. There are several example technologies from the IOS that are already making the transition to astronomical AO systems. The first is that the real time controller based on Direct Memory Access transfer between the WFS camera link frame-grabber and a DSP board is being reused on the upgrade to PALM-3000 AO system at Palomar Observatory. This enables the system to minimize latency by bypassing the CPU and its inherent timing jitter. Technologies like this will be crucial to enabling high contrast imaging on the next generation of extremely large telescopes. In addition, the IOS measures Fried’s parameter from wavefraont measures in near real time. This technology has already been deployed to PALM-3000. The main function of Laser Communication AO systems is to couple the incoming light into single mode fiber. This is the same configuration that will be used by AO coupled radial velocity spectrographs. The adaptive optics system is a woofer/tweeter design, with one deformable mirror correcting for low spatial frequencies with large amplitude and a second deformable mirror correcting for high spatial frequencies with small amplitude. The system uses a Shack-Hartmann wavefront sensor. The system has achieved first light and is undergoing commissioning. We will present an overview of the system design and initial performance.

Imaging faint objects, such as exoplanets or disks, around nearby stars is extremely challenging because host star images are dominated by the telescope diffraction pattern. Using a coronagraph is an efficient solution for removing diffraction but requires an incoming wavefront with good quality to maximize starlight rejection. On the ground, the most advanced exoplanet imagers use extreme adaptive optics (ExAO) systems that are based on a deformable mirror (DM) with a large number of actuators to efficiently compensate for high-order aberrations and provide diffraction-limited images. While several exoplanet imagers with DMs using ∼1500 actuators are now routinely operating on large telescopes to observe gas giant planets, future systems may require a tenfold increase in the number of degrees of freedom to look for rocky planets. In this paper, we explore wavefront correction with a secondary adaptive optics system that controls a very large number of degrees of freedom that are not corrected by the primary ExAO system. Using Marseille Imaging Testbed for High Contrast (MITHiC), we implement a second stage of adaptive optics with ZELDA, a Zernike wavefront sensor, and a spatial light modulator to compensate for the phase aberrations of the bench downstream residual aberrations from adaptive optics. We demonstrate that their correction up to 137 cycles per pupil with nanometric accuracy is possible, provided there is a simple distortion calibration of the pupil and a moderate wavefront low-pass filtering. We also use ZELDA for a fast compensation of ExAO residuals, showing its promising implementation as a second-stage correction for the observation of rocky planets around nearby stars. Finally, we present images with a classical Lyot coronagraph on MITHiC and validate our ability to reach its theoretical performance with our calibration.

The detection of extrasolar planets implies an extremely high-contrast, long-exposure imaging capability at near infrared and probably visible wavelengths. We present here the core of any Planet Finder instrument, that is, the extreme adaptive optics (XAO) subsystem. The level of AO correction directly impacts the exposure time required for planet detection. In addition, the capacity of the AO system to calibrate all the instrument static defects ultimately limits detectivity. Hence, the extreme AO system has to adjust for the perturbations induced by the atmospheric turbulence, as well as for the internal aberrations of the instrument itself. We propose a feasibility study for an extreme AO system in the frame of the SPHERE (Spectro-Polarimetry High-contrast Exoplanet Research) instrument, which is currently under design and should equip one of the four VLT 8-m telescopes in 2010.

The next generation of adaptive optics (AO) systems, often referred to as extreme adaptive optics (ExAO), will use higher numbers of actuators to achieve wavefront correction levels below 100 nm, and so enable a host of new observations such as high-contrast coronagraphy. However, the number of potential coronagraph types is increasing rapidly, and selection of the most advantageous coronagraph is subject to many factors. Here it is pointed out that experiments in the ExAO regime can already be carried out with existing hardware, by using a well-corrected subaperture on an existing telescope. For example, by magnifying a 1.5 m diameter off-axis subaperture onto the AO system’s deformable mirror (DM) on the Palomar Hale telescope, we have recently achieved stellar Strehl ratios as high as 92% to 94%, corresponding to wavefront errors of 85 - 100 nm. Using this approach, a wide variety of ExaO experiments can thus be carried out well before “next genera tion” ExAO systems are deployed on large telescopes. The potential experiments include infrared ExAO imaging and performance optimization, a comparison of coronagraphic approaches in the ExAO regime, visi ble wavelength AO, and predictive AO. Keywords: Extreme adaptive optics

