Adaptive optics
Encyclopedia
Adaptive optics is a technology used to improve the performance of optical systems
by reducing the effect of wavefront
distortions. It is used in astronomical
telescope
s and laser communication systems to remove the effects of atmospheric distortion
, and in retina
l imaging systems to reduce the impact of optical aberrations. Adaptive optics works by measuring the distortions in a wavefront
and compensating for them with a device that corrects those errors such as a deformable mirror
or a liquid crystal
array. AO was first envisioned by Horace W. Babcock
in 1953, but it did not come into common usage until advances in computer
technology during the 1990s made the technique practical.
Adaptive optics should not be confused with active optics
, which works on a longer timescale to correct the primary mirror geometry.
Other approaches that can yield resolving power exceeding the limits of atmospheric seeing include speckle imaging
, aperture synthesis
, lucky imaging
, and space telescopes, such as the Hubble Space Telescope
.
of the wavefront in two dimensions (equivalent to correction of the position offsets for the image). This is performed using a rapidly moving tip–tilt mirror that makes small rotations around two of its axes. A significant fraction of the aberration introduced by the atmosphere
can be removed in this way.
Tip–tilt mirrors are effectively segmented mirror
s having only one segment which can tip and tilt, rather than having an array of multiple segments that can tip and tilt independently. Due to the relative simplicity of such mirrors, they have a large stroke, meaning they have large correcting power. Because of this, most AO systems use a tip–tilt mirror first, followed by a deformable mirror for higher-order correction.
When light
from a star
or another astronomical object
enters the Earth's atmosphere
, atmospheric turbulence
(introduced, for example, by different temperature
layers and different wind
speeds interacting) can distort and move the image in various ways (see astronomical seeing
for a full discussion). Images produced by any telescope larger than a few metres are blurred by these distortions.
which takes some of the astronomical light, a deformable mirror that lies in the optical path, and a computer that receives input from the detector. The wavefront sensor measures the distortions the atmosphere has introduced on the timescale of a few millisecond
s; the computer
calculates the optimal mirror shape to correct the distortions and the surface of the deformable mirror
is reshaped accordingly. For example, an 8–10 m telescope (like the VLT
or Keck) can produce AO-corrected images with a angular resolution
of 30–60 milliarcsecond (mas) resolution
at infrared wavelengths, while the resolution without correction is of the order of 1 arcsecond.
In order to perform adaptive optics correction, the shape of the incoming wavefronts must be measured as a function of position in the telescope aperture plane. Typically the circular telescope aperture is split up into an array of pixel
s in a wavefront sensor, either using an array of small lenslet
s (a Shack–Hartmann sensor), or using a curvature or pyramid sensor which operates on images of the telescope aperture. The mean wavefront perturbation in each pixel is calculated. This pixelated map of the wavefronts is fed into the deformable mirror
and used to correct the wavefront errors introduced by the atmosphere. It is not necessary for the shape or size of the astronomical object
to be known – even Solar System
objects which are not point-like can be used in a Shack–Hartmann wavefront sensor, and time-varying structure on the surface of the Sun is commonly used for adaptive optics at solar telescopes. The deformable mirror corrects incoming light so that the images appear sharp.
can be used instead. The light from the science target has passed through approximately the same atmospheric turbulence as the reference star's light and so its image is also corrected, although generally to a lower accuracy.
The necessity of a reference star means that an adaptive optics system cannot work everywhere on the sky, but only where a guide star
of sufficient luminosity
(for current systems, about magnitude
12–15) can be found very near to the object of the observation. This severely limits the application of the technique for astronomical observations. Another major limitation is the small field of view over which the adaptive optics correction is good. As the distance from the guide star
increases, the image quality degrades. A technique known as "multiconjugate adaptive optics" uses several deformable mirrors to achieve a greater field of view.
