Adaptive Secondary Mirror Research Articles

The European Solar Telescope

The European Solar Telescope (EST) is a project aimed at studying the magnetic connectivity of the solar atmosphere, from the deep photosphere to the upper chromosphere. Its design combines the knowledge and expertise gathered by the European solar physics community during the construction and operation of state-of-the-art solar telescopes operating in visible and near-infrared wavelengths: the Swedish 1m Solar Telescope, the German Vacuum Tower Telescope and GREGOR, the French Télescope Héliographique pour l’Étude du Magnétisme et des Instabilités Solaires, and the Dutch Open Telescope. With its 4.2 m primary mirror and an open configuration, EST will become the most powerful European ground-based facility to study the Sun in the coming decades in the visible and near-infrared bands. EST uses the most innovative technological advances: the first adaptive secondary mirror ever used in a solar telescope, a complex multi-conjugate adaptive optics with deformable mirrors that form part of the optical design in a natural way, a polarimetrically compensated telescope design that eliminates the complex temporal variation and wavelength dependence of the telescope Mueller matrix, and an instrument suite containing several (etalon-based) tunable imaging spectropolarimeters and several integral field unit spectropolarimeters. This publication summarises some fundamental science questions that can be addressed with the telescope, together with a complete description of its major subsystems.

Lab tests of segment/petal phasing with a pyramid wavefront sensor and a holographic dispersed fringe sensor in turbulence with the Giant Magellan Telescope high contrast adaptive optics phasing testbed

The Giant Magellan Telescope (GMT) design consists of seven circular 8.4-m diameter mirrors, together forming a single 25.4-m diameter primary mirror. This large aperture and collecting area can help extreme adaptive optics (ExAO) systems such as GMT’s GMagAO-X achieve the small angular resolutions and contrasts required to image habitable zone earth-like planets around late type stars and possibly lead to the discovery of life outside of our solar system. However, the GMT primary mirror segments are separated by large >30 cm gaps, creating the possibility of fluctuations in optical path differences (piston) due to flexure, segment vibrations, wind buffeting, temperature effects, and atmospheric seeing. To utilize the full diffraction-limited aperture of the GMT for high-contrast, natural guide star-adaptive optics science, the seven mirror segments must be co-phased to well within a fraction of a wavelength. The current design of the GMT involves seven adaptive secondary mirrors, a slow (∼0.03 Hz) off-axis dispersed fringe sensor (part of the acquisition guiding and wavefront sensing system’s active optics off-axis guider), and a pyramid wavefront sensor [PyWFS; part of the natural guide star wavefront sensor (NGWS) adaptive optics] to measure and correct the total path length between segment pairs, but these methods have yet to be tested “end-to-end” in a lab environment. We present the design and working prototype of a “GMT high contrast adaptive optics phasing testbed” that leverages the existing MagAO-X ExAO instrument to demonstrate segment phase sensing and simultaneous AO-control for high-contrast GMT natural guide star science [i.e., testing the NGWS wavefront sensor (WFS) architecture]. We present the first test results of closed-loop piston control with one GMT segment using MagAO-X’s PyWFS with and without simulated atmospheric turbulence. We show that the PyWFS was able to successfully control segment piston without turbulence within 12- to 33-nm RMS for 0 λ / D to 5 λ / D modulation, but was unsuccessful at controlling segment piston with generated ∼0.6 arcsec (median seeing conditions at the GMT site) and ∼1.2 arcsec seeing turbulence due to nonlinear modal cross-talk and poor pixel sampling of the segment gaps on the PyWFS detector. These results suggest that a PyWFS alone is not an ideal piston sensor for the GMT (and likely the TMT and ELT). Hence, a dedicated “second channel” piston sensor is required. We report the success of an alternate solution to control piston using a holographic dispersed fringe sensor (HDFS) while controlling all other modes with the PyWFS purely as a slope sensor (piston mode removed). This “second channel” WFS method worked well to control segment piston to within 50 nm RMS and ±10 μm dynamic range under simulated 0.6 arcsec atmospheric seeing (median seeing conditions at the GMT site). These results led to the inclusion of the HDFS as the official second channel piston sensor for the GMT NGWS WFS. This HDFS + PyWFS architecture should also work well to control piston petal modes on the ELT and TMT telescopes.

