ADAPTIVE OPTICS, a branch of optics that deals with the development of methods and means for controlling the shape of the wavefront (WF) in order to eliminate distortions (aberrations) that arise when a light beam propagates in an optically inhomogeneous medium (for example, a turbulent atmosphere) or due to imperfections in the elements of the optical system.

The purpose of adaptive correction is to increase the resolution of optical instruments, increase the concentration of radiation at the receiver, achieve the most acute focusing of the light beam on the target, or obtain a given distribution of radiation intensity. The possibility of using active methods in optics began to be discussed since the early 1950s in connection with the problem of increasing the resolution of ground-based telescopes, but the intensive development of adaptive optics began after the creation of sufficiently effective correctors (controlled mirrors) and WF meters (sensors). The simplest adaptive system contains one flat mirror, the tilt of which can be changed, which eliminates the “jitter” of the image when observing through a turbulent atmosphere. More complex systems use correctors with a large number of degrees of freedom to compensate for higher order aberrations. A typical scheme for organizing control in an adaptive system (figure) is built on the principle of feedback. After the corrector, part of the light flux branches off and goes to the WF sensor, where residual aberrations are measured. This information is used to generate signals in the control unit that influence the corrector and reduce residual aberrations. They become minimal and image quality improves.

There are systems that do not require the use of VF sensors. In this case, the minimization of distortions is carried out by deliberately introducing test disturbances into the WF (aperture probing method). Then the influence of test disturbances on the quality of system operation is analyzed in the control unit, after which control signals are generated that optimize the WF. Aperture sensing systems require a lot of time to set up the corrector, since the process is repeated several times to noticeably reduce distortion.

The effectiveness of an adaptive optical system is largely determined by the perfection of the corrector used. At first, mainly composite (segmented) mirrors were used, consisting of several segments that could be shifted relative to each other using piezo actuators or in another way. Subsequently, flexible (“membrane”) mirrors with a continuously deformable surface became widespread. By the beginning of the 21st century, the technique for correcting VF had significantly improved. In addition to controlled mirrors of various types, liquid crystal phase modulators are used, which can operate both for reflection (like mirrors) and for transmission. A number of designs allow for their miniaturization and the creation of devices integrated into a single unit with control electronics, which makes it possible to create compact and relatively inexpensive adaptive systems. However, despite the development of a new generation of phase correctors, traditional flexible mirrors retain their importance due to low losses of luminous flux and relatively simple design. Nonlinear optical distortion correction methods based on the phenomenon of wavefront reversal are also used in laser systems. This approach is sometimes called nonlinear adaptive optics.

Lit.: Vorontsov M. A., Shmalgauzen V. I. Principles of adaptive optics. M., 1985; Taranenko V. G., Shanin O. I. Adaptive optics. M., 1990; Lukin V.P., Fortes B.V. Adaptive formation of beams and images in the atmosphere. Novosibirsk, 1999.

V. I. Shmalgauzen.

Section prepared by Nikolay Nosyrev and Oleg Vilkov

Adaptive optics(AO) - a branch of optics that deals with the development of optical systems with dynamic control of the wavefront shape to compensate for random disturbances and increase the resolution limit of observational instruments, the degree of radiation concentration at the receiver or target.

The main problem that can be solved by an adaptive optics system is to eliminate wavefront disturbances caused by uncontrolled random influences. The most famous systems of this type include:

· ground-based telescopes, due to the heterogeneity of the earth’s atmosphere, the resolution of these systems is reduced

systems for generating and focusing laser radiation

laser measuring systems operating in the atmosphere

· optical systems of high-power lasers.

The implementation of adaptive optical systems is determined by the specific range of problems it solves. However, the general principles for constructing such systems are the same.

There are systems with an outgoing wave, in which the wavefront of the light source is corrected, and systems with a received wave, in which the light field coming from the observed object is corrected. In turn, both of them can be implemented on the principles of phase conjugation and aperture sensing.

In a phase conjugation system, a beam of light is reflected from a small area of ​​an object (target), forming a spherical wave that travels back along the path of light and undergoes the same distortions as the emitted wave. The incoming reflected wave enters the wavefront sensor, where distortions on the path are detected. The data processing device calculates the necessary wavefront correction, which is carried out by the wavefront influencing device.

The principle of aperture sensing is based on the possibility of introducing test disturbances into the wavefront, which are transformed into amplitude disturbances of the signal. By analyzing changes in the intensity of light reflected from the target, a conclusion is made about the sign of the phase change and the wavefront is deformed until focusing on the object is optimized.

