Improvement of phase diversity algorithm for non-common path calibration in extreme AO context

The paper deals with with the on-sky performance of the pyramid wavefront sensor-based Adaptive Optics (AO) systems. These wavefront sensors are of great importance, being used in all first light AO systems of the ELTs (E-ELT, GMT, and TMT), currently in design phase. In particular, non-common path aberrations (NCPAs) are a critical issue encountered when using an AO system to produce corrected images in an associated astronomical instrument. The AO wavefront sensor (WFS) and the supported scientific instrument typically use a series of different optical elements, thus experiencing different aberrations. The usual way to correct for such NCPAs is to introduce a static offset in the WFS signals. In this way, when the AO loop is closed the sensor offsets are zeroed and the deformable mirror converges to the shape required to null the NCPA. The method assumes that the WFS operation is linear and completely described by some pre-calibrated interaction matrix. This is not the case for some frequently used wavefront sensors like the Pyramid sensor or a quad-cell Shack-Hartmann sensor. Here we present a method to work in closed-loop with a pyramid wavefront sensor, or more generally a non-linear WFS, introducing a wavefront offset that remains stable when AO correction quality changes due to variations in external conditions like star brightness, seeing, and wind speed. The paper details the methods with analytical and numerical considerations. Then we present results of tests executed at the LBT telescope, in daytime and on sky, using the FLAO system and LUCI2 facility instrument. The on-sky results clearly show the successful operation of the method that completely nulls NCPA, recovering diffraction-limited images with about 70% Strehl ratio in H band in variable seeing conditions. The proposed method is suitable for application to the above-mentioned ELT AO systems.

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

It is well known that using two or more deformable mirrors (DMs) can improve the compensation ability of an adaptive optics (AO) system. However, to keep the stability of an AO system, the correlation between the multiple DMs must be suppressed during the correction. In this paper, we proposed a slope-based decoupling algorithm to simultaneous control the multiple DMs. In order to examine the validity and practicality of this algorithm, a typical woofer-tweeter (W-T) AO system was set up. For the W-T system, a theory model was simulated and the results indicated in theory that the algorithm we presented can selectively make woofer and tweeter correct different spatial frequency aberration and suppress the cross coupling between the dual DMs. At the same time, the experimental results for the W-T AO system were consistent with the results of the simulation, which demonstrated in practice that this algorithm is practical for the AO system with dual DMs.

Wavefront sensorless adaptive optics system for extended objects based on linear phase diversity technique

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.

A key challenge of high contrast imaging (HCI) is to differentiate a speckle from an exoplanet signal. The sources of speckles are a combination of atmospheric residuals and aberrations in the non-common path. Those non-common path aberrations (NCPA) are particularly challenging to compensate for as they are not directly measured, and because they include static, quasi-static and dynamic components. The proposed method directly addresses the challenge of compensating the NCPA. The algorithm DrWHO - Direct Reinforcement Wavefront Heuristic Optimisation - is a quasi-real-time compensation of static and dynamic NCPA for boosting image contrast. It is an image-based lucky imaging approach, aimed at finding and continuously updating the ideal reference of the wavefront sensor (WFS) that includes the NCPA, and updating this new reference to the WFS. Doing so changes the point of convergence of the AO loop. We introduce here the upgrade concept of the algorithm. DrWHO does not rely on any model nor requires accurate wavefront sensor calibration, and is applicable to non-linear wavefront sensing situations. We present on-sky performances using a pyramid WFS sensor with the Subaru coronagraph extreme AO (SCExAO) instrument.

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 Programmable Liquid-crystal Active Coronagraphic Imager for the DAG telescope (PLACID) will be the world’s first ever “adaptive stellar coronagraph” facility – with “adaptive” referring to the instrument’s unique ability to adjust in real-time to changing observing conditions.The PLACID project achieved a major milestone by being delivered to the Turkish National Observatories (TNO) facilities at Atatürk University, Erzurum in March of 2024 and subsequently being installed on the diffraction-limited Nasmyth platform of the new national observatory, the 4-m DAG (Doğu Anadolu Gözlemevi) telescope during the first semester of 2025. The Assembly, Integration and Validation (AIV) activities are currently ongoing with the internal light source, with first light anticipated by the fall/end of 2025.PLACID consists of a fore-optics coronagraphic intermediate stage platform, positioned between the TROIA extreme adaptive optics (XAO) system and the DIRAC HAWAII-1RG focal-plane array. Its core component is a customized spatial light modulator (SLM) functioning as a programmable scalar focal-plane phase mask (FPM) coronagraph, operating from H- to Ks-band. The dynamically addressed SLM enables software-only adjustments or re-alignments of the FPM pattern, entirely free of actuator motion, offering observers a highly flexible and reconfigurable system.Preparing for the first observations, the PLACID graphics user interface (GUI) is currently being finalized, while the PLACID on-sky discovery space was estimated, based on laboratory measurements at factory acceptance, as well as simulations with post-AO residual wavefront errors. This was conceived to enable the Turkish and visitor astronomers to plan future high-contrast imaging observations. The science discovery space estimate has been derived using an up-to-date exposure time calculator for PLACID, considering key parameters such as adaptive optics performance, contrast levels, limiting magnitudes, and coronagraphic inner working angle.The instrument is particularly promising for exoplanet and circumstellar disk imaging, while new developments of the PLACID GUI are in the works for coronagraphic imaging of binary or multiple star systems — a valuable niche science case for PLACID. Development of the PLACID data reduction software is also currently underway, based on existing high-contrast imaging software architecture (PynPoint).In parallel to PLACID first science observations in 2026, future development avenues for the instrument include:Self-calibration of non-common path aberrations (NCPAs) using a phase-shifting Zernike wavefront sensor, Coronagraphic nulling tailored for binary or triple star systems, compatible with angular differential imaging (ADI), Time-domain coherent differential imaging (CDI). We present the delivered PLACID instrument and its discovery space, detailing its current on-site commissioning status, and — if available at the time — the first on-sky results.

