A deformable mirror can be used to correct wavefront errors in an astronomical telescope.

Adaptive optics is a technology to improve the performance of optical systems by reducing the effects of rapidly changing optical distortion. It is commonly used on astronomical telescopes to remove the effects of atmospheric distortion, or astronomical seeing. Adaptive optics works by measuring the distortion and rapidly compensating for it either using deformable mirrors or material with variable refractive properties. While the technique was theoretically understood for some time, it was only advances in computer technology during the 1990s that finally 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 itself. The simplest form of adaptive optics is tip-tilt correction, which corresponds to correction of the tilts 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 which 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 widely used in night time and solar telescopes, to correct the aberration introduced by the atmosphere on the light path and improve image quality over what would be possible according to the atmospheric seeing. Tip-tilt mirrors are effectively segmented adaptive optics mirrors having only one segment which can tip and tilt, rather than having an array of multiple segments which can tip and tilt independently. Image File history File links Diagram demonstrating the use of a deformable mirror to correct for wavefront errors. ... Image File history File links Diagram demonstrating the use of a deformable mirror to correct for wavefront errors. ... By the mid 20th century humans had achieved a level of technological mastery sufficient to leave the surface of the planet for the first time and explore space. ... Table of Opticks, 1728 Cyclopaedia Optics (appearance or look in ancient Greek) is a branch of physics that describes the behavior and properties of light and the interaction of light with matter. ... Schematic diagram illustrating how optical wavefronts from a distant star may be perturbed by a turbulent layer in the atmosphere. ... A mirror is a surface with good specular reflection that is smooth enough to form an image. ... A computer is a machine designed for manipulating data according to a list of instructions known as a program. ... The 1990s decade refers to the years from 1990 to 1999, inclusive. ... Active optics is a relatively new technology for reflecting telescopes. ... In optics, the term tilt simply refers to a deviation in the direction a beam of light propagates. ... 50 cm refracting telescope at Nice Observatory. ... Layers of Atmosphere (NOAA) Air redirects here. ... Schematic diagram illustrating how optical wavefronts from a distant star may be perturbed by a turbulent layer in the atmosphere. ...

The Shack Hartmann sensor is one type of wavefront sensor used for adaptive optics. The telescope aperture is subdivided using a lenslet array into a large number of sub-apertures and imaged on a CCD. The tilt of the optical wavefronts is measured at each sub-aperture, and the tilts are used to reconstruct a coarse (pixellated) model for the shape of the original wavefront.

Image File history File links Download high resolution version (847x800, 63 KB)A Shack Hartmann wavefront sensor (commonly used in Adaptive optics). ... Image File history File links Download high resolution version (847x800, 63 KB)A Shack Hartmann wavefront sensor (commonly used in Adaptive optics). ... An example of pixelation. ...

Introduction

When light from a star or another astronomical object enters the Earth's atmosphere, turbulence introduced (for example, by different temperature layers and different wind speeds interacting) distort and move the image in various ways (see astronomical seeing for a full discussion). Images produced by any telescope larger than a few centimeters are blurred by these distortions. For example, an 8-10 m telescope (like the VLT or Keck) can produce AO-corrected images with a resolution of 30-60 milli-arcsecond resolution at infrared wavelengths, while the resolution without correction is of the order of 1 arcsecond. Prism splitting light Light is electromagnetic radiation with a wavelength that is visible to the eye (visible light) or, in a technical or scientific context, electromagnetic radiation of wavelengths that are studied in the field of optics. ... The Pleiades star cluster A star is a massive body of plasma in outer space that is currently producing or has produced energy through nuclear fusion. ... See also Lists of astronomical objects Category: ... Layers of Atmosphere (NOAA) Air redirects here. ... In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic, stochastic property changes. ... Schematic diagram illustrating how optical wavefronts from a distant star may be perturbed by a turbulent layer in the atmosphere. ... This article is being considered for deletion in accordance with Wikipedias deletion policy. ... One of the four telescopes that make up the VLT, named Kueyen. ... The Mauna Kea Observatory, an institute of the University of Hawaii, is considered one of the most important land-based observatories in the world for its isolated, unobstructed views of space without interference from man-made light sources. ... A second of arc or arcsecond is a unit of angular measurement which comprises one-sixtieth of an arcminute, or 1/3600 of a degree of arc or 1/1296000 ≈ 7. ... The word resolution has several meanings, depending on context. ... A second of arc or arcsecond is a unit of angular measurement which comprises one-sixtieth of an arcminute, or 1/3600 of a degree of arc or 1/1296000 ≈ 7. ...

