Adaptive optics in the context of Retina

A laser guide star is an artificial star image created for use in astronomical adaptive optics systems, which are employed in large telescopes in order to correct atmospheric distortion of light (called astronomical seeing). Adaptive optics (AO) systems require a wavefront reference source of light called a guide star. Natural stars can serve as point sources for this purpose, but sufficiently bright stars are not available in all parts of the sky, which greatly limits the usefulness of natural guide star adaptive optics. Instead, one can create an artificial guide star by shining a laser into the atmosphere. Light from the beam is reflected by components in the upper atmosphere back into the telescope. This star can be positioned anywhere the telescope desires to point, opening up much greater amounts of the sky to adaptive optics.

Because the laser beam is deflected by astronomical seeing on the way up, the returning laser light does not move around in the sky as astronomical sources do. In order to keep astronomical images steady, a natural star nearby in the sky must be monitored in order that the motion of the laser guide star can be subtracted using a tip-tilt mirror. However, this star can be much fainter than is required for natural guide star adaptive optics because it is used to measure only tip and tilt, and all higher-order distortions are measured with the laser guide star. This means that many more stars are suitable, and a correspondingly larger fraction of the sky is accessible.

In astronomy, seeing is the degradation of the image of an astronomical object due to turbulence in the atmosphere of Earth that may become visible as blurring, twinkling or variable distortion. The origin of this effect is rapidly changing variations of the optical refractive index along the light path from the object to the detector.Seeing is a major limitation to the angular resolution in astronomical observations with telescopes that would otherwise be limited through diffraction by the size of the telescope aperture.Today, many large scientific ground-based optical telescopes include adaptive optics to overcome seeing.

The strength of seeing is often characterized by the angular diameter of the long-exposure image of a star (seeing disk) or by the Fried parameter r₀. The diameter of the seeing disk is the full width at half maximum of its optical intensity. An exposure time of several tens of milliseconds can be considered long in this context. The Fried parameter describes the size of an imaginary telescope aperture for which the diffraction limited angular resolution is equal to the resolution limited by seeing. Both the size of the seeing disc and the Fried parameter depend on the optical wavelength, but it is common to specify them for 500 nanometers.A seeing disk smaller than 0.4 arcseconds or a Fried parameter larger than 30 centimeters can be considered excellent seeing. The best conditions are typically found at high-altitude observatories on small islands, such as those at Mauna Kea or La Palma.

A nuclear star cluster (NSC) or compact stellar nucleus (sometimes called young stellar nucleus) is a star cluster with high density and high luminosity near the center of mass of most galaxies.

NSCs are the central massive objects of fainter, low-mass galaxies where supermassive black holes (SMBHs) are not present or are of negligible mass. In the most massive galaxies, NSCs are entirely absent. Some galaxies, including the Milky Way, are known to contain both a NSC and a SMBH of comparable mass.

Neptune has been directly explored by one space probe, Voyager 2, in 1989. As of 2025, there are no confirmed future missions to visit the Neptunian system. NASA, ESA, CNSA and independent academic groups have proposed future scientific missions to visit Neptune. Some mission plans are still active, while others have been abandoned or put on hold.

Since the mid-1990s, Neptune has been studied from afar with telescopes, including the Hubble Space Telescope and the ground-based Keck telescope using adaptive optics.

Roque de los Muchachos Observatory (Spanish: Observatorio del Roque de los Muchachos, ORM) is an astronomical observatory located in the municipality of Garafía on the island of La Palma in the Canary Islands, Spain. The observatory site is operated by the Instituto de Astrofísica de Canarias, based on nearby Tenerife. ORM is part of the European Northern Observatory.

The seeing statistics at ORM make it the second-best location for optical and infrared astronomy in the Northern Hemisphere, after Mauna Kea Observatory, Hawaii. The site also has some of the most extensive astronomical facilities in the Northern Hemisphere; its fleet of telescopes includes the 10.4 m Gran Telescopio Canarias, the world's largest single-aperture optical telescope as of July 2009, the William Herschel Telescope (second largest in Europe), and the adaptive optics corrected Swedish 1-m Solar Telescope.

Deformable mirrors (DM) are mirrors whose surface can be deformed, in order to achieve wavefront control and correction of optical aberrations. Deformable mirrors are used in combination with wavefront sensors and real-time control systems in adaptive optics. In 2006 they found a new use in femtosecond pulse shaping.

The shape of a DM can be controlled with a speed that is appropriate for compensation of dynamic aberrations present in the optical system. In practice the DM shape should be changed much faster than the process to be corrected, as the correction process, even for a static aberration, may take several iterations.

When observing a star through a telescope, the atmosphere distorts the incoming light, making images blurry and causing stars to twinkle. The Fried parameter, or Fried's coherence length, is a quantity that measures the strength of this optical distortion. It is denoted by the symbol ${\displaystyle r_{0}}$ and has units of length, usually expressed in centimeters.