Received wave systems work in a similar way. In phase-coupling systems, a portion of the received light with a distorted wavefront is directed to a wavefront sensor. The information obtained is used to create a compensating effect on the received wavefront. As a result, an image limited only by diffraction is ideally formed at the receiver.

In aperture sensing systems, test disturbances are introduced into the received wavefront, and their influence is assessed using a receiver located in the image plane.

St. Petersburg National Research University of Information Technologies, Mechanics and Optics

Faculty of Photonics and Optoinformatics

Department of Computer Photonics and Video Informatics

in the discipline of Systems Theory and System Analysis

« ANALYTICAL REVIEW OF CHARACTERISTICS OF MODERN COMPONENTS OF ADAPTIVE OPTICAL SYSTEMS»

Student: Romanov I.E.

Group: 4352

Teacher: Gurov I.P.

Saint Petersburg

Introduction…………………………………………………….……………………….2

Adaptive optical system………………………………………………………3

Wavefront sensors…………………………………………..………..5

Wavefront correctors…………………………………….………..9

1) Segmented mirrors ...................................................... ..............10

2) Mirrors with a solid surface………………………………...11

2.1) Bimorph mirrors…………………………………….....12

2.2) Membrane mirrors……………………..………………….14

3) MOEMS (silicon technology)………………..………………...14

Conclusion……………………………………………………..………………...15

References………………………………………………………...16

Additional sources of information…………………………………..17

Introduction

Adaptive optics (AO) is a branch of optics that deals with the development of optical systems with dynamic control of the wavefront shape to compensate for random disturbances and increase the resolution limit of observational instruments, the degree of radiation concentration at the receiver or target. Adaptive optics began to develop intensively in the 1950s. in connection with the task of compensating for front distortions caused by atmospheric turbulence and imposing the main limitation on the resolution of ground-based telescopes. Later, the problems of creating orbital telescopes and powerful laser emitters, subject to other types of interference, were added to this.

Adaptive optics finds application in various fields of science and technology. For example, in the design of ground-based astronomical telescopes, in optical communication systems, in industrial laser technology, in medicine, etc., where it allows, respectively, to compensate for atmospheric distortions and aberrations of optical systems, including the optical elements of the human eye.

The purpose of this work is to study adaptive optical systems, as well as to conduct an analytical review of the characteristics of their components.

Adaptive optical system

The possibility of correcting atmospheric image distortions using a deformable mirror was first pointed out in 1953 by the American astronomer Horace H.W. Babcock. He proposed the creation of an instrument that would measure dynamic atmospheric distortions in real time and correct them using quickly tunable shape-changing optical elements. However, it was not possible to implement his ideas at that time due to limited technology.

The main problem that can be solved by an adaptive optics system is to eliminate wavefront disturbances caused by uncontrolled random influences. The most famous systems of this type include:

Ground-based telescopes, due to the heterogeneity of the earth's atmosphere, the resolution of these systems is reduced.

Systems for forming and focusing laser radiation.

Laser measuring systems operating in the atmosphere.

Optical systems of high-power lasers.

The implementation of adaptive optical systems is determined by the specific range of problems it solves. However, the general principles for constructing such systems are the same. Structurally, an adaptive optical system usually consists of a sensor that measures distortion (wavefront sensor), a wavefront corrector, and a control system that communicates between the sensor and the corrector. The general diagram of the adaptive optical design is shown in Fig. 1. .

Rice. 1. General diagram of an adaptive optical system

Wavefront sensors

The wavefront sensor (WFS) is one of the elements of the adaptive system for correcting laser radiation. Its task is to measure the curvature of the wavefront and transmit these measurements to the processing device (Fig. 2).

Rice. 2. Image of a distorted wavefront obtained using a microlens array.

The main reasons for wavefront curvature are:

Atmospheric turbulence.

Non-ideal shapes of the optical elements of the system.

Errors when adjusting the system, etc.

Today there is a wide variety of DVFs. However, the most common one is based on the Shack-Hartmann scheme (Fig. 3.).

Rice. 3. Typical Hartmann sensor circuit

The history of such a sensor dates back to the 1900s, when German physicist and astronomer Johannes Franz Hartmann decided to use many small apertures to track the path of individual light rays through a large telescope, allowing him to check the quality of the image. Later, in the 1960s, Roland Schuck and Ben Platt modified this technology by replacing the apertures with multiple lenses (lens raster).

