![Atmos struct imaging](https://upload.wikimedia.org/wikipedia/commons/3/3b/Atmos_struct_imaging.svg)
In
astronomy
Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...
, seeing is the degradation of the
image of an
astronomical object due to
turbulence
In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between ...
in the
atmosphere of Earth
The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing for ...
that may become visible as blurring,
twinkling
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.Wang, Ting-I; Williams, Donn"Scintillation technology bests NIST" , ''InTe ...
or variable
distortion
In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signal ...
. The origin of this effect are rapidly changing variations of the optical
refractive index
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, or ...
along the light path of the object.
Seeing is a major limitation to the
angular resolution
Angular resolution describes the ability of any image-forming device such as an 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. ...
in astronomical observations with
telescopes
A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...
that would otherwise be
limited through
diffraction
Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...
by the size of the telescope
aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane.
An opt ...
.
Today, many large scientific ground-based
optical telescopes
An optical telescope is a 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 electr ...
include
adaptive optics
Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effect of incoming wavefront distortions by deforming a mirror in order to compensate for the distortion. It is used in astronomical tele ...
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
The Fried parameter or Fried's coherence length (commonly designated as r_0) is a measure of the quality of optical transmission through the atmosphere due to random inhomogeneities in the atmosphere's refractive index. In practice, such inhomogene ...
''r''
0. The diameter of the seeing disk is the
full width at half maximum
In a distribution, full width at half maximum (FWHM) is the difference between the two values of the independent variable at which the dependent variable is equal to half of its maximum value. In other words, it is the width of a spectrum curve mea ...
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
A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The na ...
or a Fried parameter larger than 30 centimeters can be considered excellent seeing. The best conditions are typically found at high-altitude
observatories
An observatory is a location used for observing terrestrial, marine, or celestial events. Astronomy, climatology/meteorology, geophysical, oceanography and volcanology are examples of disciplines for which observatories have been constructed. His ...
on small islands such as
Mauna Kea
Mauna Kea ( or ; ; abbreviation for ''Mauna a Wākea''); is a dormant volcano on the island of Hawaii. Its peak is above sea level, making it the highest point in the state of Hawaii and second-highest peak of an island on Earth. The peak is ...
or
La Palma
La Palma (, ), also known as ''La isla bonita'' () and officially San Miguel de La Palma, is the most north-westerly island of the Canary Islands, Spain. La Palma has an area of making it the fifth largest of the eight main Canary Islands. The ...
.
Effects
![Zeta bootis short exposure](https://upload.wikimedia.org/wikipedia/commons/8/81/Zeta_bootis_short_exposure.png)
Astronomical seeing has several effects:
# It causes the images of
point source
A point source is a single identifiable ''localised'' source of something. A point source has negligible extent, distinguishing it from other source geometries. Sources are called point sources because in mathematical modeling, these sources can ...
s (such as stars), which in the absence of atmospheric turbulence would be steady
Airy patterns produced by diffraction, to break up into speckle patterns, which change very rapidly with time (the resulting speckled images can be processed using
speckle imaging
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. They can be divided into the shift-and-add ("'' ...
)
# Long exposure images of these changing speckle patterns result in a blurred image of the point source, called a ''seeing disc''
# The brightness of stars appears to fluctuate in a process known as
scintillation or twinkling
# Atmospheric seeing causes the fringes in an
astronomical interferometer
An astronomical interferometer or telescope array is a set of separate telescopes, mirror segments, or radio telescope antenna (radio), antennas that work together as a single telescope to provide higher resolution images of astronomical objects ...
to move rapidly
# The distribution of atmospheric seeing through the atmosphere (the C
N2 profile described below) causes the image quality in
adaptive optics
Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effect of incoming wavefront distortions by deforming a mirror in order to compensate for the distortion. It is used in astronomical tele ...
systems to degrade the further you look from the location of reference star
The effects of atmospheric seeing were indirectly responsible for the belief that there were
canals on Mars
During the late 19th and early 20th centuries, it was erroneously believed that there were "canals" on the planet Mars. These were a network of long straight lines in the equatorial regions from 60° north to 60° south latitude on Mars, observed ...
. In viewing a bright object such as Mars, occasionally a
still patch of air will come in front of the planet, resulting in a brief moment of clarity. Before the use of
charge-coupled device
A charge-coupled device (CCD) is an integrated circuit containing an array of linked, or coupled, capacitors. Under the control of an external circuit, each capacitor can transfer its electric charge to a neighboring capacitor. CCD sensors are a ...
s, there was no way of recording the image of the planet in the brief moment other than having the observer remember the image and draw it later. This had the effect of having the image of the planet be dependent on the observer's memory and preconceptions which led the belief that Mars had linear features.
The effects of atmospheric seeing are qualitatively similar throughout the visible and near
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
wavebands. At large telescopes the long exposure image resolution is generally slightly higher at longer wavelengths, and the timescale (''t''
0 - see below) for the changes in the dancing speckle patterns is substantially lower.
