Astronomical seeing

Schematic diagram illustrating how optical wavefronts from a distant star may be perturbed by a layer of turbulent mixing in the atmosphere. The vertical scale of the wavefronts plotted is highly exaggerated.
Simulated negative image showing what a single (point-like) star would look like through a ground-based telescope with a diameter of 2r0. The blurred look of the image is because of diffraction, which causes the appearance of the star to be an Airy pattern with a central disk surrounded by hints of faint rings. The atmosphere would make the image move around very rapidly, so that in a long-exposure photograph it would appear more blurred.
Simulated negative image showing what a single (point-like) star would look like through a ground-based telescope with a diameter of 7r0, on the same angular scale as the 2r0 image above. The atmosphere makes the image break up into several blobs (speckles). The speckles move around very rapidly, so that in a long-exposure photograph the star would appear as a single blurred blob.
Simulated negative image showing what a single (point-like) star would look like through a ground-based telescope with a diameter of 20r0. The atmosphere makes the image break up into several blobs (speckles). The speckles move around very rapidly, so that in a long-exposure photograph the star would appear as a single blurred blob.
Astronomical observatories are generally situated on mountaintops, as the air at ground level is usually more convective. A light wind bringing stable air from high above the clouds and ocean generally provides the best seeing conditions (telescope shown: NOT).
An animated image of the Moon's surface showing the effects of Earth's atmosphere on the view
Astronomers can make use of an artificial star by shining a powerful laser to correct for the blurring caused by the atmosphere.
This amateur lucky imaging stack using the best of 1800 frames of Jupiter captured using a relatively small telescope approaches the theoretical maximum resolution for the telescope, rather than being limited by seeing.
Slow motion movie of the image seen at a telescope when looking at a star at high magnification (negative images). The telescope used had a diameter of about 7r{{sub|0}} (see definition of r{{sub|0}} below, and example simulated image through a 7r{{sub|0}} telescope). The star breaks up into multiple blobs (speckles) -- entirely an atmospheric effect. Some telescope vibration is also noticeable.

Astronomical object due to turbulent airflows in the atmosphere of Earth that may become visible as blurring, twinkling or variable distortion.

- Astronomical seeing

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Fluid motion characterized by chaotic changes in pressure and flow velocity.

Laminar and turbulent water flow over the hull of a submarine. As the relative velocity of the water increases turbulence occurs.
Turbulence in the tip vortex from an airplane wing passing through coloured smoke
Flow visualization of a turbulent jet, made by laser-induced fluorescence. The jet exhibits a wide range of length scales, an important characteristic of turbulent flows.
The plume from this candle flame goes from laminar to turbulent. The Reynolds number can be used to predict where this transition will take place

Clear-air turbulence experienced during airplane flight, as well as poor astronomical seeing (the blurring of images seen through the atmosphere).


Location used for observing terrestrial, marine, or celestial events.

The Sphinx Observatory on a mountain top in the Swiss Alps at 3,571 m
The Hubble Space Telescope in Earth's orbit
SOFIA on board a Boeing 747SP

At high elevations, the Earth's atmosphere is thinner, thereby minimizing the effects of atmospheric turbulence and resulting in better astronomical "seeing".

Roque de los Muchachos Observatory

Astronomical observatory located in the municipality of Garafía on the island of La Palma in the Canary Islands, Spain.

The former Royal Greenwich Observatory at Herstmonceux in East Sussex demonstrating its cloudy viewing conditions, which led to a search for a better location for the large telescopes
Several of the helicopter pads built for the inauguration ceremony can be seen under the now dismantled HEGRA array
Telescopes at the observatory at sunset, from left to right: the NOT, the WHT, the DOT, the SST, the Mercator Telescope and the ING
Picture of the 10m Gran Telescopio Canarias building under construction in March 2003
Picture of the Nordic Optical Telescope (NOT)
Large-Sized-Telescope 1 of the Cherenkov Telescope Array (MAGIC)
360 degrees panorama as taken on 2011 January 28

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.

Angular resolution

Optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object, thereby making it a major determinant of image resolution.

Airy diffraction patterns generated by light from two point sources passing through a circular aperture, such as the pupil of the eye. Points far apart (top) or meeting the Rayleigh criterion (middle) can be distinguished. Points closer than the Rayleigh criterion (bottom) are difficult to distinguish.
Log-log plot of aperture diameter vs angular resolution at the diffraction limit for various light wavelengths compared with various astronomical instruments. For example, the blue star shows that the Hubble Space Telescope is almost diffraction-limited in the visible spectrum at 0.1 arcsecs, whereas the red circle shows that the human eye should have a resolving power of 20 arcsecs in theory, though normally only 60 arcsecs.

A single optical telescope may have an angular resolution less than one arcsecond, but astronomical seeing and other atmospheric effects make attaining this very hard.


Generic term for variations in apparent brightness, colour, or position of a distant luminous object viewed through a medium.

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.

Speckle imaging

Typical short-exposure image of a binary star (ζ Boötis) as seen through atmospheric turbulence. Each star should appear as a single point, but the atmosphere causes the images of the two stars to break up into two patterns of speckles. The speckles move around rapidly, so that each star appears as a single fuzzy blob in long exposure images.</gallery>

Speckle imaging describes a range of high-resolution astronomical imaging techniques based on the analysis of large numbers of short exposures that freeze the variation of atmospheric turbulence.

Minute and second of arc

Unit of angular measurement equal to 1⁄60 of one degree.