Such a sensor is most often used in wavefront correction systems due to its advantages. One of the main advantages of the Shack-Hartmann sensor is its ability to measure a wide range of wavefront slopes when distortion cannot be measured by other methods (for example, interference). Such a sensor can be used to determine aberrations in the profile of a non-collimated laser beam. In addition, it has low sensitivity to mechanical vibrations and can operate with high-power pulses and femtosecond durations.

A Shack-Hartmann type sensor consists of an array of microlenses and a photodetector located in their focal plane. Each lens typically measures 1mm or less. The sensor lenses divide the wavefront under study into subapertures (the aperture of one microlens), forming a set of focal spots in the focal plane. The position of each spot depends on the local inclination of the wavefront of the beam arriving at the sensor input. By measuring the transverse displacements of the focal spots, it is possible to calculate the average angles of inclination of the wavefront within each of the subapertures. From these values, the wavefront profile is calculated over the entire sensor aperture.

Rice. 4. Operating principle of the wavefront sensor

When the incoming wavefront is flat, all images are arranged in a regular grid determined by the geometry of the lens array. Once the wavefront is distorted, the images are displaced from their nominal positions. The displacements of the image centroids in two orthogonal directions are proportional to the average wavefront slopes in these directions along the sub-apertures. Thus, the Shack-Hartmann WF (Sh-H WF) measures the slopes of the wavefront. The wavefront itself is reconstructed (restored) from an array of measured slopes accurate to a constant, which does not play a role for the image.

Characteristics of the Shack-Harman DWF:

The amplitude of measured aberrations is up to 15 microns.

Measurement accuracy - λ/100 (RMS).

The diameter of the input radiation is 8...100 mm.

However, Shack-Hartmann WEFs have one significant drawback: crosstalk on CCD matrices. They arise when a sufficiently distorted wavefront falls on the matrix, since with strong deviations it can go beyond the limits of its subarray and end up on a neighboring matrix. This creates a false spot.

But today, errors due to crosstalk are eliminated using complex algorithms. They allow you to accurately track and display the true location of the spot. Modern development of algorithms and manufacturing precision make it possible to expand the scope of application of these sensors. Today they have found application in various image verification systems.

Wavefront correctors

An adaptive mirror is an executive active element of an adaptive optical system that has a reflective surface with a deformable profile. Deformable mirrors are the most convenient tool for wavefront control and correction of optical aberrations.

Main characteristics of adaptive mirrors:

Range of movements (characterized by the sensitivity of the drive as part of the mirror (usually sensitivity is expressed in surface movements in micrometers when the control voltage increases by 1 V)).

Area of ​​local deformation (reflects the number of degrees of freedom of the mirror (can be specified by the effective width of deformation of a unit amplitude caused by the action of one drive; the function describing this deformation is called the response function)).

Frequency bandwidth (determined by the speed of the drive used (limited above by mechanical resonances of the mirror design itself)).

Structurally, adaptive mirrors can be divided into two large groups:

1) Segmented mirrors.

2) Mirrors with a solid surface.

In segmented mirrors, each individual section allows it to be moved and tilted (or just moved). A solid mirror experiences complex deformations under the influence of special drives.

The choice of one design or another is determined by the specifics of the system in which it will be used. The main factors that are taken into account in this case include the overall size, weight and quality of the mirror surface.

Segmented mirrors

Segmented mirrors are made up of individual, independent segments of flat mirrors. Each segment can be moved a short distance and back to adjust the average wavefront value.

Sectioned adaptive mirrors with translational movement of sections (Fig. 5, a) allow you to change only the temporary phase relationships between signals from individual sections (the length of the optical path), and mirrors with movement and tilt of sections (Fig. 5, b) also allow the spatial phase .

Rice. 5. Sectioned adaptive mirrors: a) with translational movement of sections, b) with movement and tilt of sections

Significant disadvantages of sectioned mirrors are the need to control the position of a separate section and the state of its surface, as well as the complexity of implementing a thermal stabilization system for such mirrors.

1) Number of actuators - 100 – 1500.

2) The gaps between the actuators are 2-10 mm.

3) The shape of the electrodes is rectangular or hexagonal.

5) The amplitude of movement is several microns.

6) Resonant frequency - several kilohertz.

7) Cost - high.

Solid surface mirrors

Mirrors with discrete drives (Fig. 6.) are formed on the front surface of a thin deformable membrane. The shape of the plate is controlled by a series of separate actuators that are attached to its rear wall. The shape of the mirror depends on a combination of forces acting on the front panel, boundary conditions (how the plate is attached to the mirror), and the geometry and material of the plate.

