Wavefront Sensor For Adaptive Optics Research Articles

On-sky validation of image-based adaptive optics wavefront sensor referencing

Context. Differentiating between a true exoplanet signal and residual speckle noise is a key challenge in high-contrast imaging (HCI). Speckles result from a combination of fast, slow, and static wavefront aberrations introduced by atmospheric turbulence and instrument optics. While wavefront control techniques developed over the last decade have shown promise in minimizing fast atmospheric residuals, slow and static aberrations such as non-common path aberrations (NCPAs) remain a key limiting factor for exoplanet detection. NCPAs are not seen by the wavefront sensor (WFS) of the adaptive optics (AO) loop, hence the difficulty in correcting them. Aims. We propose to improve the identification and rejection of slow and static speckles in AO-corrected images. The algorithm known as the Direct Reinforcement Wavefront Heuristic Optimisation (DrWHO) performs a frequent compensation operation on static and quasi-static aberrations (including NCPAs) to boost image contrast. It is applicable to general-purpose AO systems as well as HCI systems. Methods. By changing the WFS reference at every iteration of the algorithm (a few tens of seconds), DrWHO changes the AO system point of convergence to lead it towards a compensation mechanism for the static and slow aberrations. References are calculated using an iterative lucky-imaging approach, where each iteration updates the WFS reference, ultimately favoring high-quality focal plane images. Results. We validated this concept through both numerical simulations and on-sky testing on the SCExAO instrument at the 8.2-m Subaru telescope. Simulations show a rapid convergence towards the correction of 82% of the NCPAs. On-sky tests were performed over a 10 min run in the visible (750 nm). We introduced a flux concentration (FC) metric to quantify the point spread function (PSF) quality and measure a 15.7% improvement compared to the pre-DrWHO image. Conclusions. The DrWHO algorithm is a robust focal-plane wavefront sensing calibration method that has been successfully demonstrated on-sky. It does not rely on a model and does not require wavefront sensor calibration or linearity. It is compatible with different wavefront control methods, and can be further optimized for speed and efficiency. The algorithm is ready to be incorporated in scientific observations, enabling better PSF quality and stability during observations.

On-sky correction of non-common path aberration with the pyramid wavefront sensor

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.

Crossed-sine wavefront sensor for adaptive optics, metrology and ophthalmology applications

Wavefront Sensors (WFS) have now become core components in the fields of adaptive optics for astronomy, biomedical optics, or metrology of optical systems. However, none of the designs used or proposed so far seems to achieve simultaneously a high spatial resolution at the pupil of the tested optics and absolute measurement accuracy comparable to that attained by laser-interferometers. This paper presents a new WFS concept susceptible to achieving both previous goals. This device is named crossed-sine wavefront sensor and is based on a gradient transmission filter located near the image plane of the tested system. The theoretical principle of the sensor is fully described in a Fourier optics framework. Numerical simulations confirm that the achievable measurement accuracy can reach λ/80 RMS, which is significantly higher than achieved by other types of WFS. The crossed-sine WFS also offers the advantages of being quasi-achromatic and working on slightly extended, natural or artificial light sources.

Auto gain control of EMCCD in Shack–Hartmann wavefront sensor for adaptive optics

Electron multiplying charge-coupled-device (EMCCD) applied in Shack–Hartmann wavefront sensor (S–H WFS) makes the wavefront sensing more efficient for adaptive optics (AO). However when the brightness of the observed target changes in large ranges in a few minutes, a fixed electron multiplying (EM) gain may not be optimum. Thus an auto-gain-control (AGC) method based on the spots image of the S–H WFS is proposed. The designed control value is the average value of the maximum signals of all the light spots in a frame. It has been demonstrated in the experiments that the control value is sensitive to the change of the target brightness, and is stable in the presence of detecting noises and turbulence influence. The goal value for control is predetermined based on the linear relation of the signal with the EM gain and the number of photons collected in sub-apertures. The conditions of the self-protection of the EMCCD are also considered for the goal value. Simulations and experiments indicate that the proposed control method is efficient, and keeps the sensing in a high SNR which reaches the upper SNR limit when sensing with EMCCD. The self-protection of the EMCCD is avoided during the whole sensing process.

An application of adaptive techniques to vibration rejection in adaptive optics systems

In modern Adaptive Optics (AO) systems, lightly damped sinusoidal oscillations resulting from telescope structural vibrations have a significant deleterious impact on the quality of the image collected at the detector plane. These oscillations can be observed in any mode of a generic modal representation of the AO wave-front sensor. A technique for the rejection of periodic disturbances recently presented in the literature has been adapted to the problem of rejecting vibrations in AO loops. The proposed methodology aims at estimating the harmonic disturbance together with the response of the plant at the vibration frequency. The algorithm has been tested in simulation on realistic scenarios and at the telescope.

Study of a MEMS-based Shack-Hartmann wavefront sensor with adjustable pupil sampling for astronomical adaptive optics

We introduce a Shack-Hartmann wavefront sensor for adaptive optics that enables dynamic control of the spatial sampling of an incoming wavefront using a segmented mirror microelectrical mechanical systems (MEMS) device. Unlike a conventional lenslet array, subapertures are defined by either segments or groups of segments of a mirror array, with the ability to change spatial pupil sampling arbitrarily by redefining the segment grouping. Control over the spatial sampling of the wavefront allows for the minimization of wavefront reconstruction error for different intensities of guide source and different atmospheric conditions, which in turn maximizes an adaptive optics system's delivered Strehl ratio. Requirements for the MEMS devices needed in this Shack-Hartmann wavefront sensor are also presented.