, LGS) in the atmosphere. LGSs come in two flavors: Rayleigh guide stars and sodium
guide stars. Rayleigh guide stars work by propagating a laser
, usually at near ultraviolet
wavelengths, and detecting the backscatter from air at altitudes between 15–25 km. Sodium guide stars use laser light at 589 nm to excite sodium atoms in the mesosphere
and thermosphere
, which then appear to "glow". The LGS can then be used as a wavefront reference
in the same way as a natural guide star – except that (much fainter) natural reference stars are still required for image position (tip/tilt) information. The lasers are often pulsed, with measurement of the atmosphere
being limited to a window occurring a few microsecond
s after the pulse has been launched. This allows the system to ignore most scattered light at ground level; only light which has travelled for several microseconds high up into the atmosphere and back is actually detected.
s in the wavefront passing through the pupil of the eye
. These aberrations diminish the quality of the image formed on the retina, sometimes necessitating the wearing of spectacles or contact lens
es. In the case of retinal imaging, light passing out of the eye carries similar wavefront distortions, leading to an inability to resolve the microscopic structure (cells and capillaries) of the retina. Spectacles and contact lenses correct "low-order aberrations", such as defocus and astigmatism, which tend to be stable in humans for long periods of time (months or years). While correction of these is sufficient for normal visual functioning, it is generally insufficient to achieve microscopic resolution. Additionally, "high-order aberrations", such as coma, spherical aberration
, and trefoil, must also be corrected in order to achieve microscopic resolution. High-order aberrations, unlike low-order, are not stable over time, and may change with frequencies between 10 Hz and 100 Hz. The correction of these aberrations requires continuous, high-frequency measurement and compensation.
, and the most commonly used type of wavefront sensor is the Shack-Hartmann
. Ocular aberrations are caused by spatial phase nonuniformities in the wavefront exiting the eye. In a Shack-Hartmann wavefront sensor, these are measured by placing a two-dimensional array of small lenses (lenslets) in a pupil plane conjugate to the eye's pupil, and a CCD chip at the back focal plane of the lenslets. The lenslets cause spots to be focused onto the CCD chip, and the positions of these spots are calculated using a centroiding algorithm. The positions of these spots are compared with the positions of reference spots, and the displacements between the two are used to determine the local curvature of the wavefront—an estimate of the phase nonuniformities causing aberration.
to produce (also in living human eyes) the first images of retinal microvasculature and associated blood flow and retinal pigment epithelium cells in addition to single cones. Combined with optical coherence tomography
, adaptive optics has allowed the first three-dimensional images of living cone photoreceptors to be collected.
. It is also expected to play a military role by allowing ground-based and airborne laser
weapons to reach and destroy targets at a distance including satellite
s in orbit
. The Missile Defense Agency
Airborne Laser program is the principal example of this.
Adaptive optics has been used to enhance the performance of free space optical communication systems. Medical applications include imaging of the retina
, where it has been combined with optical coherence tomography
. Development of an Adaptive Scanning Optical Microscope (ASOM) was announced by Thorlabs in April 2007. Adaptive and active optics
are also being developed for use in glasses to achieve better than 20/20 vision
, initially for military applications.
After propagation of a wavefront, parts of it may overlap leading to interference and preventing adaptive optics from correcting it. Propagation of a curved wavefront always leads to amplitude variation. This needs to be considered if a good beam profile is to be achieved in laser applications.
is used to control both direction and position. The actual beam is measured by photo diodes. This signal is fed into some Analog-to-digital converter
s and a microcontroller
runs a PID controller
algorithm. The controller drives some digital-to-analog converter
s which drive stepper motor
s attached to mirror mount
s.
If the beam is to be centered onto 4-quadrant diodes, no Analog-to-digital converter
is needed. Operational amplifier
s are sufficient.
Optics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...
by reducing the effect of wavefront
Wavefront
In physics, a wavefront is the locus of points having the same phase. Since infrared, optical, x-ray and gamma-ray frequencies are so high, the temporal component of electromagnetic waves is usually ignored at these wavelengths, and it is only the phase of the spatial oscillation that is described...
distortions. It is used in astronomical
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
s and laser communication systems to remove the effects of atmospheric distortion
Astronomical seeing
Astronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by turbulent mixing in the Earth's atmosphere varying the optical refractive index...