These mirrors allow smooth adjustment of the wavefront with a very large number (up to several thousand) degrees of freedom.

Rice. 6. Diagram of a mirror with discrete drives.

Bimorph mirrors

A bimorph mirror (Fig. 7.) consists of two piezoelectric plates, which are fastened together and polarized in opposite directions (parallel to the axes). Between these plates there is an array of electrodes. The front and back surfaces are grounded. The front side of the mirror is used as a reflective surface.

Fig.7. Schematic of a bimorph mirror.

At the moment when voltage is applied to the electrode, one of the plates is compressed, and the opposite one is stretched, which leads to local curvature. The local curvature of the mirror is proportional to the applied voltage, so these deformable mirrors are also called curvature mirrors.

Typical parameters of segmented deformable mirrors:

1) Number of actuators – 18 - 35

2) The gaps between the actuators are 30-200 mm.

3) The shape of the electrodes is radial.

5) Resonant frequency – more than 500 Hz.

6) Cost - moderate.

Membrane mirrors.

The deformation of the membrane of these mirrors is achieved due to the action of a magnetic field. A set of magnets is attached to the membrane directly opposite the solenoids. When current flows through the solenoids, Laplace forces arise, which deform the membrane.

MOEMS (silicon technology)

MOEMS (Fig. 8.) - micro-opto-electro-mechanical systems. Such adaptive mirrors are made using microlithography, like electronic chips, the deflection of small mirror elements is carried out by electrostatic forces. The disadvantages of MOEMS are insufficient movements and small size of the mirror elements.

Fig.8. Operating principle of MOEMS mirror

Another method of controlling the phase of light is the use of liquid crystals, as in monitors, which have up to a million controllable elements. Until recently, liquid crystals were very slow, but this limitation has now been overcome. Although the phase shift introduced by liquid crystals remains very small and, moreover, we should not forget that it depends on the wavelength.

Conclusion

Having studied in the course of this work the structure and characteristics of the components of adaptive optical systems, we can conclude that the development of new types of AOS components does not stand still. New developments in photonics and optical materials are making it possible to create more advanced adaptive system components with better performance than their predecessors.

Bibliography:

Wirth A., Gonsirovsky T. Adaptive optics: matching atmospheric turbulence // Fotnika, 2007, number 6, pp. 10 – 15.

Berchenko E.A., Kalinin Yu.A., Kiselev V.Yu., Polynkin M.A. Wavefront sensors // Laser-optical systems and technologies, 2009, pp. 64–69.

A.G. Aleksandrov, V.E. Zavalova, A.V. Kudryashov, A.L. Rukosuev, P.N. Romanov, V.V. Samarkin, Yu.V. Sheldakova, "Shack - Hartmann wavefront sensor for measuring the parameters of high-power pulsed solid-state lasers", QUANTUM ELECTRON, 2010, 40 (4), 321–326.

Alikhanov A.N., Berchenko E.A., Kiselev V.Yu., Kuleshov V.N., Kurchanov M.S., Narusbek E.A., Otsechkin A.G., Prilepsky B.V., Son V. .G., Filatov A.S., Deformable mirrors for power and information laser systems //Laser-optical systems and technologies, FSUE "NPO ASTROPHYSICS", M., 2009, pp. 54–58

Vorontsov M.A., Shmalgauzen V.I., Principles of adaptive optics, //Moscow, Science, (1985), pp. 336.

Vorontsov M.A., Koryabin A.V., Shmalgauzen V.I., Controlled optical systems. //Moscow, Science, (1988), p. 275.

Krasheninnikov V. R. Estimation of Parameters of Geometric Transformation of Images by Fixed-Point Method / V. R. Krasheninnikov, M. A. Potapov // Pattern Recognition and Image Analysis. – 2012. – Vol. 22, No. 2. – P. 303 –317.

Additional sources of information:

Laser Portal: http://www.laserportal.ru//

Wikipedia: https://en.wikipedia.org/wiki/Adaptive_optics

Astronet: http://www.astronet.ru/db/msg/1205112/part2/dm.html#SEC2.2

Duration:

Listeners:

5th year students of the Department of Physics and Physics, Faculty of Physics, Moscow State University. M.V. Lomonosov (about 15 students)

Description:

The course introduces the basic principles of adaptive optics, including problems of light transmission through a distorting medium, phase correction, and statistical analysis of phase distortions. The problem of anisoplanatism in adaptive optics is also considered. The course introduces students to the basics of phase measurements and phase correction techniques in adaptive optics, as well as some of its applications.