Negative images of a star through a telescope. The left-hand panel shows the slow-motion movie of a star when the adaptive optics system is switched off. The right-hand panel shows the slow motion movie of the same star when the adaptive optics system is switched on. The image of the star is much more compact, and breaks up less with adaptive optics switched on. However, the image of the star also changes shape much more quickly when the adaptive optics system is switched on.

An adaptive optics system tries to correct these distortions, using a wavefront sensor 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 milliseconds; the computer calculates the optimal mirror shape to correct the distortions and the surface of the deformable mirror is reshaped accordingly. Image File history File links Ao_movie. ... Image File history File links Ao_movie. ... In optics, a wavefront is the locus (a line or surface in an electromagnetic wave) of points having the same phase. ...

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 pixels in a wavefront sensor, either using an array of small lenslets (Shack-Hartmann), 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 pixellated 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. Because a science target is often too faint to be used as a reference star a nearby brighter guide star 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. A pixel (short for picture element, using the common abbreviation pix for picture) is one of the many tiny dots that make up the representation of a picture in a computers memory. ... The solar system comprises the Earths Sun and the retinue of celestial objects gravitationally bound to it. ... 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. ...

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. 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. ... Luminosity has different meanings in several different fields of science. ... // Headline text HEY!! HOW ARE YOU ALL?? Its nice of you to come read this page. ... 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. ...

An alternative is the use of a laser beam to generate a target (a Laser guide star, 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 microseconds 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. For alternative meanings see laser (disambiguation). ... Lick Observatory laser guide star, built by LLNL. Laser guide stars are a form of artificial star created for use in astronomical adaptive optics imaging. ... General Name, Symbol, Number sodium, Na, 11 Chemical series alkali metals Group, Period, Block 1, 3, s Appearance silvery white Atomic mass 22. ... A nanometre (American spelling: nanometer) is 1. ... The mesosphere (from the Greek words mesos = middle and sphaira = ball) is the layer of the Earths atmosphere that is directly above the stratosphere and directly below the thermosphere. ... The thermosphere is the layer of the Earths atmosphere directly above the mesosphere and directly below the exosphere. ...

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 NASA's Hubble Space Telescope. Speckle imaging (also known as video astronomy) describes a range of high-resolution astronomical imaging techniques based either on the shift-and-add (image stacking) method or on speckle interferometry methods. ... Aperture synthesis is a type of interferometry that mixes signals from a collection instruments to produce measurements having the same angular resolution as an instrument the size of the entire collection. ... The best 1% of exposures of the 0. ... A space observatory is any object in outer space which is used for observation of distant planets, galaxies, and other outer space objects. ... This article or section is missing references or citation of sources. ... The Hubble Space Telescope (HST) is a telescope in orbit around the Earth. ...

Uses of adaptive optics

Lick Observatory laser guide star, built by LLNL.