An illustration of the size of an arcminute (not to scale). A standard association football (soccer) ball (with a diameter of 22 cm) subtends an angle of 1 arcminute at a distance of approximately 756 m.
Comparison of angular diameter of the Sun, Moon, planets and the International Space Station. True represent&shy;ation of the sizes is achieved when the image is viewed at a distance of 103 times the width of the "Moon: max." circle. For example, if the "Moon: max." circle is 10 cm wide on a computer display, viewing it from 10.3 m away will show true representation of the sizes.
Example ballistic table for a given 7.62×51mm NATO load. Bullet drop and wind drift are shown both in mrad and minute of angle.
Comparison of milliradian (mrad) and minute of arc (MOA).

Because of the effects of atmospheric blurring, ground-based telescopes will smear the image of a star to an angular diameter of about 0.5″; in poor conditions this increases to 1.5″ or even more.

Martian canal

Erroneously believed that there were "canals" on the planet Mars.

1877 map of Mars by Giovanni Schiaparelli.
Mars as seen through a 6-inch (15 cm) aperture reflecting telescope, as Schiaparelli may have seen it.
Martian canals depicted by Percival Lowell

Astronomers had to stare for hours through their telescopes, waiting for a moment of still air when the image was clear, and then draw a picture of what they had seen.

Optical telescope

Telescope that gathers and focuses light mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct visual inspection, to make a photograph, or to collect data through electronic image sensors.

The Large Binocular Telescope uses two curved mirrors to gather light
Schematic of a Keplerian refracting telescope. The arrow at (4) is a (notional) representation of the original image; the arrow at (5) is the inverted image at the focal plane; the arrow at (6) is the virtual image that forms in the viewer's visual sphere. The red rays produce the midpoint of the arrow; two other sets of rays (each black) produce its head and tail.
Eight-inch refracting telescope at Chabot Space and Science Center
The Keck II telescope gathers light by using 36 segmented hexagonal mirrors to create a 10 m (33 ft) aperture primary mirror
These eyes represent a scaled figure of the human eye where 15 px = 1 mm, they have a pupil diameter of 7 mm. Figure A has an exit pupil diameter of 14 mm, which for astronomy purposes results in a 75% loss of light. Figure B has an exit pupil of 6.4 mm, which allows the full 100% of observable light to be perceived by the observer.
Two of the four Unit Telescopes that make up the ESO's VLT, on a remote mountaintop, 2600 metres above sea level in the Chilean Atacama Desert.
Comparison of nominal sizes of primary mirrors of some notable optical telescopes
Harlan J. Smith Telescope reflecting telescope at McDonald Observatory, Texas

The late 20th century has seen the development of adaptive optics and space telescopes to overcome the problems of astronomical seeing.

Hubble Space Telescope

Space telescope that was launched into low Earth orbit in 1990 and remains in operation.

Astronaut Owen Garriott working next to Skylab's crewed solar space observatory, 1973.
Dr. Nancy Grace Roman with a model of the Large Space Telescope that was eventually developed as the Hubble Space Telescope. While listed as a 1966 photo, this design was not the standard until the mid-1970s.
Grinding of Hubble's primary mirror at Perkin-Elmer, March 1979.
The backup mirror, by Kodak. Its inner support structure can be seen because it is not coated with a reflective surface.
The OTA, metering truss, and secondary baffle are visible in this image of Hubble during early construction.
DF-224 in Hubble, before it was replaced in 1999.
Exploded view of the Hubble Space Telescope
Hubble Control Center at Goddard Space Flight Center, 1999
STS-31 lifting off, carrying Hubble into orbit
Hubble being deployed from Discovery in 1990
An extract from a WF/PC image shows the light from a star spread over a wide area instead of being concentrated on a few pixels.
Optical evolution of Hubble's primary camera system. These images show spiral galaxy M100 as seen with WFPC1 in 1993 before corrective optics (left), with WFPC2 in 1994 after correction (center), and with WFC3 in 2018 (right).
COSTAR being removed in 2009
Astronauts Musgrave and Hoffman install corrective optics during SM1
Hubble as seen from Discovery during its second servicing mission
Nickel–hydrogen battery pack for Hubble
One of Hubble's most famous images, Pillars of Creation, shows stars forming in the Eagle Nebula.
The Frontier Fields program studied MACS0416.1-2403.
Star cluster Pismis 24 with nebula
The HST is sometimes visible from the ground, as in this 39-second exposure when it is in Orion. Maximum brightness about magnitude 1.
Hubble Extreme Deep Field image of space in the constellation Fornax
An illustration of a black hole
Hubble's STIS UV and ACS visible light combined to reveal Saturn's southern aurora
Brown spots mark Comet Shoemaker–Levy 9 impact sites on Jupiter's southern hemisphere. Imaged by Hubble.
Hubble and ALMA image of MACS J1149.5+2223
Evolution of detecting the early Universe
Some of the Carina nebula by WFC3
Hubble precision stellar distance measurement has been extended ten times further into the Milky Way.
Data analysis of a spectrum revealing the chemistry of hidden clouds
In 2001, NASA polled internet users to find out what they would most like Hubble to observe; they overwhelmingly selected the Horsehead Nebula.
One-quarter scale model at the courthouse in Marshfield, Missouri, a hometown of Edwin Hubble
A pillar of gas and dust in the Carina Nebula. This Wide Field Camera 3 image, dubbed Mystic Mountain, was released in 2010 to commemorate Hubble's 20th anniversary in space.
Hubble views the Fomalhaut system. This false-color image was taken in October 2004 and July 2006 with the Advanced Camera for Surveys.
Illustration of the Soft Capture Mechanism (SCM) installed on Hubble
Hubble and JWST mirrors (4.0 m2 and 25 m2 respectively)

First, the angular resolution (the smallest separation at which objects can be clearly distinguished) would be limited only by diffraction, rather than by the turbulence in the atmosphere, which causes stars to twinkle, known to astronomers as seeing.