Current high-contrast "extreme" adaptive optics (ExAO) systems are partially limited by deformable mirror technology. Mirror requirements specify thousands of actuators, all of which must be functional within the clear aperture, and which give nanometer flatness yet micron stroke when operated in closed loop.¹ Micro-electrical mechanical-systems (MEMS) deformable mirrors have been shown to meet ExAO actuator yield, wavefront error, and cost considerations. This study presents the performance of Boston Micromachines' 1024-actuator continuous-facesheet MEMS deformable mirrors under tests for actuator stability, position repeatability, and practical operating stroke. To explore whether MEMS actuators are susceptible to temporal variation, a series of long-term stability experiments were conducted. Each actuator was held fixed and the motion over 40 minutes was measured. The median displacement of all the actuators tested was 0.08 nm surface, inclusive of system error. MEMS devices are also appealing for adaptive optics architectures based on open-loop correction. In experiments of actuator position repeatability, 100% of the tested actuators returned repeatedly to their starting point with a precision of < 1 nm surface. Finally, MEMS devices were tested for maximum stroke achieved under application of spatially varying one-dimensional sinusoids. Given a specified amplitude in voltage, the measured stroke was 1 &#956;m surface at the low spatial frequencies, decreasing to 0.2 &#956;m surface for the highest spatial frequency. Stroke varied somewhat linearly as inverse spatial frequency, with a flattening in the relation at the high spatial frequency end.

One of the top priorities in observational astronomy is the direct imaging and characterization of extrasolar planets (exoplanets) and planetary systems. Direct images of rocky exoplanets are of particular interest in the search for life beyond the Earth, but they tend to be rather challenging targets since they are orders-of-magnitude dimmer than their host stars and are separated by small angular distances that are comparable to the classical λ/D diffraction limit, even for the coming generation of 30 m class telescopes. Current and planned efforts for ground-based direct imaging of exoplanets combine high-order adaptive optics (AO) with a stellar coronagraph observing at wavelengths ranging from the visible to the mid-IR. The primary barrier to achieving high contrast with current direct imaging methods is quasi-static speckles, caused largely by non-common path aberrations (NCPAs) in the coronagraph optical train. Recent work has demonstrated that millisecond imaging, which effectively "freezes" the atmosphere's turbulent phase screens, should allow the wavefront sensor (WFS) telemetry to be used as a probe of the optical system to measure NCPAs. Starting with a realistic model of a telescope with an AO system and a stellar coronagraph, this paper provides simulations of several closely related regression models that take advantage of millisecond telemetry from the WFS and coronagraph's science camera. The simplest regression model, called the naïve estimator, does not treat the noise and other sources of information loss in the WFS. Despite its flaws, in one of the simulations presented herein, the naïve estimator provides a useful estimate of an NCPA of ∼0.5 radian RMS (≈λ/13), with an accuracy of ∼0.06 radian RMS in 1 min of simulated sky time on a magnitude 8 star. The bias-corrected estimator generalizes the regression model to account for the noise and information loss in the WFS. A simulation of the bias-corrected estimator with 4 min of sky time included an NCPA of ∼0.05 radian RMS (≈λ/130) and an extended exoplanet scene. The joint regression of the bias-corrected estimator simultaneously achieved an NCPA estimate with an accuracy of ∼5×10-3 radian RMS and an estimate of the exoplanet scene that was free of the self-subtraction artifacts typically associated with differential imaging. The 5σ contrast achieved by imaging of the exoplanet scene was ∼1.7×10-4 at a distance of 3λ/D from the star and ∼2.1×10-5 at 10λ/D. These contrast values are comparable to the very best on-sky results obtained from multi-wavelength observations that employ both angular differential imaging (ADI) and spectral differential imaging (SDI). This comparable performance is despite the fact that our simulations are quasi-monochromatic, which makes SDI impossible, nor do they have diurnal field rotation, which makes ADI impossible. The error covariance matrix of the joint regression shows substantial correlations in the exoplanet and NCPA estimation errors, indicating that exoplanet intensity and NCPA need to be estimated self-consistently to achieve high contrast.