DO-CRIME: dynamic on-sky covariance random interaction matrix evaluation, a novel method for calibrating adaptive optics systems

ABSTRACT Adaptive optics (AO) systems require a calibration procedure to operate, whether in closed loop or even more importantly in forward control. This calibration usually takes the form of an interaction matrix and is a measure of the response on the wavefront sensor (WFS) to wavefront corrector stimulus. If this matrix is sufficiently well conditioned, it can be inverted to produce a control matrix, which allows to compute the optimal commands to apply to the wavefront corrector for a given WFS measurement vector. Interaction matrices are usually measured by means of an artificial source at the entrance focus of the AO system; however, adaptive secondary mirrors (ASMs) on Cassegrain telescopes offer no such focus and the measurement of their interaction matrices becomes more challenging and needs to be done on-sky using a natural star. The most common method is to generate a theoretical or simulated interaction matrix and adjust it parametrically (e.g. magnification, rotation) using on-sky measurements. We propose a novel method of measuring on-sky interaction matrices ab initio from the telemetry stream of the AO system using random patterns on the deformable mirror with diagonal commands covariance matrices. The approach, being developed for the ASM upgrade for the imaka wide-field AO system on the UH2.2m telescope project, is shown to work on-sky using the current imaka testbed.

ARGOS at the LBT

Having completed its commissioning phase, the Advanced Rayleigh guided Ground-layer adaptive Optics System (ARGOS) facility is coming online for scientific observations at the Large Binocular Telescope (LBT). With six Rayleigh laser guide stars in two constellations and the corresponding wavefront sensing, ARGOS corrects the ground-layer distortions for both LBT 8.4 m eyes with their adaptive secondary mirrors. Under regular observing conditions, this set-up delivers a point spread function (PSF) size reduction by a factor of 2–3 compared to a seeing-limited operation. With the two LUCI infrared imaging and multi-object spectroscopy instruments receiving the corrected images, observations in the near-infrared can be performed at high spatial and spectral resolution. We discuss the final ARGOS technical set-up and the adaptive optics performance. We show that imaging cases with ground-layer adaptive optics (GLAO) are enhancing several scientific programmes, from cluster colour magnitude diagrams and Milky Way embedded star formation, to nuclei of nearby galaxies or extragalactic lensing fields. In the unique combination of ARGOS with the multi-object near-infrared spectroscopy available in LUCI over a 4 × 4 arcmin field of view, the first scientific observations have been performed on local and high-z objects. Those high spatial and spectral resolution observations demonstrate the capabilities now at hand with ARGOS at the LBT.

Optical calibration and performance of the adaptive secondary mirror at the Magellan telescope

In this paper we describe the procedure for the optical calibration of large size deformable mirrors, acting as wavefront correctors for adaptive optics systems. Adaptive optics compensate the disturbance due to the atmospheric turbulence to restore the telescope resolution. We will showcase in particular the activities performed for the Adaptive Secondary Mirror (ASM) of the Magellan Adaptive Optics system (MagAO), which is an instrument for the 6.5 m Magellan Clay Telescope, located at Las Campanas Observatory, in Chile. The MagAO ASM calibration is part of the MagAO-2K project, a major MagAO upgrade that started in 2016 with the goal of boosting adaptive optics (AO) correction at visible wavelengths to image exoplanets. For the first time, the optical quality of MagAO mirror is reported. We describe the procedures developed to achieve high SNR interferometric measurements of the ASM modes under the presence of dome convection noise and telescope vibrations. These measurements were required to produce an improved control matrix with up to 500 modes to close the AO loop on sky with enhanced performances. An updated slaving algorithm was developed to improve the control of actuators vignetted by the central obscuration. The calibrations yielded also a new ASM flattening command, updating the one in use since the MagAO commissioning in 2013. With the new flattening command, a 22 nm RMS surface error was achieved. Finally, we present on-sky results showing the MagAO performance achieved with the new calibrations.

First on-sky demonstration of the piezoelectric adaptive secondary mirror.

We propose using a piezoelectric adaptive secondary mirror (PASM) in the medium-sized adaptive telescopes with a 2-4m aperture for structure and control simplification by utilizing the piezoelectric actuators in contrast with the voice-coil adaptive secondary mirror. A closed-loop experimental setup was built for on-sky demonstration of the 73-element PASM developed by our laboratory. In this Letter, the PASM and the closed-loop adaptive optics system are introduced. High-resolution stellar images were obtained by using the PASM to correct high-order wavefront errors in May 2016. To the best of our knowledge, this is the first successful on-sky demonstration of the PASM. The results show that with the PASM as the deformable mirror, the angular resolution of the 1.8m telescope can be effectively improved.