Measures
There are three common descriptions of the astronomical seeing conditions at an observatory:
* The full width at half maximum (FWHM) of the seeing disc
* ''r''
0 (the size of a typical "lump" of uniform air within the turbulent atmosphere) and ''t''
0 (the time-scale over which the changes in the turbulence become significant)
*The C
N2 profile
These are described in the sub-sections below:
The full width at half maximum (FWHM) of the seeing disc
Without an atmosphere, a small star would have an apparent size, an "
Airy disk
In optics, the Airy disk (or Airy disc) and Airy pattern are descriptions of the best- focused spot of light that a perfect lens with a circular aperture can make, limited by the diffraction of light. The Airy disk is of importance in physics, ...
", in a telescope image determined by
diffraction
Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...
and would be inversely proportional to the diameter of the telescope. However, when light enters the
Earth's atmosphere
The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing for ...
, the different temperature layers and different wind speeds distort the light waves, leading to distortions in the image of a star. The effects of the atmosphere can be modeled as rotating cells of air moving turbulently. At most observatories, the turbulence is only significant on scales larger than ''r''
0 (see below—the seeing parameter ''r''
0 is 10–20 cm at visible wavelengths under the best conditions) and this limits the resolution of telescopes to be about the same as given by a space-based 10–20 cm telescope.
The distortion changes at a high rate, typically more frequently than 100 times a second. In a typical astronomical image of a star with an
exposure time
In photography, shutter speed or exposure time is the length of time that the film or digital sensor inside the camera is exposed to light (that is, when the camera's shutter is open) when taking a photograph.
The amount of light that re ...
of seconds or even minutes, the different distortions average out as a filled disc called the "seeing disc". The
diameter
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle. Both definitions are also valid for ...
of the seeing disk, most often defined as the
full width at half maximum
In a distribution, full width at half maximum (FWHM) is the difference between the two values of the independent variable at which the dependent variable is equal to half of its maximum value. In other words, it is the width of a spectrum curve mea ...
(FWHM), is a measure of the astronomical seeing conditions.
It follows from this definition that seeing is always a variable quantity, different from place to place, from night to night, and even variable on a scale of minutes. Astronomers often talk about "good" nights with a low average seeing disc diameter, and "bad" nights where the seeing diameter was so high that all observations were worthless.
Image:Eps_aql_movie_not_2000.gif, 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 7''r''0 (see definition of ''r''0 below, and example simulated image through a 7''r''0 telescope). The star breaks up into multiple blobs (speckles) -- entirely an atmospheric effect. Some telescope vibration is also noticeable.
The FWHM of the seeing disc (or just "seeing") is usually measured in
arcsecond
A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The na ...
s, abbreviated with the symbol (″). A 1.0″ seeing is a good one for average astronomical sites. The seeing of an urban environment is usually much worse. Good seeing nights tend to be clear, cold nights without wind gusts. Warm air rises (
convection
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...
), degrading the seeing, as do wind and clouds. At the best high-altitude mountaintop
observatories
An observatory is a location used for observing terrestrial, marine, or celestial events. Astronomy, climatology/meteorology, geophysical, oceanography and volcanology are examples of disciplines for which observatories have been constructed. His ...
, the wind brings in stable air which has not previously been in contact with the ground, sometimes providing seeing as good as 0.4".
''r''0 and ''t''0
The astronomical seeing conditions at an observatory can be conveniently described by the parameters ''r''
0 and ''t''
0.
For telescopes with diameters smaller than ''r''
0, the resolution of long-exposure images is determined primarily by diffraction and the size of the Airy pattern and thus is inversely proportional to the telescope diameter.
For telescopes with diameters larger than ''r''
0, the image resolution is determined primarily by the atmosphere and is independent of telescope diameter, remaining constant at the value given by a telescope of diameter equal to ''r''
0. ''r''
0 also corresponds to the length-scale over which the turbulence becomes significant (10–20 cm at visible wavelengths at good observatories), and ''t''
0 corresponds to the time-scale over which the changes in the turbulence become significant. ''r''
0 determines the spacing of the actuators needed in an
adaptive optics
Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effect of incoming wavefront distortions by deforming a mirror in order to compensate for the distortion. It is used in astronomical tele ...
system, and ''t''
0 determines the correction speed required to compensate for the effects of the atmosphere.
The parameters ''r''
0 and ''t''
0 vary with the wavelength used for the astronomical imaging, allowing slightly higher resolution imaging at longer wavelengths using large telescopes.
The seeing parameter ''r''
0 is often known as the
Fried parameter
The Fried parameter or Fried's coherence length (commonly designated as r_0) is a measure of the quality of optical transmission through the atmosphere due to random inhomogeneities in the atmosphere's refractive index. In practice, such inhomogene ...
(pronounced "freed"), named after
David L. Fried. The atmospheric time constant ''t''
0 is often referred to as the
Greenwood time constant, after
Darryl Greenwood.