A Shack-Hartmann wavefront sensor projected on to the sky with reduced focal anisoplanatism

A method for producing a laser guide star wavefront sensor for adaptive optics with reduced focal anisoplanatism is presented. A theoretical analysis and numerical simulations have been carried out and the results are presented. The technique, named Sky-Projected Laser Array Shack–Hartmann (SPLASH), is shown to suffer considerably less from focal anisoplanatism than a conventional laser guide star system. The method is potentially suitable for large telescope apertures (∼8 m), and possibly for extremely large telescopes.

Design of an image projection correlating wavefront sensor for adaptive optics

A new wavefront sensor data processing algorithm is described and analyzed. The wavefront sensing concept is similar to a Shack-Hartmann type wavefront sensor, but uses an image projection correlation algorithm as opposed to a centroiding approach to estimate optical tilt. This allows the wavefront sensor to estimate tilt parameters while guiding off of point sources and extended objects such as the surface granulation of the sun. The projection-based cross-correlating scheme differs from a 2-D correlation-based tilt estimation approach in that the images are vectorized on the focal plane array itself prior to readout. This on-chip preprocessing approach allows the wavefront sensor data to be compressed, which results in a large reduction in the amount of data read out of the focal plane array while maintaining the desired bandwidth of the adaptive optical system. An implementation of the projection-based wavefront sensor algorithm is described in detail, showing important signal processing steps on and off of the focal plane array of the sensor. The algorithm design is tested in simulation for speed and accuracy by processing simulated solar and astronomical datasets. Timing analysis is presented, which shows how the collection and processing of image projections is computationally efficient and lends itself to a wavefront sensor design that can produce both competitive speed and tilt estimation accuracy.

Characterisation of adaptive optic pyramid wavefront sensors fabricated by deep X-ray lithography

A novel type of wavefront sensor for adaptive optics, utilizing a pyramidal prism in the focus of a telescope, has recently been reported. The key part, the pyramid wavefront sensor, requires accurate control of the quality of the turned edges, the surfaces and the size of the pyramid tip. The classical figuring and polishing techniques used so far to fabricate the pyramid are time consuming and do not guarantee uniform repeatability of the optical characteristics required for the implementation of a large number of these devices for multi-conjugated adaptive optics. We propose a new manufacturing method for the pyramid sensors using the deep X-ray lithography process, allowing very precise control of the surfaces of the components produced. The fabrication method and characterisation of the first pyramids are presented.

Calibration of NAOS and CONICA static aberrations

NAOS is the first adaptive optics (AO) system of the Very Large Telescope and will provide CONICA with diffraction-limited images. CONICA is a near infrared camera that offers a variety of imaging and spectroscopic observing modes. A technique will be described to benefit of the AO system to correct not only for atmospheric turbulence but also for the internal optical aberrations of the high-resolution camera and the beam splitters of NAOS. The aberrant optical components in the light path of CONICA as well as the beam splitters are outside of the AO loop and therefore no self-acting correction is possible. Independently of the AO wavefront sensor, a separate measurement of these aberrations using a method called phase diversity allows one to predict for a certain instrument configuration the corresponding aberrations. They are quantified by sets of Zernike coefficients that are rendered to the adaptive optics. This technique turns out to be very flexible and results in a further improvement of the optical overall performance. The application of phase diversity to the instrument is investigated in a preceding paper (Blanc et al. 2003). In the present paper we present in detail the instrumental implementation of phase diversity, the obtained calibration results, and the achieved gain in optical performance.

Hybrid curvature and gradient wave-front sensor

A new wave-front sensor for adaptive optics that gives signals for the curvature and the two gradient components of the wave front simultaneously is proposed. The sensor uses quad cells placed at the foci of an array of astigmatic lenslets, giving three intensity-normalized differential signals. The parameters of a single subdetector are optimized to maximize the curvature signal. The performance of such a sensor designed for a membrane curvature mirror is analyzed. The good match between sensor and mirror is exhibited by an almost diagonal response matrix between the mirror actuator controls and the curvature signals.

Atmospheric sodium monitor for Laser Guide Star Adaptive Optics

Laser Guide Star Adaptive Optics (AO) makes use of a laser to excite the mesospheric sodium (Na) layer and thereby creates an artificial star that can be used as reference for the AO wavefront sensor. The intensity of the created star depends strongly on the amount of Na present in the atmosphere. Experiments to measure the column density and details of the excitation and scattering properties of sodium atoms in the corresponding atmospheric layer are thus very important to refine the design parameters of laser and assess the power requirement. This article reviews the current knowledge on the mesospheric sodium derived from both astronomical and atmospherical studies. It then presents possible methods to study the sodium layer characteristics relevant for adaptive optics laser guide star systems.

Temporal analysis of stellar wave-front-tilt data

Two-dimensional fractional Brownian motion (FBM) is a good model for stellar wave fronts distorted by Kolmogorov turbulence. However, the time series generated by the movement of such a wave front past an observation point does not explain the reported predictability of measured wave-front slopes. Rescaled-range analysis, a technique for detecting dependence among samples in a series, and a correlation-dimension algorithm that tests for the presence of deterministic chaos were applied to series of real, measured, stellar wavefront slopes. These tests suggest that the source of correlation and predictability is the low-pass spatial filtering of the FBM wave front by the lenslets of an adaptive-optics wave-front sensor and that the process is not chaotic. A simple modeling procedure is described for generating time series of measured wave-front slopes with the correct temporal characteristics.