Astronomical seeing in the context of "Roque de los Muchachos Observatory"

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.

The Mauna Kea Observatories (MKO) are a group of independent astronomical research facilities and large telescope observatories that are located at the summit of Mauna Kea on Hawaiʻi, United States. The facilities are located in a 525-acre (212 ha) special land use zone known as the "Astronomy Precinct", which is located within the 11,228-acre (4,544 ha) Mauna Kea Science Reserve. The Astronomy Precinct was established in 1967 and is located on land protected by the Historical Preservation Act for its significance to Hawaiian culture. The presence and continued construction of telescopes is highly controversial due to Mauna Kea's centrality in native Hawaiian religion and culture, as well as for a variety of environmental reasons.

The location is nearly ideal because of its dark skies from lack of light pollution, good astronomical seeing due to low atmospheric turbulence, low humidity, high elevation of 4,207 m (13,802 ft), position above most of the water vapor in the atmosphere, clean air, low cloud cover and low latitude location.

Adaptive optics (AO) is a technique of precisely deforming a mirror in order to compensate for light distortion. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, in microscopy, optical fabrication and in retinal imaging systems (ophthalmoscopy) to reduce 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.

Adaptive optics should not be confused with active optics, which work on a longer timescale to correct the primary mirror geometry.

An astronomical interferometer or telescope array is a set of separate telescopes, mirror segments, or radio telescope antennas that work together as a single telescope to provide higher resolution images of astronomical objects such as stars, nebulas and galaxies by means of interferometry. The advantage of this technique is that it can theoretically produce images with the angular resolution of a huge telescope with an aperture equal to the separation, called baseline, between the component telescopes. The main drawback is that it does not collect as much light as the complete instrument's mirror. Thus it is mainly useful for fine resolution of more luminous astronomical objects, such as close binary stars. Another drawback is that the maximum angular size of a detectable emission source is limited by the minimum gap between detectors in the collector array.

Interferometry is most widely used in radio astronomy, in which signals from separate radio telescopes are combined. A mathematical signal processing technique called aperture synthesis is used to combine the separate signals to create high-resolution images. In Very Long Baseline Interferometry (VLBI) radio telescopes separated by thousands of kilometers are combined to form a radio interferometer with a resolution which would be given by a hypothetical single dish with an aperture thousands of kilometers in diameter. At the shorter wavelengths used in infrared astronomy and optical astronomy it is more difficult to combine the light from separate telescopes, because the light must be kept coherent within a fraction of a wavelength over long optical paths, requiring very precise optics. Practical infrared and optical astronomical interferometers have only recently been developed, and are at the cutting edge of astronomical research. At optical wavelengths, aperture synthesis allows the atmospheric seeing resolution limit to be overcome, allowing the angular resolution to reach the diffraction limit of the optics.

Teide Observatory (Spanish: Observatorio del Teide), IAU code 954, is an astronomical observatory on Mount Teide at 2,390 metres (7,840 ft), located on Tenerife, Spain. It has been operated by the Instituto de Astrofísica de Canarias since its inauguration in 1964. It became one of the first major international observatories, attracting telescopes from different countries around the world because of the good astronomical seeing conditions. Later, the emphasis for optical telescopes shifted more towards Roque de los Muchachos Observatory on La Palma.

Twinkling, also called scintillation, is a generic term for variations in apparent brightness, colour, or position of a distant luminous object viewed through a medium. If the object lies outside the Earth's atmosphere, as in the case of stars and planets, the phenomenon is termed astronomical scintillation; for objects within the atmosphere, the phenomenon is termed terrestrial scintillation. As one of the three principal factors governing astronomical seeing (the others being light pollution and cloud cover), atmospheric scintillation is defined as variations in illuminance only.

In simple terms, twinkling of stars is caused by the passing of light through different layers of a turbulent atmosphere. Most scintillation effects are caused by anomalous atmospheric refraction caused by small-scale fluctuations in air density usually related to temperature gradients. Scintillation effects are always much more pronounced near the horizon than near the zenith (directly overhead), since light rays near the horizon must have longer paths through the atmosphere before reaching the observer. Atmospheric twinkling is measured quantitatively using a scintillometer.

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.