Modeling pyramidal sensors in ray-tracing software by a suitable user-defined surface

Following the unprecedented results in terms of performances delivered by the first light adaptive optics system at the Large Binocular Telescope, there has been a wide-spread and increasing interest on the pyramid wavefront sensor (PWFS), which is the key component, together with the adaptive secondary mirror, of the adaptive optics (AO) module. Currently, there is no straightforward way to model a PWFS in standard sequential ray-tracing software. Common modeling strategies tend to be user-specific and, in general, are unsatisfactory for general applications. To address this problem, we have developed an approach to PWFS modeling based on user-defined surface (UDS), whose properties reside in a specific code written in C language, for the ray-tracing software ZEMAX™. With our approach, the pyramid optical component is implemented as a standard surface in ZEMAX™, exploiting its dynamic link library (DLL) conversion then greatly simplifying ray tracing and analysis. We have utilized the pyramid UDS DLL surface—referred to as pyramidal acronyms may be too risky (PAM2R)—in order to design the current PWFS-based AO system for the Giant Magellan Telescope, evaluating tolerances, with particular attention to the angular sensitivities, by means of sequential ray-tracing tools only, thus verifying PAM2R reliability and robustness. This work indicates that PAM2R makes the design of PWFS as simple as that of other optical standard components. This is particularly suitable with the advent of the extremely large telescopes era for which complexity is definitely one of the main challenges.

Self-Tuning Mechanism for the Design of Adaptive Secondary Mirror Position Control

Deformable mirrors (DMs) are electromechanical devices used in ground-based telescopes to compensate for the distortions caused by the atmospheric turbulence, the main factor limiting the resolution of astronomical imaging. Adaptive secondary mirrors (ASMs) represent a new type of DMs; two of them have been recently installed on the 8-m-class large binocular telescope (LBT). ASMs are able to jointly correct rigid and nonrigid wave-front distortions thanks to the use of force actuators distributed on the overall mirror surface. As an offset, each actuator needs to be piloted by a dedicated controller, whose parameters must be accurately tuned to obtain the desired mirror shape. At the present time, the calibration of the controller parameters is executed manually. This paper presents a novel automatic controller tuning procedure that does not rely on the modeling of the mirror dynamics. The experimental validation on a prototype reproducing the three innermost rings of the LBT ASM is reported.

Ground-layer turbulence evaluation project at Subaru Telescope

A candidate of the next-generation adaptive optics (AO) system at Subaru Telescope is a ground-layer AO (GLAO) using an adaptive secondary mirror. The performance of GLAO depends on the turbulence profile because only ground-layer turbulence is corrected. At Mouna Kea, the profile data have been obtained at the summit ridge site and TMT site. However, the height difference of the Subaru site from these sites is about 100 m, close to the scale height of the ground-layer turbulence. There is a possibility that the topographical difference affects the ground-layer turbulence property, and then the performance of GLAO. In this paper, the activity to evaluate the ground-layer turbulence at the Subaru site is introduced.

MOSE: operational forecast of the optical turbulence and atmospheric parameters at European Southern Observatory ground-based sites – II. Atmospheric parameters in the surface layer 0–30 m

This article is the second of a series of articles aiming at proving the feasibility of the forecast of all the most relevant classical atmospherical parameters for astronomical applications (wind speed and direction, temperature, relative humidity) and the optical turbulence (Cn2 and the derived astro-climatic parameters like seeing, isoplanatic angle, wavefront coherence time...). This study is done in the framework of the MOSE project, and focused above the two ESO ground-bases sites of Cerro Paranal and Cerro Armazones. In this paper we present the results related to the Meso-Nh model ability in reconstructing the surface layer atmospherical parameters (wind speed intensity, wind direction and absolute temperature, [0-30] m a.g.l.). The model reconstruction of all the atmospherical parameters in the surface layer is very satisfactory. For the temperature, at all levels, the RMSE (Root Mean Square Error) is inferior to 1{\deg}C. For the wind speed, it is ~2 m/s, and for the wind direction, it is in the range [38-46{\deg}], at all levels, that corresponds to a RMSE_relative in a range [21-26{\deg}]. If a filter is applied for the wind direction (the winds inferior to 3 m/s are discarded from the computations), the wind direction RMSE is in the range [30-41{\deg}], i.e. a RMSE_relative in the range [17-23{\deg}]. The model operational forecast of the surface layer atmospherical parameters is suitable for different applications, among others: thermalization of the dome using the reconstructed temperature, hours in advance, of the beginning the night; knowing in advance the main direction which the strong winds will come from during the night could allow the astronomer to anticipate the occurrence of a good/bad seeing night, and plan the observations accordingly; preventing adaptive secondary mirrors shake generated by the wind speed.