, and in retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
l imaging systems to reduce the impact of optical aberrations. Adaptive optics works by measuring the distortions in a wavefront
Wavefront
In physics, a wavefront is the locus of points having the same phase. Since infrared, optical, x-ray and gamma-ray frequencies are so high, the temporal component of electromagnetic waves is usually ignored at these wavelengths, and it is only the phase of the spatial oscillation that is described...
and compensating for them with a device that corrects those errors such as a deformable mirror
Deformable mirror
Deformable mirror represents the most convenient tool for wavefront control and correction of optical aberrations. Deformable mirrors are used in combination with wavefront sensors and real-time control systems in adaptive optics...
or a liquid crystal
Liquid crystal
Liquid crystals are a state of matter that have properties between those of a conventional liquid and those of a solid crystal. For instance, an LC may flow like a liquid, but its molecules may be oriented in a crystal-like way. There are many different types of LC phases, which can be...
array. AO was first envisioned by Horace W. Babcock
Horace W. Babcock
Horace Welcome Babcock was an American astronomer. He was the son of Harold D. Babcock.He invented and built a number of astronomical instruments, and in 1953 was the first to propose the idea of adaptive optics. He specialized in spectroscopy and the study of magnetic fields of stars...
in 1953, but it did not come into common usage until advances in computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
technology during the 1990s made the technique practical.
Adaptive optics should not be confused with active optics
Active optics
Active optics is a technology used with reflecting telescopes developed in the 1980s, which actively shapes a telescope's mirrors to prevent deformation due to external influences such as wind, temperature, mechanical stress...
, which works on a longer timescale to correct the primary mirror geometry.
Other approaches that can yield resolving power exceeding the limits of atmospheric seeing include speckle imaging
Speckle imaging
Speckle imaging describes a range of high-resolution astronomical imaging techniques based either on the shift-and-add method or on speckle interferometry methods...
, aperture synthesis
Aperture synthesis
Aperture synthesis or synthesis imaging is a type of interferometry that mixes signals from a collection of telescopes to produce images having the same angular resolution as an instrument the size of the entire collection...
, lucky imaging
Lucky imaging
Lucky imaging is one form of speckle imaging used for astronomical photography. Speckle imaging techniques use a high-speed camera with exposure times short enough so that the changes in the Earth's atmosphere during the exposure are minimal.With lucky imaging, those optimum exposures least...
, and space telescopes, such as the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
.
Tip–tilt correction
The simplest form of adaptive optics is tip-tilt correction, which corresponds to correction of the tiltsTilt (optics)
In optics, tilt is a deviation in the direction a beam of light propagates. Tilt quantifies the average slope in both the X and Y directions of a wavefront or phase profile across the pupil of an optical system...
of the wavefront in two dimensions (equivalent to correction of the position offsets for the image). This is performed using a rapidly moving tip–tilt mirror that makes small rotations around two of its axes. A significant fraction of the aberration introduced by the atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
can be removed in this way.
Tip–tilt mirrors are effectively segmented mirror
Segmented mirror
A segmented mirror is an array of smaller mirrors designed to act as segments of a single large curved mirror. The segments can be either spherical or asymmetric . They are used as objectives for large reflecting telescopes...
s having only one segment which can tip and tilt, rather than having an array of multiple segments that can tip and tilt independently. Due to the relative simplicity of such mirrors, they have a large stroke, meaning they have large correcting power. Because of this, most AO systems use a tip–tilt mirror first, followed by a deformable mirror for higher-order correction.
In astronomy
Atmospheric seeing
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
from a star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
or another astronomical object
Astronomical object
Astronomical objects or celestial objects are naturally occurring physical entities, associations or structures that current science has demonstrated to exist in the observable universe. The term astronomical object is sometimes used interchangeably with astronomical body...
enters the Earth's atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
, atmospheric turbulence
Turbulence
In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...
(introduced, for example, by different temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
layers and different wind
Wind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...
speeds interacting) can distort and move the image in various ways (see astronomical seeing
Astronomical seeing
Astronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by turbulent mixing in the Earth's atmosphere varying the optical refractive index...
for a full discussion). Images produced by any telescope larger than a few metres are blurred by these distortions.