We present a novel method for distributed wavefront reconstruction from spatial derivatives of the wavefront with application to large-scale adaptive optics systems, like the extreme adaptive optics (XAO) system of the EPICS instrument of the European Extremely Large Telescope (E-ELT). The new method, indicated as D-SABRE (Distributed Spline based ABerration REconstruction), is an extension of the recently presented SABRE method for wavefront reconstruction which uses B-spline basis functions defined on triangular partitions. D-SABRE aims to not only greatly increase the computational efficiency of the wavefront reconstruction operation, but also to remove the need for centralized reconstruction. The D-SABRE is demonstrated on simulated E-ELT turbulence wavefronts. We show that the D-SABRE obtains a reconstruction accuracy that is within 1% of the reconstruction accuracy of a global (non-distributed) method.

SPHERE (Spectro-Polarimetric High Contrast Exoplanet Research) is one of the first instruments which aim for the direct detection from extra-solar planets. SPHERE commissioning is foreseen in 2013 on the VLT. ZIMPOL (Zurich Imaging Polarimeter) is the high contrast imaging polarimeter subsystem of the ESO SPHERE instrument. ZIMPOL is dedicated to detect the very faint reflected and hence polarized visible light (600-900 nm) from extrasolar planets. It is located behind an extreme AO system (SAXO) and a stellar coronagraph. We present the first high contrast polarimetric results obtained for the fully integrated SPHERE-ZIMPOL system. We have measured the polarimetric high contrast performance of several coronagraphs: a Classical Lyot on substrate, a suspended Classical Lyot and two 4 Quadrant Phase Mask coronagraphs. We describe the impact of crucial system parameters – Adaptive Optics, Coronagraphy and Polarimetry - on the contrast performance.

Integral field spectroscopy coupled with an extreme adaptive optics system and coronagraphy allows a marked improvement of the standard spectroscopic simultaneous differential imaging calibration technique. Hence, with an integral field spectrograph (IFS) direct imaging of extrasolar giant planets becomes potentially feasible over a wide range of ages, masses, and separations from the hosting stars. This aim represents the prime goal of the planet finder instrument for the VLT (SPHERE). Inside SPHERE, the IFS channel exploits various spectral features of the candidate planets in the near infrared, in order to reduce the speckles noise at the level of the stellar background noise, over a field of view comprised between the coronagraphic inner working angle and the outer working angle provided by the SPHERE extreme adaptive optic system (SAXO). The IFS allows then to realize an extensive spectroscopic simultaneous differential imaging calibration technique, and at least in few cases, to get the spectrum of the candidate extrasolar giant planets. Here we present the IFS baseline design, which is based upon a new optical concept we developed for its integral field unit (BIGRE). When applied to the technical specifications of SPHERE IFS, a BIGRE integral field unit is able to take into account all the effects appearing when integral field spectroscopy is used in diffraction limited conditions and for high-contrast imaging purposes. Finally a BIGRE-oriented IFS optical design is shown here to reach the requested high optical quality by standard lenses-based optical devices.

The European Extremely Large Telescope (ELT) is a 39m large, ground-based optical and near- to mid-infrared telescope under construction in the Chilean Atacama desert. Operation is planned to start around the middle of the next decade. All first light instruments will come with wavefront sensing devices that allow control of the ELT’s intrinsic M4 and M5 wavefront correction units, thus building an adaptive optics (AO) system. To take advantage of the ELT’s optical performance, full diffraction-limited operation is required and only a high performance AO system can deliver this. Further technically challenging requirements for the AO come from the exoplanet research field, where the task to resolve the very small angular separations between host star and planet, has also to take into account the high-contrast ratio between the two objects. We present in detail the results of our simulations and their impact on high-contrast imaging in order to find the optimal wavefront sensing device for the METIS instrument. METIS is the mid-infrared imager and spectrograph for the ELT with specialised high-contrast, coronagraphic imaging capabilities, whose performance strongly depends on the AO residual wavefront errors. We examined the sky and target sample coverage of a generic wavefront sensor in two spectral regimes, visible and near-infrared, to pre-select the spectral range for the more detailed wavefront sensor type analysis. We find that the near-infrared regime is the most suitable for METIS. We then analysed the performance of Shack-Hartmann and pyramid wavefront sensors under realistic conditions at the ELT, did a balancing with our scientific requirements, and concluded that a pyramid wavefront sensor is the best choice for METIS. For this choice we additionally examined the impact of non-common path aberrations, of vibrations, and the long-term stability of the SCAO system including high-contrast imaging performance.