Course program:

1. Problems of controlling the parameters of an optical system.
Increasing the angular resolution of astronomical telescopes and limitations introduced by atmospheric turbulence. Phasing of multi-mirror telescopes. Michelson stellar interferometer. Focusing a laser beam through a turbulent atmosphere. Wavefront reversal and phase conjugation. Speckle problem. Compensation of optical intracavity inhomogeneities in lasers and the problem of forming diffraction-limited beams.

2. Aberrations of optical systems.
Linear optical systems and methods for their description. Complex amplitude transformation. Impulse response and transfer function. Accounting for aberrations. Generalized Huygens-Fresnel principle Transfer function of an optical system with aberrations. Incoherent systems. Optical transfer function (OTF) and frequency-contrast characteristics of the imaging system. Strehl number and normalized resolution of the system, their dependence on the strength of aberrations.

3. Decomposition of aberrations into orthogonal functions.
Properties of orthonormal systems of functions. Zernike polynomials [see Zernike polynomials]. Aberration coefficients. Random aberrations and ways to describe them. Correlation matrix of aberration coefficients. Average characteristics of the optical system. Mean square phase error. Approximate expressions for system resolution and Strehl number.

4. Atmospheric aberrations.
Fluctuations of the refractive index in a turbulent atmosphere. Structure function of phase fluctuations. Correlation radius (Fried radius). OPF and Strehl number in the case of phase fluctuations. Correlation of aberration coefficients in the atmosphere. Expression of correlation coefficients through the phase structure function. Dependence of coefficient dispersion on aperture size and correlation radius.

5. Compensation of aberrations with controlled phase correctors.
Types of correctors and schemes for their use. Adaptive optical systems. Ideal modal VF corrector. Potential effectiveness of a modal corrector in compensating for atmospheric distortions. Expression for the residual squared error. Distribution of residual error over the aperture depending on the number of degrees of freedom of the corrector.

6. Methods for controlling the corrector in adaptive systems.
Typical schemes of adaptive systems. Phase conjugation and aperture sensing systems. Control structure for systems with a VF sensor. Sources of errors and their contribution to the total residual error. Organization of maximum search in aperture sensing systems. Selecting a quality criterion. The problem of local extrema. Advantages and disadvantages of aperture sensing systems.

7. Anisoplanatism of adaptive systems.
Isoplanatism angle of an ideal adaptive system in a turbulent atmosphere. Influence of mean phase fluctuations and WF slopes. Anisoplanatism with modal correction. Long exposure and short exposure images. Methods for expanding the field of view of an adaptive system. Methods for improving the quality of registered images.

8. Amplitude fluctuations in adaptive systems.
Intensity fluctuations in the atmosphere. Speckles and features of speckle fields. Weak amplitude fluctuations and their description. Wave structure function. Influence of amplitude fluctuations on OPF and Strehl number. Residual error and accuracy of phase measurements in the presence of amplitude fluctuations.

9. Measuring waveform distortion in adaptive optics 1.
Measuring local slopes. Fundamental limitations: photon shot noise, photodetector noise. Shear interferometers: rotating diffraction gratings, two-channel and combined schemes; sensitivity assessments.

10. Measuring waveform distortion in adaptive optics 2.
Transverse shear interferometer with holographic filter; radial shear interferometer. Shark-Hartmann sensor. Positional characteristics; assessments of accuracy and sensitivity. VF curvature sensor. Characteristics of modern VF sensor circuits.

11. Reconstruction of the WF from measured local slopes.
Reconstruction of the WF profile using the least squares method. Calculation of aberration coefficients; expansion in terms of corrector response functions. Reconstruction of the WF taking into account the statistics of phase distortions (Bayesian approach).

12. Methods of high-resolution phase correction.
Liquid crystal spatial phase modulators and adaptive systems with optical feedback. Basic equation of the system; fundamental restrictions. Methods for visualizing phase distortions: defocusing and free propagation; Hilbert transform; transverse shear interferometer and holographic filter; radial shear interferometer.

13. The problem of a reference source in astronomy.
Methods for creating artificial reference sources: Rayleigh scattering in the atmosphere; the use of sodium layers excited by laser radiation. The problem of measuring average slopes. Anisoplanatism of WF measurements using an artificial reference source. Systems with many reference sources.

