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Adaptive Optics Systems for Correcting Atmosphere Turbulence in Astronomy

Adaptive optics (AO) systems for correcting atmospheric turbulence in astronomy utilize an imaging system with a wavefront sensor and device to correct aberrations in light entering a telescope; typically this involves deforming mirrors; however other forms may also work.

An AO system must be calibrated using a known initial phase source, such as a nearby star. Furthermore, an accurate sensor must be in place to accurately record any signals coming into the system.

Adaptive mirrors

Astronomers seek to capture every fine detail possible of celestial objects, but atmospheric turbulence leads to distortions. Adaptive optics systems correct these distortions with deformable mirrors which correct for wavefront distortion entering telescope lenses and produce sharp, clear images of distant objects. First proposed by Horace Babcock in 1953 but developed gradually over a decade due to their complex architecture including sensitive wavefront sensors, computational power and high speed deformable mirrors.

Adaptive optics rely on sensors that measure and map out the distortion of light waves approaching a telescope, taking hundreds to several thousand frames per second to keep up with changes in atmospheric pressure and achieve diffraction-limited image quality. Sensors either use bright reference stars in their field of view for this process – which may be difficult to locate at some observatories – or they create artificial references with laser beams as an artificial reference star source.

Deformable mirrors must respond rapidly to changing distortion, sometimes as frequently as updating their shape multiple times per second – this high frequency of update allows the system to keep pace with atmospheric turbulence above telescope observatories.

As well as its use for astronomy, this technology also finds many other uses outside the realm of astronomy. It can help compensate for wavefront distortions caused by firing lasers over longer distances in military applications – compensating for these distortions beforehand makes it easier for lasers to hit their targets with an eye for precise optical effects.

Adaptive optics technology is being employed to monitor satellites orbiting Earth – known as space situational awareness – with an aim of maintaining their safety as manmade and natural space objects increase in numbers. A major concern with increasing numbers of satellites colliding could create an unsafe debris cloud which could prevent future missions from taking off; adaptive optics technology may provide an answer by tracking all debris.

Wavefront sensors

Wavefront sensors are vital components of adaptive optics systems because they measure distortion in incoming light waves and use that data to compensate for atmospheric turbulence distortions. To do so, these sensors compare output from telescopes with an ideal flat wavefront signal before sending correction signals directly to deformable mirrors to produce corrected light beams that reach cameras without incurring diffraction limitations and provide corrected images that meet or surpass them.

For most applications of adaptive optics (AO), the most prevalent and widely utilized solution is known as single conjugate AO systems; however, various other designs exist for measuring and controlling wavefront distortions – each offering their own set of benefits and drawbacks.

Shack-Hartmann wavefront sensors are commonly used to measure optical distortion of incoming light. This type of sensor consists of an array of lenslets with equal focal length that focus the beam into spots on a sensor surface; then using software, a computer calculates each spot’s position relative to the entrance area of the sensor and determines wavefront orientation accordingly.

As the Shack-Hartmann sensor features a fixed lens focal length, it can only measure a certain number of wavefront positions per second. To gain more comprehensive measurements of an incoming wavefront, other types of sensors are typically utilized.

These sensors offer much higher frame rates than Shack-Hartmann sensors; many can measure thousands of frames per second, making them popular choices in larger observatories. Furthermore, they are generally cheaper to produce and require less complex computer processing for processing purposes.

Wavefront reconstruction sensors use point sources to reconstruct incoming wavefronts. Although more expensive than other sensor types, this one provides highly accurate measurements of initial phase of wavefront. This enables AO systems to quickly correct for distortions caused by atmospheric effects or other sources of error while also being capable of detecting aberrations not caused by atmosphere alone.

Control systems

Control systems are essential components of adaptive optics. They monitor wavefront changes and send corrections directly to a deformable mirror based on type of turbulence; correcting small or large errors depending on its magnitude, while simultaneously shaping an input beam for specific types of turbulence. They must be reliable and possess high bandwidth in order to respond rapidly and accurately when input signal changes occur.

An AO system’s control system consists of various components, including a feedback path, summing junction and error detector. The feedback path transfers portions of its output back to summing junction, which in turn sends it onward to error detector. Error detector then compares output with input and produces an actuating signal which sends back to control element which adjusts mirror accordingly to correct any discrepancies or deviations in output or input data.

Adaptive optics is now widely employed at large observatories around the globe, where atmospheric seeing limits the resolution of telescopes. Furthermore, adaptive optics is vital in developing extremely large telescopes such as Hawaii’s Thirty Meter Telescope or Chile’s European Extremely Large Telescope; regardless of any challenges associated with adaptive optics astronomers have produced astounding scientific results with adaptive optics.

Adaptive optics technology involves sophisticated electronics and fast computers, with calculations performed at thousands of frames per second while sensors must operate with very low noise levels. One such adaptive optics system for Chile’s Magellan 6.5-meter telescope includes over 85 actuators to correct its wavefront while using laser guide stars as calibrators stars for calibrating its sensor system.

This system is currently the most advanced AO system available, yet still suffers from limitations. One such limitation is that it requires a very bright reference source – something not available everywhere astronomical sites – thus restricting what objects can be studied with it.

Researchers have developed multi-conjugate AO systems, combining two or more sensors, in order to overcome this limitation. Astronomers can then measure turbulence across various layers of atmosphere using this type of system; some models even come equipped with laser beacons which detect clouds as an impediment to performance of these AO systems.


Adaptive optics is an innovative system which can enhance telescope imaging quality significantly and unleash their full scientific potential. Light from an object is transmitted to a deformable mirror of the telescope for processing before being sent onward to be displayed as images onscreen. If there were no atmospheric turbulence, the wavefront of a mirror would be perfectly straight and parallel (see Figure 11). Light is then reflected to a beam splitter where some is directed back toward a wavefront sensor. The wavefront sensor measures distortions to the wavefront and transmits an adaptive mirror a correction signal for correcting it. In turn, this alters its shape to remove distortions caused by atmospheric turbulence; then this corrected light reaches a high-resolution camera where its image meets or surpasses diffraction limits for telescope optical systems.

Astronomers utilize adaptive optics as a technique for compensating against atmospheric turbulence’s blurring of telescope images at short wavelengths. While initially developed for infrared light sources, its application to optical telescopes has also become widespread.

Astronomers need a way to correct for distortions caused by telescope images that reflect off space objects – this is known as a “guide star”. Astronomers may use natural or artificial guide stars – both must be bright enough that their instruments can detect them.

Single Conjugated Adaptive Optics (SCAO) systems provide one of the simplest adaptive optics solutions. Turbulence can be corrected for by directing a laser beam from a telescope’s aperture toward a very bright natural guide star or artificial “fiducial marker”, which will reflect light off atmospheric surfaces in such a way as to be easily measured and corrected for.

Multi-Conjugate Adaptive Optics (MACAO) is another type of adaptive optics. MACAO systems developed at ESO can accurately characterize an extended field of view using multiple reference stars to characterize air column conditions over 1-2 arcmin compared to 15 arcsec offered by existing adaptive optics facilities.
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