Wavefront sensing and correction
An adaptive optics system tries to correct these distortions, using a wavefront sensorWavefront sensor
A wavefront sensor is a device for measuring the aberrations of an optical wavefront. Although an amplitude splitting interferometer such as the Michelson interferometer could be called a wavefront sensor, the term is normally applied to instruments that do not require an unaberrated reference...
which takes some of the astronomical light, a deformable mirror that lies in the optical path, and a computer that receives input from the detector. The wavefront sensor measures the distortions the atmosphere has introduced on the timescale of a few millisecond
Millisecond
A millisecond is a thousandth of a second.10 milliseconds are called a centisecond....
s; the computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
calculates the optimal mirror shape to correct the distortions and the surface of the deformable mirror
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...
is reshaped accordingly. For example, an 8–10 m telescope (like the VLT
Very Large Telescope
The Very Large Telescope is a telescope operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. The VLT consists of four individual telescopes, each with a primary mirror 8.2m across, which are generally used separately but can be used together to...
or Keck) can produce AO-corrected images with a angular resolution
Angular resolution
Angular resolution, or spatial resolution, describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object...
of 30–60 milliarcsecond (mas) resolution
Image resolution
Image resolution is an umbrella term that describes the detail an image holds. The term applies to raster digital images, film images, and other types of images. Higher resolution means more image detail....
at infrared wavelengths, while the resolution without correction is of the order of 1 arcsecond.
In order to perform adaptive optics correction, the shape of the incoming wavefronts must be measured as a function of position in the telescope aperture plane. Typically the circular telescope aperture is split up into an array of pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
s in a wavefront sensor, either using an array of small lenslet
Lenslet
A lenslet is literally a small lens. The fact that distinguishes it from a small lens is that it is part of a lenslet array. A lenslet array consists of a set of lenslets in the same plane. Each lenslet normally has the same focal length....
s (a Shack–Hartmann sensor), or using a curvature or pyramid sensor which operates on images of the telescope aperture. The mean wavefront perturbation in each pixel is calculated. This pixelated map of the wavefronts is fed into the deformable mirror
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...
and used to correct the wavefront errors introduced by the atmosphere. It is not necessary for the shape or size of the astronomical object
Astronomical object
Astronomical objects or celestial objects are naturally occurring physical entities, associations or structures that current science has demonstrated to exist in the observable universe. The term astronomical object is sometimes used interchangeably with astronomical body...
to be known – even Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
objects which are not point-like can be used in a Shack–Hartmann wavefront sensor, and time-varying structure on the surface of the Sun is commonly used for adaptive optics at solar telescopes. The deformable mirror corrects incoming light so that the images appear sharp.
Natural guide stars
Because a science target is often too faint to be used as a reference star for measuring the shape of the optical wavefronts, a nearby brighter guide starGuide star
In astronomy, a guide star is a reference star used to accurately maintain the tracking by a telescope of a heavenly body, whose motion across the sky is primarily due to the rotation of the Earth....
can be used instead. The light from the science target has passed through approximately the same atmospheric turbulence as the reference star's light and so its image is also corrected, although generally to a lower accuracy.
The necessity of a reference star means that an adaptive optics system cannot work everywhere on the sky, but only where a guide star
Guide star
In astronomy, a guide star is a reference star used to accurately maintain the tracking by a telescope of a heavenly body, whose motion across the sky is primarily due to the rotation of the Earth....
of sufficient luminosity
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...
(for current systems, about magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
12–15) can be found very near to the object of the observation. This severely limits the application of the technique for astronomical observations. Another major limitation is the small field of view over which the adaptive optics correction is good. As the distance from the guide star
Guide star
In astronomy, a guide star is a reference star used to accurately maintain the tracking by a telescope of a heavenly body, whose motion across the sky is primarily due to the rotation of the Earth....
increases, the image quality degrades. A technique known as "multiconjugate adaptive optics" uses several deformable mirrors to achieve a greater field of view.
Artificial guide stars
An alternative is the use of a laser beam to generate a reference light source (a laser guide starLaser guide star
Laser guide stars are a form of artificial star created for use in astronomical adaptive optics imaging.Adaptive optics systems require a wavefront reference source in order to correct atmospheric distortion of light...
, LGS) in the atmosphere. LGSs come in two flavors: Rayleigh guide stars and sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
guide stars. Rayleigh guide stars work by propagating a laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
, usually at near ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
wavelengths, and detecting the backscatter from air at altitudes between 15–25 km. Sodium guide stars use laser light at 589 nm to excite sodium atoms in the mesosphere
Mesosphere
The mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. In the mesosphere temperature decreases with increasing height. The upper boundary of the mesosphere is the mesopause, which can be the coldest naturally occurring...
and thermosphere
Thermosphere
The thermosphere is the biggest of all the layers of the Earth's atmosphere directly above the mesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causes ionization. The International Space Station has a stable orbit within the middle of the thermosphere, between...