The detection and characterization of Earth-like exoplanets is one of the major science drivers for the next generation of telescopes. Current direct imaging instruments are limited by evolving Non Common Path Aberrations (NCPA). A promising sensor is the Self Coherent Camera (SCC) that uses a pinhole in the Lyot plane to modulate the focal plane electric field. However, the SCC has low throughput due to the large separation between the pinhole and the edge of the pupil. The spectrally modulated Self Coherent Camera solves this issue by modulating the pinhole along the spectral direction. The SM-SCC can place the pinhole directly at the edge of the pupil which increases the throughput by a factor of 100. This allows the SM-SCC to go faster or use it on fainter objects that the SCC. Here we present the concept of the SM-SCC and show that it can be used for focal plane wavefront control, Coherence Differential Imaging (CDI) and Spectral Differential Imaging (SDI).

This paper presents the design of an Adaptive Optics (AO) system for retinal imaging applications. The development of retinal imaging systems allows for early diagnosis of eye diseases. Such systems can increase the quality of life of patients as well as curtail increasing health care costs through early eye disease detection and treatment. Until recently, AO systems have been prohibitively expensive and cumbersome. This has been mainly due to the size and cost of flexible membrane mirrors normally used as the aberration correction device. Recent developments in the technology of Microelectromechanical System (MEMS) based actuators allow the implementation of AO systems which would have been difficult to implement a few years ago due to exorbitant costs. The aim of this paper is to present the design of a compact and flexible low cost AO system using off the shelf components to measure and compensate for the aberrations of the eye. The design is based around the system’s main components which include a 52 channel magnetically actuated deformable membrane mirror, a Shack Hartmann wavefront sensor and a control system which runs on a single processor personal computer. All the components are commercially available. The use of the MEMS-based magnetically actuated mirror allows for increased resolution and force compared to conventional membrane mirrors designed mainly for use in astronomical applications. The performance of the closed-loop system is evaluated through experiments. Although designed as a diagnostic tool for eye diseases, such a system will find a number of applications in basic research in the visual sciences, including the study of microscopic structures in the living retina that could not be seen before. Optometrists, retinal surgeons, and ophthalmologists will also benefit from using such a system, through potential improvements on commonly used instruments such as phoropters and fundus cameras.

We present preliminary findings on the characteristic behavior and initial performance of Boston Micromachine Corporations' (BMC) 4096-actuator micro-electrical mechanical systems (MEMS) deformable mirror (DM). This device is examined for its application in the Gemini Planet Imager high-contrast adaptive optics (AO) system. It is also being considered for use in next generation AO systems on the extremely large telescopes. Testing of this device has been in progress at the Laboratory for Adaptive Optics (LAO) on the Extreme Adaptive Optics (ExAO) testbed in experiments designed to qualify performance for imaging extrasolar planets. In this paper we present first test results including actuator stroke (3.0 microns at 200 V), individual actuator RMS surface (10.3 nm surface), actuator yield for two DM arrays (94.4% and 98.8%), actuator crosstalk (no more than 32%), stroke at the highest spatial frequency (1.2 nm surface), and sub-nanometer closed loop flattening capabilities over a 30-actuator diameter.

High-contrast imaging is a growing observational technique aimed at discovering and characterizing extrasolar planets. The Gemini Planet Imager (GPI) is designed to achieve contrast ratios of 10^-6 - 10^-7 and requires unprecedented wavefront correction and coronagraphic control of diffraction. G PI is a facility instrument now undergoing integration and testing and is scheduled for first light on the 8-m Gemini South telescope towards the end of 2012. In this paper, we focus on the wavefront sensing and correction aspects of the instrument. To measure the wavefront, GPI combines a Shack-Hartmann wavefront sensor and a high-accuracy infrared interferometric wavefront calibration system. The Shack-Hartmann wavefront sensor uses 1700 subapertures to precisely sample the wavefront at 1.5 kHz and features a spatial filter to prevent aliasing. The wavefront calibration system measures the slower temporal frequency errors as well as non-common path aberrations. The wavefront correction is performed using a two-stage adaptive optics system employing a 9x9 piezoelectric deformable mirror and a 43x43 actuators MEMS deformable mirror operating in a woofer-tweeter configuration. Finally, an image sharpening technique is used to further increase the contrast of the final image. In this paper, we describe the three wavefront sensing methods and how we combine their respective information to achieve the best possible contrast.

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.