The Fried parameter can be thought of as the diameter of a "tube" of relatively calm air through the turbulent atmosphere. Within this area, the seeing is good. A telescope with an aperture diameter ${\displaystyle D}$ that is smaller than ${\displaystyle r_{0}}$ can achieve a resolution close to its theoretical best (the diffraction limit). However, for telescopes with apertures much larger than ${\displaystyle r_{0}}$—which includes all modern professional telescopes—the image resolution is limited by the atmosphere, not the telescope's size. The angular resolution of a large telescope without adaptive optics is limited to approximately ${\displaystyle \lambda /r_{0}}$, where ${\displaystyle \lambda }$ is the wavelength of the light observed. At good observatory sites, ${\displaystyle r_{0}}$ is typically 10–20 cm at visible wavelengths. Large ground-based telescopes use adaptive optics to compensate for atmospheric effects and reach the diffraction limit.

An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal. An optical amplifier may be thought of as a laser without an optical cavity, or one in which feedback from the cavity is suppressed. Optical amplifiers are important in optical communication and laser physics. They are used as optical repeaters in the long distance fiber-optic cables which carry much of the world's telecommunication links.

There are several different physical mechanisms that can be used to amplify a light signal, which correspond to the major types of optical amplifiers. In doped fiber amplifiers and bulk lasers, stimulated emission in the amplifier's gain medium causes amplification of incoming light. In semiconductor optical amplifiers (SOAs), electron–hole recombination occurs. In Raman amplifiers, Raman scattering of incoming light with phonons in the lattice of the gain medium produces photons coherent with the incoming photons. Parametric amplifiers use parametric amplification.

Hiʻiaka, formal designation (136108) Haumea I, is the larger, outer moon of the trans-Neptunian dwarf planet Haumea. Discovered by Michael E. Brown and the Keck Observatory adaptive optics team on 26 January 2005, it is named after Hiʻiaka, the patron goddess of the Big Island of Hawaii and one of the daughters of Haumea. The moon follows a slightly elliptical orbit around Haumea every 49.5 days, at a distance of 49,400 km (30,700 mi).

Hiʻiaka is an elongated and irregularly shaped body with a mean diameter of 369 km (229 mi), making it the sixth-largest known moon of a trans-Neptunian object. It has a very low bulk density between 0.46 g/cm and 0.69 g/cm, which indicates it is mostly made of loosely-packed water ice and rock. Telescope observations have shown that Hiʻiaka has a highly reflective surface made of crystalline water ice, much like Haumea itself. Hiʻiaka rotates about its axis every 9.68 hours, and appears to rotate sideways with respect to its orbit around Haumea. Like its smaller sibling moon Namaka, Hiʻiaka is believed to be a fragment of Haumea that was ejected in the aftermath of a giant impact 4.4 billion years ago.

Namaka (full designation (136108) Haumea II) is the smaller, inner moon of the trans-Neptunian dwarf planet Haumea. Discovered by Michael E. Brown and the Keck Observatory adaptive optics team in the fall of 2005, it is named after Nāmaka, a water spirit and one of the daughters of Haumea in Hawaiian mythology. Namaka follows a highly elliptical orbit that is highly tilted by roughly 13 degrees with respect to Haumea's equator. Namaka is heavily perturbed by both the gravitational influence of Haumea's larger, outer moon Hiʻiaka and the variable gravitational field of Haumea's elongated shape.

With a diameter of around 150 km (93 mi), Namaka is predicted to have an irregular shape and a chaotic rotation. It has a reflective surface made of fresh water ice, similar to that of Haumea and Hiʻiaka. Like Hiʻiaka, Namaka is believed to be a fragment of Haumea that was ejected in the aftermath of a giant impact 4.4 billion years ago.

The Multi-Unit Spectroscopic Explorer (MUSE) is an integral field spectrograph installed at the Very Large Telescope (VLT) of the European Southern Observatory (ESO). It operates in the visible wavelength range, and combines a wide field of view with a high spatial resolution and a large simultaneous spectral range (480-930 nm). It is specifically designed to take advantage of the improved spatial resolution provided by adaptive optics, offering diffraction-limited performance in specific configurations. MUSE had first light on the VLT’s Unit Telescope 4 (UT4) on 31 January 2014.

The Extremely Large Telescope (ELT) is an astronomical observatory under construction. When completed, it will be the world's largest optical and near-infrared extremely large telescope. Part of the European Southern Observatory (ESO) agency, it is located on top of Cerro Armazones in the Atacama Desert of northern Chile, 23 km from the existing facilities at Paranal Observatory.

The design consists of a reflecting telescope with a 39.3-metre-diameter (130-foot) segmented primary mirror and a 4.25 m (14 ft) diameter secondary mirror. The telescope is equipped with adaptive optics, six laser guide star units, and various large-scale scientific instruments. The observatory's design will gather 100 million times more light than the human eye, equivalent to about 10 times more light than the largest optical telescopes in existence as of 2025, with the ability to correct for atmospheric distortion. It has around 250 times the light-gathering area of the Hubble Space Telescope and, according to the ELT's specifications, will provide images 15 times sharper than those from Hubble.