, which then appear to "glow". The LGS can then be used as a wavefront reference
Reference
Reference is derived from Middle English referren, from Middle French rèférer, from Latin referre, "to carry back", formed from the prefix re- and ferre, "to bear"...
in the same way as a natural guide star – except that (much fainter) natural reference stars are still required for image position (tip/tilt) information. The lasers are often pulsed, with measurement of the atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
being limited to a window occurring a few microsecond
Microsecond
A microsecond is an SI unit of time equal to one millionth of a second. Its symbol is µs.A microsecond is equal to 1000 nanoseconds or 1/1000 millisecond...
s after the pulse has been launched. This allows the system to ignore most scattered light at ground level; only light which has travelled for several microseconds high up into the atmosphere and back is actually detected.
In retinal imaging
Ocular aberrations are distortionDistortion
A distortion is the alteration of the original shape of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted, and often many methods are employed to minimize it in practice...
s in the wavefront passing through the pupil of the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
. These aberrations diminish the quality of the image formed on the retina, sometimes necessitating the wearing of spectacles or contact lens
Contact lens
A contact lens, or simply contact, is a lens placed on the eye. They are considered medical devices and can be worn to correct vision, for cosmetic or therapeutic reasons. In 2004, it was estimated that 125 million people use contact lenses worldwide, including 28 to 38 million in the United...
es. In the case of retinal imaging, light passing out of the eye carries similar wavefront distortions, leading to an inability to resolve the microscopic structure (cells and capillaries) of the retina. Spectacles and contact lenses correct "low-order aberrations", such as defocus and astigmatism, which tend to be stable in humans for long periods of time (months or years). While correction of these is sufficient for normal visual functioning, it is generally insufficient to achieve microscopic resolution. Additionally, "high-order aberrations", such as coma, spherical aberration
Spherical aberration
thumb|right|Spherical aberration. A perfect lens focuses all incoming rays to a point on the [[Optical axis|optic axis]]. A real lens with spherical surfaces suffers from spherical aberration: it focuses rays more tightly if they enter it far from the optic axis than if they enter closer to the...
, and trefoil, must also be corrected in order to achieve microscopic resolution. High-order aberrations, unlike low-order, are not stable over time, and may change with frequencies between 10 Hz and 100 Hz. The correction of these aberrations requires continuous, high-frequency measurement and compensation.
Measurement of ocular aberrations
Ocular aberrations are generally measured using a wavefront sensorWavefront sensor
A wavefront sensor is a device for measuring the aberrations of an optical wavefront. Although an amplitude splitting interferometer such as the Michelson interferometer could be called a wavefront sensor, the term is normally applied to instruments that do not require an unaberrated reference...
, and the most commonly used type of wavefront sensor is the Shack-Hartmann
Shack-Hartmann
A Shack–Hartmann wavefront sensor is an optical instrument used to characterize an imaging system. It is a wavefront sensor commonly used in adaptive optics systems. It consists of an array of lenses of the same focal length. Each is focused onto a photon sensor...
. Ocular aberrations are caused by spatial phase nonuniformities in the wavefront exiting the eye. In a Shack-Hartmann wavefront sensor, these are measured by placing a two-dimensional array of small lenses (lenslets) in a pupil plane conjugate to the eye's pupil, and a CCD chip at the back focal plane of the lenslets. The lenslets cause spots to be focused onto the CCD chip, and the positions of these spots are calculated using a centroiding algorithm. The positions of these spots are compared with the positions of reference spots, and the displacements between the two are used to determine the local curvature of the wavefront—an estimate of the phase nonuniformities causing aberration.