14. Modern applications of adaptive optics.
Correction of phase distortions of laser beams in LTS problems and systems for generating femtosecond laser pulses; systems for intracavity correction of thermal aberrations in active elements of medium-power technological lasers. Formation of a given intensity distribution in a technological CO2 laser beam. Use of adaptive optics in ophthalmology: measurement of aberrations of the human eye; increasing the resolution of retinal images in retinoscopy; multispectral retinoscopy.

Lectures:

· No. 1. Introductory.
· No. 2. Imaging systems with a lens.
· No. 3. Incoherent systems.
· No. 4. Measurement of waveform distortion in adaptive optics. Part I.
· No. 5. Measurement of waveform distortion in adaptive optics. Part II.
· No. 6. Measurement of waveform distortion in adaptive optics. Part III.

A scattering of stars, as if winking at the observer, looks very romantic. But for astronomers, this beautiful twinkling does not evoke admiration at all, but completely opposite feelings. Fortunately, there is a way to remedy the situation.

Alexey Levin

The experiment that breathed new life into space science was not carried out at a famous observatory or on a giant telescope. Experts learned about it from the article Successful Tests of Adaptive Optics, published in the astronomical journal The Messenger in 1989. There, the results of tests of the Come-On electro-optical system, designed to correct atmospheric distortions of light from cosmic sources, were presented. They were carried out from October 12 to 23 on the 152-cm reflector of the French observatory OHP (Observatoire de Haute-Province). The system worked so well that the authors began the article by stating that “a long-time dream of astronomers working with ground-based telescopes has finally come true thanks to the creation of a new optical observing technology—adaptive optics.”

A few years later, adaptive optics (AO) systems began to be installed on large instruments. In 1993, they were equipped with the 360-cm telescope of the European Southern Observatory (ESO) in Chile, a little later - the same instrument in Hawaii, and then 8-10-meter telescopes. Thanks to AO, ground-based instruments can observe luminaries in visible light with a resolution that was only the province of the Hubble Space Telescope, and in infrared rays with even higher resolution. For example, in the very useful astronomical region of the near-infrared wavelength of 1 μm, Hubble provides a resolution of 110 arcms, and ESO's 8-meter telescopes provide up to 30 ms.

In fact, when French astronomers were testing their AO system, similar devices already existed in the United States. But they were not created for the needs of astronomy. The customer for these developments was the Pentagon.

The Scheck-Hartmann sensor works like this: after leaving the telescope's optical system, light passes through an array of small lenses that direct it to a CCD matrix. If the radiation from a cosmic source or an artificial star propagated in a vacuum or in an ideally calm atmosphere, then all the mini-lenses would focus it strictly in the center of the pixels allocated to them. Due to atmospheric turbulence, the convergence points of the rays “walk” along the surface of the matrix, and this makes it possible to reconstruct the disturbances themselves.

When air is a problem

If you observe through a telescope two stars located very close to each other in the sky, their images will merge into one luminous point. The minimum angular distance between such stars, due to the wave nature of light (diffraction limit), is the resolution of the device, and it is directly proportional to the wavelength of light and inversely proportional to the diameter (aperture) of the telescope. So, for a three-meter reflector when observing in green light, this limit is about 40 angular ms, and for a 10-meter reflector - a little more than 10 ms (at this angle, a small coin is visible from a distance of 2000 km).

However, these estimates are valid only for observations in vacuum. In the earth's atmosphere, areas of local turbulence constantly appear, which changes the density and temperature of the air and, consequently, its refractive index several hundred times per second. Therefore, in the atmosphere, the front of a light wave from a cosmic source inevitably spreads out. As a result, the real resolution of conventional telescopes is at best 0.5−1 arcsecond and falls far short of the diffraction limit.

Previously, the size of the corrected sky zones was limited to cells with a side of 15 arcms. In March 2007, multi-coupled adaptive optics (MCAO) was tested for the first time on one of ESO's telescopes. It probes turbulence at different altitudes, which made it possible to increase the size of the corrected field of view to two or more arc minutes. “The capabilities of AO have expanded greatly this century,” Claire Max, a professor of astronomy and astrophysics and director of the Center for Adaptive Optics at the University of California, Santa Cruz, tells PM. — Large telescopes have systems with two and three deformable mirrors, which include MCAO. New wavefront sensors and more powerful computer programs have appeared. Mirrors with microelectromechanical actuators have been created that make it possible to change the shape of the reflecting surface better and faster than piezoelectric actuators. In recent years, experimental multi-object adaptive optics (MOAO) systems have been developed and tested, with the help of which up to ten or more sources can be simultaneously tracked in a field of view with a diameter of 5-10 arc minutes. They will be installed on the next generation of telescopes that will begin operation in the next decade.”