Servo-fluid-elastic modeling of contactless levitated adaptive secondary mirrors

The paper develops a comprehensive approach for the servo-fluid-elastic modeling of electromagnetically levitated secondary adaptive mirrors. The system modeling includes the mirror structural dynamics model, its interaction with the fluid film interposed between the mirror and its reference backplate and disturbances due to local air turbulence. The accuracy of the proposed fluid dynamic model is assessed by comparing it with a significant sample of high fidelity 3D Navier---Stokes analyses. The simulation tool is then used to demonstrate the effectiveness and robustness of a recently proposed control, based on a system dynamics cancellation and an iterated steady state feedforward contribution. The simulations clearly highlights the improvements achievable implementing the new control scheme.

Stress polishing of thin shells for adaptive secondary mirrors

Context. Adaptive secondary mirrors (ASM) are, or will be, key components on all modern telescopes, providing improved seeing conditions or diffraction limited images, thanks to the high-order atmospheric turbulence correction obtained by controlling the shape of a thin mirror. Their development is a key milestone towards future extremely large telescopes (ELT) where this technology is mandatory for successful observations.

Automatic Tuning of the Internal Position Control of an Adaptive Secondary Mirror

One of the key components of an Adaptive Optics system is the deformable mirror. This mirror can correct the atmospheric turbulence effects by changing its shape. In the last years Adaptive Secondary Mirrors (ASM) have been developed and the Large Binocular Telescope (LBT) will be soon equipped with two ASMs. Each LBT ASM unit has 672 voice-coil force actuators to change the shape of the mirror shell. Because the actuators apply force, an internal position control is required. The LBT ASM internal control uses a local feedback of position and velocity, then the control law for each actuator has two characteristic parameters which define the closed-loop shell dynamics. In this paper, an analysis of the dynamical behaviour of the ASM under the proposed control law is provided. Then an algorithm is suggested that, based on a simplified model of the shell dynamics, tunes the controller parameters by means of an automatic procedure, as a replacement for a manual procedure based on the operator skill.

Model-Based Feedforward Control of Large Deformable Mirrors

In this paper, a model-based feedforward control concept for fast set-point changes of large deformable mirrors is proposed. It takes into account local position control loops of excited actuators and mode-dependent stiffness variations of the mirror shell. Based on partial differential equations for the temporal and spatial behavior of the deformable mirror, a modal approximation of the mirror dynamics is performed. It is shown that a second order approximation of the eigenmode dynamics is appropriate for low-order modes even when additional system components as delays, digital-to-analog converters, current drivers, actuator-magnet efficiencies, capacitive sensors, and analog-to-digital converters are included. In particular, appropriate transfer functions with identified coefficients of the proposed modal models are presented for a 45-actuator prototype of the Large Binocular Telescope adaptive secondary mirror (P45). Additionally, experimental results with model-based feedforward control are presented. The identified dynamic model of the P45 is finally used to derive flatness based feedforward commands for chopping.

Laboratory characterization and performance of the high-order adaptive optics system for the Large Binocular Telescope

We present the laboratory characterization and performance evaluation of the First Light Adaptive Optics (FLAO), the natural guide star adaptive optics system for the Large Binocular Telescope (LBT). The system uses an adaptive secondary mirror with 672 actuators and a pyramid wavefront sensor with adjustable sampling of the telescope pupil from 30×30 down to 7×7 subapertures. The system was fully assembled in the Arcetri Observatory laboratories and passed its acceptance test in December 2009. The performance measured at different star magnitudes during the acceptance test is shown to lie between the baseline and the goal specifications. In particular, FLAO obtained a 77% Strehl ratio (SR) in the bright end (8.5 magnitude star in the R band) using an H-band filter and correcting 495 modes with 30×30 subaperture sampling. In the faint end (16.4 magnitude), a 5% SR correcting 36 modes with 7×7 subapertures was measured. The seeing conditions for these tests were 0.8 arcsec (r0=0.14 m at 550 nm) and an average wind speed of 15 m/s. The results for other seeing conditions up to 1.5 arcsec are also presented. The system has been shipped to the LBT site in Arizona, and its commissioning started in March 2010.