Correction of ocular aberrations
Once the local phase errors in the wavefront are known, they can be corrected by placing a phase modulator such as a deformable mirror at yet another plane in the system conjugate to the eye's pupil. The phase errors can be used to reconstruct the wavefront, which can then be used to control the deformable mirror. Alternatively, the local phase errors can be used directly to calculate the deformable mirror instructions.Open loop vs. closed loop operation
If the wavefront error is measured once and corrected once, followed by the acquisition of retinal images, operation is said to be in open loop. If the residual wavefront error (i.e. the wavefront error which remains after correction) is continually measured and used to reshape the mirror, operation is said to be closed loop. The latter is necessary in the case of rapidly changing aberrations, and as such is generally used in retinal imaging systems.Applications
Adaptive optics was first applied to flood-illumination retinal imaging to produce images of single cones in the living human eye. It has also been used in conjunction with scanning laser ophthalmoscopyOphthalmoscopy
Ophthalmoscopy is a test that allows a health professional to see inside the fundus of the eye and other structures using an ophthalmoscope . It is done as part of an eye examination and may be done as part of a routine physical examination...
to produce (also in living human eyes) the first images of retinal microvasculature and associated blood flow and retinal pigment epithelium cells in addition to single cones. Combined with optical coherence tomography
Optical coherence tomography
Optical coherence tomography is an optical signal acquisition and processing method. It captures micrometer-resolution, three-dimensional images from within optical scattering media . Optical coherence tomography is an interferometric technique, typically employing near-infrared light...
, adaptive optics has allowed the first three-dimensional images of living cone photoreceptors to be collected.
Other uses
Besides its use for improving nighttime astronomical imaging and retinal imaging, adaptive optics technology has also been used in other settings. Adaptive optics is used for solar astronomy at observatories such as the Swedish 1-m Solar TelescopeSwedish Solar Telescope
The Swedish 1-m Solar Telescope is a refracting solar telescope at Roque de los Muchachos Observatory, La Palma in the Canary Islands. It is run by the Institute for Solar Physics of the Royal Swedish Academy of Sciences. The primary element is a single fused silica lens, making it the second...
. It is also expected to play a military role by allowing ground-based and airborne laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
weapons to reach and destroy targets at a distance including satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
s in orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
. The Missile Defense Agency
Missile Defense Agency
The Missile Defense Agency is the section of the United States government's Department of Defense responsible for developing a layered defense against ballistic missiles. The agency has its origins in the Strategic Defense Initiative, which was established in 1983 and was headed by Lt...
Airborne Laser program is the principal example of this.
Adaptive optics has been used to enhance the performance of free space optical communication systems. Medical applications include imaging of the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
, where it has been combined with optical coherence tomography
Optical coherence tomography
Optical coherence tomography is an optical signal acquisition and processing method. It captures micrometer-resolution, three-dimensional images from within optical scattering media . Optical coherence tomography is an interferometric technique, typically employing near-infrared light...
. Development of an Adaptive Scanning Optical Microscope (ASOM) was announced by Thorlabs in April 2007. Adaptive and active optics
Active optics
Active optics is a technology used with reflecting telescopes developed in the 1980s, which actively shapes a telescope's mirrors to prevent deformation due to external influences such as wind, temperature, mechanical stress...
are also being developed for use in glasses to achieve better than 20/20 vision
Visual acuity
Visual acuity is acuteness or clearness of vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain....
, initially for military applications.
After propagation of a wavefront, parts of it may overlap leading to interference and preventing adaptive optics from correcting it. Propagation of a curved wavefront always leads to amplitude variation. This needs to be considered if a good beam profile is to be achieved in laser applications.
Beam stabilization
A rather simple example is the stabilization of the position and direction of laser beam between modules in a large free space optical communication system. Fourier opticsFourier optics
Fourier optics is the study of classical optics using Fourier transforms and can be seen as the dual of the Huygens-Fresnel principle. In the latter case, the wave is regarded as a superposition of expanding spherical waves which radiate outward from actual current sources via a Green's function...
is used to control both direction and position. The actual beam is measured by photo diodes. This signal is fed into some Analog-to-digital converter
Analog-to-digital converter
An analog-to-digital converter is a device that converts a continuous quantity to a discrete time digital representation. An ADC may also provide an isolated measurement...
s and a microcontroller
Microcontroller
A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM...
runs a PID controller
PID controller
A proportional–integral–derivative controller is a generic control loop feedback mechanism widely used in industrial control systems – a PID is the most commonly used feedback controller. A PID controller calculates an "error" value as the difference between a measured process variable and a...
algorithm. The controller drives some digital-to-analog converter
Digital-to-analog converter
In electronics, a digital-to-analog converter is a device that converts a digital code to an analog signal . An analog-to-digital converter performs the reverse operation...
s which drive stepper motor
Stepper motor
A stepper motor is a brushless, electric motor that can divide a full rotation into a large number of steps. The motor's position can be controlled precisely without any feedback mechanism , as long as the motor is carefully sized to the application...
s attached to mirror mount
Mirror mount
A mirror mount is a device that holds a mirror. In optics research, these can be quite sophisticated devices, due to the need to be able to tip and tilt the mirror by controlled amounts, while still holding it in a precise position when it is not being adjusted.Precision mirror mounts can be quite...
s.