Guiding Stars

Let’s imagine a device that analyzes light waves passing through a telescope hundreds of times per second to identify traces of atmospheric turbulence and, based on these data, changes the shape of a deformable mirror placed at the focus of the telescope in order to neutralize atmospheric interference and, ideally, make the image of the object “vacuum.” In this case, the resolution of the telescope is limited solely by the diffraction limit.

However, there is one subtlety. Typically, the light from distant stars and galaxies is too weak for reliable wavefront reconstruction. It’s another matter if there is a bright source near the observed object, the rays from which go to the telescope along almost the same path - they can be used to read atmospheric interference. It was precisely this scheme (in a slightly reduced form) that French astronomers tested in 1989. They selected several bright stars (Deneb, Capella and others) and, using adaptive optics, really improved the quality of their images when observed in infrared light. Soon such systems, using guide stars in the earth's sky, began to be used on large telescopes for real observations.

But there are few bright stars in the earth’s sky, so this technique is suitable for observing only 10% of the celestial sphere. But if nature has not created a suitable star in the right place, you can create an artificial star - using a laser to cause a glow in the atmosphere at a high altitude, which will become a reference light source for the compensating system.

This method was proposed in 1985 by French astronomers Renaud Foix and Antoine Labeyrie. Around the same time, their US colleagues Edward Kibblewhite and Laird Thomson came to similar conclusions. In the mid-1990s, laser emitters paired with JSC equipment appeared on medium-sized telescopes at the Lick Observatory in the USA and at the Calar Alto Observatory in Spain. However, it took about ten years for this technique to find application on 8-10 meter telescopes.

The actuator element of an adaptive optics system is a deformable mirror that is bent using piezoelectric or electromechanical actuators (actuators) according to commands from a control system that receives and analyzes distortion data from wavefront sensors.

Military interest

The history of adaptive optics has not only an obvious side, but also a secret side. In January 1958, the Pentagon established a new structure, the Advanced Research Projects Agency, ARPA (now DARPA), responsible for developing technologies for new generations of weapons. This department played a primary role in the creation of adaptive optics: to observe Soviet orbital vehicles, telescopes with the highest possible resolution insensitive to atmospheric interference were required, and in the future the task of creating laser weapons to destroy ballistic missiles was considered.

In the mid-1960s, under the control of ARPA, a program was launched to study atmospheric disturbances and the interaction of laser radiation with air. This was done at the RADC (Rome Air Development Center) research center located at Griffis Air Force Base in New York State. Powerful spotlights mounted on bombers flying over the test site were used as a reference light source, and it was so impressive that frightened residents sometimes contacted the police!

In the spring of 1973, ARPA and RADC contracted the private corporation Itec Optical Systems to participate in the development of devices that compensate for light scattering under the influence of atmospheric disturbances as part of the RTAC (Real-Time Atmospheric Compensation) program. Itec employees created all three main components of the AO - an interferometer to analyze light front disturbances, a deformable mirror to correct them, and a control system. Their first mirror, two inches in diameter, was made of glass coated with a reflective film of aluminum. Piezoelectric actuators (21 pieces) were built into the support plate, capable of contracting and lengthening by 10 microns under the influence of electrical impulses. Already the first laboratory tests carried out in the same year indicated success. And the following summer, a new series of tests demonstrated that experimental equipment could correct a laser beam at distances of several hundred meters.

These purely scientific experiments were not yet classified. However, in 1975, the closed CIS (Compensating Imaging System) program was approved for the development of JSC in the interests of the Pentagon. In accordance with it, more advanced wavefront sensors and deformable mirrors with hundreds of actuators were created. This equipment was installed on a 1.6-meter telescope located on the top of Mount Haleakala on the Hawaiian island of Maui. In June 1982, with its help, it was possible for the first time to obtain photographs of an artificial Earth satellite of acceptable quality.