Direct detections of Earth-like extrasolar planets are extremely challenging and require to overcome the huge brightness contrast between two sources that have a very small angular separation. One possible solution to this problem is nulling interferometry at mid-infrared wavelengths where the flux ratio between host star and planet is more favorable than in the visible. The beams of an array of telescopes are combined so that the light from the on-axis direction (the star) is canceled by destructive interference, while the light from an off-axis direction (the planet) is kept. The global performance of such a system depends strongly on the accuracy and stability of the achromatic phase shift and the beam combination. To assess the technological feasibility of this technique, the European Space Agency (ESA) and IAS Paris have initiated a study of different physical concepts and technical realizations of achromatic phase shifters (APS) that fulfill the following requirements: allowing a >10^-6 rejection rate or better over a wavelength range 6-20μm and providing a transmission better than 95%. MPIA, in collaboration with the Kayser-Threde GmbH in Munich and the IOF Fraunhofer institute for applied optics in Jena has breadboarded and studied a phase shifter that is based on the geometric reversal of the electric field vectors (pupil flip) at two successive antisymmetric 90 degree reflections. In this paper we describe the bread-boarded phase-shifter device and the results of our characterization measurements in the Lab.

An Adaptive Optic (AO) system is a fundamental requirement of 8m-class telescopes. We know that in order to obtain the maximum possible resolution allowed by these telescopes we need to correct the atmospheric turbulence. Thanks to adaptive optic systems we are able to use all the effective potential of these instruments, drawing all the information from the universe sources as best as possible. In an AO system there are two main components: the wavefront sensor (WFS) that is able to measure the aberrations on the incoming wavefront in the telescope, and the deformable mirror (DM) that is able to assume a shape opposite to the one measured by the sensor. The two subsystem are connected by the reconstructor (REC). In order to do this, the REC requires a “common language" between these two main AO components. It means that it needs a mapping between the sensor-space and the mirror-space, called an interaction matrix (IM). Therefore, in order to operate correctly, an AO system has a main requirement: the measure of an IM in order to obtain a calibration of the whole AO system. The IM measurement is a 'mile stone' for an AO system and must be done regardless of the telescope size or class. Usually, this calibration step is done adding to the telescope system an auxiliary artificial source of light (i.e a fiber) that illuminates both the deformable mirror and the sensor, permitting the calibration of the AO system. For large telescope (more than 8m, like Extremely Large Telescopes, ELTs) the fiber based IM measurement requires challenging optical setups that in some cases are also impractical to build. In these cases, new techniques to measure the IM are needed. In this PhD work we want to check the possibility of a different method of calibration that can be applied directly on sky, at the telescope, without any auxiliary source. Such a technique can be used to calibrate AO system on a telescope of any size. We want to test the new calibration technique, called “sinusoidal modulation technique”, on the Large Binocular Telescope (LBT) AO system, which is already a complete AO system with the two main components: a secondary deformable mirror with by 672 actuators, and a pyramid wavefront sensor. My first phase of PhD work was helping to implement the WFS board (containing the pyramid sensor and all the auxiliary optical components) working both optical alignments and tests of some optical components. Thanks to the “solar tower” facility of the Astrophysical Observatory of Arcetri (Firenze), we have been able to reproduce an environment very similar to the telescope one, testing the main LBT AO components: the pyramid sensor and the secondary deformable mirror. Thanks to this the second phase of my PhD thesis: the measure of IM applying the sinusoidal modulation technique. At first we have measured the IM using a fiber auxiliary source to calibrate the system, without any kind of disturbance injected. After that, we have tried to use this calibration technique in order to measure the IM directly “on sky”, so adding an atmospheric disturbance to the AO system. The results obtained in this PhD work measuring the IM directly in the Arcetri solar tower system are crucial for the future development: the possibility of the acquisition of IM directly on sky means that we are able to calibrate an AO system also for extremely large telescope class where classic IM measurements technique are problematic and, sometimes, impossible. Finally we have not to forget the reason why we need this: the main aim is to observe the universe. Thanks to these new big class of telescopes and only using their full capabilities, we will be able to increase our knowledge of the universe objects observed, because we will be able to resolve more detailed characteristics, discovering, analyzing and understanding the behavior of the universe components.