If the beam is to be centered onto 4-quadrant diodes, no Analog-to-digital converter
Analog-to-digital converter
An analog-to-digital converter is a device that converts a continuous quantity to a discrete time digital representation. An ADC may also provide an isolated measurement...
is needed. Operational amplifier
Operational amplifier
An operational amplifier is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output...
s are sufficient.
See also
- Active opticsActive opticsActive optics is a technology used with reflecting telescopes developed in the 1980s, which actively shapes a telescope's mirrors to prevent deformation due to external influences such as wind, temperature, mechanical stress...
- Nonlinear optics: optical phase conjugation
- WavefrontWavefrontIn physics, a wavefront is the locus of points having the same phase. Since infrared, optical, x-ray and gamma-ray frequencies are so high, the temporal component of electromagnetic waves is usually ignored at these wavelengths, and it is only the phase of the spatial oscillation that is described...
- Wavefront sensorWavefront sensorA wavefront sensor is a device for measuring the aberrations of an optical wavefront. Although an amplitude splitting interferometer such as the Michelson interferometer could be called a wavefront sensor, the term is normally applied to instruments that do not require an unaberrated reference...
- Deformable mirrorDeformable mirrorDeformable mirror represents the most convenient tool for wavefront control and correction of optical aberrations. Deformable mirrors are used in combination with wavefront sensors and real-time control systems in adaptive optics...
- Holography: real-time holography
- Image stabilizationImage stabilizationImage stabilization is a family of techniques used to reduce blurring associated with the motion of a camera during exposure. Specifically, it compensates for pan and tilt of a camera or other imaging device. It is used in image-stabilized binoculars, still and video cameras, and astronomical...
- Angular diameterAngular diameterThe angular diameter or apparent size of an object as seen from a given position is the “visual diameter” of the object measured as an angle. In the vision sciences it is called the visual angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its...
- Angular size
- Greenwood frequencyGreenwood frequencyIn adaptive optics, the Greenwood frequency is the frequency or bandwidth required for optimal correction with an adaptive optics system. It depends on the transverse windspeed and the turbulence strength in the atmosphere...
- ADONISADONIS: ADaptive Optics Near Infrared SystemADONIS was a second-generation adaptive optics system for the astronomical community. It was mounted on the ESO 3.6m telescope at La Silla Observatory...
- Adjustable-focus eyeglassesAdjustable-focus eyeglassesAdjustable focus eyeglasses are prescription eyeglasses with an adjustable focal length. They compensate for refractive errors by providing variable focusing, allowing users to adjust them for desired distance or prescription, or both....
- List of telescope parts and construction
- CILASCILASCILAS is a French company, a subsidiary of EADS Astrium, specialized in laser and optics technology, founded in 1966. This high-technology engineering company was the inventor of the particle size analyzer. Today, it develops, manufactures and produces systems combining laser and precision optics...
- Boston Micromachines CorporationBoston Micromachines CorporationBoston Micromachines Corporation is a US company operating out of Cambridge, Massachusetts. Boston Micromachines manufactures and develops MEMS deformable mirrors to perform open- and closed- loop adaptive optics. The technology is applied in Beam Shaping, Astronomy, Vision Science, and general...
- Bates methodBates MethodThe Bates method is an alternative therapy aimed at improving eyesight. Eye-care physician William Horatio Bates attributed nearly all sight problems to habitual strain of the eyes, and felt that glasses were harmful and never necessary...
External links
- Adaptive Optics Tutorial at CTIO A. Tokovinin
- Research groups and companies with interests in Adaptive Optics
- Saturn's moon TitanTitan (moon)Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....
occulting NV0435215+200905 (December 20, 2001) - Space-based vs. Ground-based telescopes with Adaptive Optics
- Comparison of M92 view by HST and LBT adaptive optics