With laser sight

Although the experiments on Maui continued for several more years, the development center moved to a special area of ​​Kirtland Air Force Base in New Mexico, to the secret Sandia Optical Range (SOR), where they had long been working on laser weapons. In 1983, a group led by Robert Fugate began experiments in which they were to study laser scanning of atmospheric inhomogeneities. This idea was put forward by American physicist Julius Feinleib in 1981, and now it had to be tested in practice. Feinleib proposed using elastic (Rayleigh) scattering of light quanta on atmospheric inhomogeneities in AO systems. Some of the scattered photons return to the point from which they left, and in the corresponding part of the sky a characteristic glow of an almost point source appears - an artificial star. Fugate and his colleagues recorded distortions in the wavefront of reflected radiation on its way to Earth and compared them with similar disturbances in starlight coming from the same part of the sky. The disturbances turned out to be almost identical, which confirmed the possibility of using lasers to solve AO problems.

These measurements did not require complex optics—simple mirror systems were sufficient. However, for more reliable results, they had to be repeated on a good telescope, which was installed at SOR in 1987. Fugate and his assistants conducted experiments on it, during which adaptive optics with man-made stars was born. In February 1992, the first significantly improved image of a celestial body, Betelgeuse (the brightest luminary in the constellation Orion), was obtained. Soon, the capabilities of the method were demonstrated in photographs of a number of other stars, the rings of Saturn and other objects.

Fugate's team lit artificial stars with powerful copper vapor lasers that generated 5,000 pulses per second. Such a high flash frequency makes it possible to scan even the shortest-lived turbulences. Interferometric wavefront sensors were replaced by the more advanced Scheck-Hartmann sensor, invented in the early 1970s (by the way, also commissioned by the Pentagon). The mirror with 241 actuators, supplied by Itec, could change shape 1664 times per second.

Raise it higher

Rayleigh scattering is quite weak, so it is excited in the altitude range of 10−20 km. The rays from the artificial reference star diverge, while the rays from a much more distant cosmic source are strictly parallel. Therefore, their wave fronts are not quite equally distorted in the turbulent layer, which affects the quality of the corrected image. It is better to light beacon stars at a higher altitude, but the Rayleigh mechanism is unsuitable here.

In the spring of 1991, the Pentagon decided to declassify most of the work on adaptive optics. The declassified results of the 1980s became the property of astronomers.

This problem was solved in 1982 by Princeton University professor Will Harper. He proposed to take advantage of the fact that in the mesosphere at an altitude of about 90 km there are many sodium atoms accumulated there due to the combustion of micrometeorites. Harper proposed to excite the resonant glow of these atoms using laser pulses. The intensity of such a glow at equal laser power is four orders of magnitude higher than the light intensity during Rayleigh scattering. It was just a theory. Its practical implementation became possible thanks to the efforts of the staff of the Lincoln Laboratory, located at Hanscom Air Force Base in Massachusetts. In the summer of 1988, they received the first images of stars taken using mesospheric beacons. However, the quality of the photographs was not high, and the implementation of Harper's method required many years of polishing.

In 2013, the unique Gemini Planet Imager device for photographing and spectrographing exoplanets, designed for eight-meter Gemini telescopes, was successfully tested. It allows using AO to observe planets whose apparent brightness is millions of times less than the brightness of the stars around which they orbit.

In the spring of 1991, the Pentagon decided to declassify most of the work on adaptive optics. The first reports about it were made in May at the American Astronomical Association conference in Seattle. Magazine publications soon followed. Although the US military continued to work on adaptive optics, declassified results from the 1980s became available to astronomers.

The Great Leveler

“AO made it possible for the first time for ground-based telescopes to obtain data on the structure of very distant galaxies,” says professor of astronomy and astrophysics Claire Max from the University of Santa Cruz. — Before the advent of the AO era, they could be observed in the optical range only from space. All ground-based observations of the motion of stars near the supermassive black hole in the center of the Galaxy are also carried out using AO.

JSC also contributed a lot to the study of the Solar System. With its help, extensive information was obtained about the asteroid belt - in particular, about binary asteroid systems. JSC has enriched knowledge about the atmospheres of the planets of the Solar System and their satellites. Thanks to it, observations of the gaseous shell of Titan, the largest satellite of Saturn, have been carried out for fifteen years now, making it possible to track daily and seasonal changes in its atmosphere. So a vast amount of data has already been accumulated on weather conditions on the outer planets and their satellites.

In a certain sense, adaptive optics has equalized the capabilities of terrestrial and space astronomy. Thanks to this technology, the largest stationary telescopes with their giant mirrors provide much better resolution than Hubble or the yet-to-launch James Webb IR Telescope. In addition, measuring instruments for ground-based observatories do not have the strict weight and size restrictions that apply to the design of space equipment. So it would not be an exaggeration to say,” Professor Max concluded, “that adaptive optics has radically transformed many branches of modern science about the Universe.”