Besides its obvious use for improving nightime astronomical imaging, adaptive optics technology has also been used recently to image individual cone photoreceptors in the living, human eye. 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 satellites in orbit. Adaptive optics is used for solar astronomy at observatories such as the Swedish Solar Telescope. Adaptive and active optics are also being developed for use in glasses to achieve better than 20/20 vision, initially for military applications[1]. Download high resolution version (483x640, 36 KB)Lick observatory laser guide star image from Lawrence Livermore National Laboratory File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Download high resolution version (483x640, 36 KB)Lick observatory laser guide star image from Lawrence Livermore National Laboratory File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Aerial view of the lab and surrounding area. ... Normalised absorption spectra of human cone (S,M,L) and rod (R) cells Cone cells, or cones, are cells in the retina of the eye which only function in relatively bright light. ... A photoreceptor is a specialized form of cell (specifically, neuron) that is capable of phototransduction. ... This article or section does not cite its references or sources. ... Lasers range in size from microscopic diode lasers (top) with numerous applications, to football field sized neodymium glass lasers (bottom) used for inertial confinement fusion, nuclear weapons research and other high energy density physics experiments. ... A satellite is any object that orbits another object (which is known as its primary). ... In physics, an orbit is the path that an object makes, around another object, whilst under the influence of a source of centripetal force, such as gravity. ... The Swedish Solar Telescope (or SST) is a 1m telescope on La Palma in the Canary Islands. ... Active optics is a relatively new technology for reflecting telescopes. ...

It is also reasonable to presume that adaptive optics have been in use for some time in spy satellites to improve the resolution of ground pictures. KH-4B Corona satellite Lacrosse radar spy satellite under construction A spy satellite (officially referred to as a reconnaissance satellite or recon sat) is an Earth observation satellite or communications satellite deployed for military or intelligence 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 optics 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 converters and a microcontroller runs a PID controller algorithm. The controller drives some digital-to-analog converters which drive stepper motors attached to mirror mounts. To explain the basic principle of Fourier optics, it might be useful to consider the analogue with acoustics: If an arbitrary sound is analyzed through a spectrum analyzer, you will get all the different frequencies and their individual amplitudes that, together, form the sound. ... A photodiode is an electronic component and a type of photodetector. ... An analog-to-digital converter (abbreviated ADC, A/D or A to D) is an electronic circuit that converts continuous signals to discrete digital numbers. ... A microcontroller (or MCU) is a computer-on-a-chip used to control electronic devices. ... Please wikify (format) this article as suggested in the Guide to layout and the Manual of Style. ... In electronics, a digital-to-analog converter (DAC or D-to-A) is a device for converting a digital (usually binary) code to an analog signal (current, voltage or charges). ... The top electromagnet (1) is charged, attracting the topmost four teeth of a sprocket. ... Two kinematic mirror mounts, with mirrors. ...

If the beam is to be centered onto 4-quadrant diodes, no Analog-to-digital converter is needed. Operational amplifiers are sufficient. An analog-to-digital converter (abbreviated ADC, A/D or A to D) is an electronic circuit that converts continuous signals to discrete digital numbers. ... A 741 operational amplifier in a TO-5 metal can package An operational amplifier, often referred to as an op-amp, is a DC-coupled electronic differential voltage amplifier, usually of very high gain, with one inverting and one non-inverting input. ...

See also

• Active optics

• Nonlinear optics (4.4 Optical Phase Conjugation)

• Holography (3.0 Real-time holography)

Active optics is a relatively new technology for reflecting telescopes. ... Nonlinear optics is the branch of optics that describes the behaviour of light in nonlinear media, that is, media in which the polarization P responds nonlinearly to the electric field E of the light. ... Holography (from the Greek, Όλος-holos whole + γραφή-graphe writing) is the science of producing holograms, an advanced form of photography that allows an image to be recorded in three dimensions. ...

External links

• Description of Active & Adaptive Optics
• NSF Center for Adaptive Optics
• Using Adaptive Optics to image photoreceptors
• Ball aerospace Tip-tilt mirrors (accessed June 2005).
• PI piezoelectric tip-tilt nirrors (accessed June 2005).
• OKO Closed-loop AO systems and deformable mirrors.

• MEMS Deformable Mirrors for AO applications