High precision massive shape control of magnetically levitated adaptive mirrors

The paper presents a control scheme that provides precise active shape control of deformable mirror shells using a very large number of control points. The proposed controller combines a low frequency centralized feedforward and a high frequency fully decentralized feedback, with each single actuator mated to a single sensor. To grant a precisely controlled shape the feedforward part requires an accurate knowledge of the system stiffness. For this purpose a viable experimental identification procedure for high dimension steady state response matrices is described and verified through realistic simulations. Finally the control methodology is applied to the model of an adaptive secondary mirror with 3360 control points. Numerical results confirm the validity of the proposed approach.

MMT/AO 5 μm IMAGING CONSTRAINTS ON THE EXISTENCE OF GIANT PLANETS ORBITING FOMALHAUT AT ∼13-40 AU

A candidate < 3 Jupiter mass, extrasolar planet was recently imaged by Kalas et al. (2008) using HST/ACS at 12.7" (96 AU) separation from the nearby (d = 7.7 pc) young (~200 Myr) A2V star Fomalhaut. Here we report results from M-band (4.8 micron) imaging of Fomalhaut on 5 Dec 2006 using the Clio IR imager on the 6.5-m MMT with the adaptive secondary mirror. Our images are sensitive to giant planets at orbital radii comparable to the outer solar system (~5-40 AU). Comparing our 5-sigma M-band photometric limits to theoretical evolutionary tracks for substellar objects, our results rule out the existence of planets with masses greater than 2 Jupiter masses, from ~13-40 AU and objects greater than 13 Jupiter masses from ~8-40 AU.

ON-SKY WIDE-FIELD ADAPTIVE OPTICS CORRECTION USING MULTIPLE LASER GUIDE STARS AT THE MMT

We describe results from the first astronomical adaptive optics system to use multiple laser guide stars, located at the 6.5-m MMT telescope in Arizona. Its initial operational mode, ground-layer adaptive optics (GLAO), provides uniform stellar wavefront correction within the 2 arc minute diameter laser beacon constellation, reducing the stellar image widths by as much as 53%, from 0.70 to 0.33 arc seconds at lambda = 2.14 microns. GLAO is achieved by applying a correction to the telescope's adaptive secondary mirror that is an average of wavefront measurements from five laser beacons supplemented with image motion from a faint stellar source. Optimization of the adaptive optics system in subsequent commissioning runs will further improve correction performance where it is predicted to deliver 0.1 to 0.2 arc second resolution in the near-infrared during a majority of seeing conditions.

A nickel-carbon-fibre composite for large adaptive mirrors: fabrication methods and properties

We present results from our recent research into carbon-fibre composite (CFC) mirror fabrication for optical and infra-red applications. In particular this research is aimed towards the next generation of extremely large telescopes to offer an alternative to thin glass shell adaptive secondary mirrors. We address the issues involved with CFC mirror production, in particular the accuracy of the form replication process, a suitable surface for polishing to optical quality, no fibre print-through, environmental stability (shape change due to thermal and moisture variations), material uniformity and lifetime. We have performed experiments into the effectiveness of cold electroplating thick nickel coatings to totally encapsulate the CFC base substrate; the manufacturing procedure and properties of the Ni-CFC mirror are described here.

PILOT – the Pathfinder for an International Large Optical Telescope

PILOT is proposed as a partnership between Australia and Europe to develop a 2.4 m optical/infrared telescope for Dome C, Antarctica. Funding for a detailed designed study is being sought from Australian sources, with a view to commencing construction in early 2008. The current “strawman” design is for an f/10 dual Nasmyth configuration with provision for both a silicon carbide fast tip-tilt secondary mirror for the thermal infrared, and an adaptive secondary mirror to achieve diffraction-limited imaging at wavelengths as short as V -band.