Mathematical description of ''r''0 and ''t''0
Mathematical models can give an accurate model of the effects of astronomical seeing on images taken through ground-based telescopes. Three simulated short-exposure images are shown at the right through three different telescope diameters (as negative images to highlight the fainter features more clearly—a common astronomical convention). The telescope diameters are quoted in terms of the Fried parameter
(defined below).
is a commonly used measurement of the astronomical seeing at observatories. At visible wavelengths,
varies from 20 cm at the best locations to 5 cm at typical sea-level sites.
In reality, the pattern of blobs (''speckles'') in the images changes very rapidly, so that long-exposure photographs would just show a single large blurred blob in the center for each telescope diameter. The diameter (FWHM) of the large blurred blob in long-exposure images is called the seeing disc diameter, and is independent of the telescope diameter used (as long as adaptive optics correction is not applied).
It is first useful to give a brief overview of the basic theory of optical propagation through the atmosphere. In the standard classical theory, light is treated as an oscillation in a field
. For monochromatic plane waves arriving from a distant point source with wave-vector
:
where
is the complex field at position
and
time
, with real and imaginary parts corresponding to the electric and magnetic field components,
represents a phase offset,
is the frequency of the light determined by
, and
is the amplitude of the light.
The photon flux in this case is proportional to the square of the amplitude
, and the optical phase corresponds to the complex argument of
. As wavefronts pass through the Earth's atmosphere they may be perturbed by refractive index variations in the atmosphere. The diagram at the top-right of this page shows schematically a turbulent layer in the Earth's atmosphere perturbing planar wavefronts before they enter a telescope. The perturbed wavefront
may be related at any given instant to the original planar wavefront
in the following way:
where
represents the fractional change in wavefront amplitude and
is the change in wavefront phase introduced by the atmosphere. It is important to emphasise that
and
describe the effect of the Earth's atmosphere, and the timescales for any changes in these functions will be set by the speed of refractive index fluctuations in the atmosphere.
The Kolmogorov model of turbulence
A description of the nature of the wavefront perturbations introduced by the atmosphere is provided by the
Kolmogorov model developed by Tatarski, based partly on the studies of
turbulence
In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between ...
by the Russian mathematician
Andrey Kolmogorov
Andrey Nikolaevich Kolmogorov ( rus, Андре́й Никола́евич Колмого́ров, p=ɐnˈdrʲej nʲɪkɐˈlajɪvʲɪtɕ kəlmɐˈɡorəf, a=Ru-Andrey Nikolaevich Kolmogorov.ogg, 25 April 1903 – 20 October 1987) was a Sovi ...
. This model is supported by a variety of experimental measurements and is widely used in simulations of astronomical imaging. The model assumes that the
wavefront perturbations are brought about by variations in the refractive index of the atmosphere. These refractive index variations lead directly to phase fluctuations described by
, but any amplitude fluctuations are only brought about as a second-order effect while the perturbed wavefronts propagate from the perturbing atmospheric layer to the telescope. For all reasonable models of the Earth's atmosphere at optical and
infrared wavelengths the instantaneous imaging performance is dominated by the phase fluctuations
. The amplitude fluctuations described by
have negligible effect on the structure of the images seen in the focus of a large telescope.
For simplicity, the phase fluctuations in Tatarski's model are often assumed to have a Gaussian random distribution with the following second-order structure function:
where
is the atmospherically induced variance between the phase at two parts of the wavefront separated by a distance
in the aperture plane, and
represents the ensemble average.
For the Gaussian random approximation, the structure function of Tatarski (1961) can be described in terms of a single parameter
:
indicates the ''strength'' of the phase fluctuations as it corresponds to the diameter of a circular telescope aperture at which atmospheric phase perturbations begin to seriously limit the image resolution. Typical
values for I band (900 nm wavelength) observations at good sites are 20–40 cm.
also corresponds to the aperture diameter for which the variance
of the wavefront phase averaged over the aperture comes approximately to unity:
This equation represents a commonly used definition for
, a parameter frequently used to describe the atmospheric conditions at astronomical observatories.
can be determined from a measured C
N2 profile (described below) as follows:
where the turbulence strength
varies as a function of height
above the telescope, and
is the angular distance of the
astronomical source
Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, galaxies, ...
from the
zenith
The zenith (, ) is an imaginary point directly "above" a particular location, on the celestial sphere. "Above" means in the vertical direction (plumb line) opposite to the gravity direction at that location (nadir). The zenith is the "highest" ...
(from directly overhead).
If turbulent evolution is assumed to occur on slow timescales, then the timescale ''t''
0 is simply proportional to ''r''
0 divided by the mean wind speed.
The refractive index fluctuations caused by Gaussian random turbulence can be simulated using the following algorithm:
where
is the optical phase error introduced by atmospheric turbulence, R (k) is a two-dimensional square array of independent random complex numbers which have a Gaussian distribution about zero and white noise spectrum, K (k) is the (real) Fourier amplitude expected from the Kolmogorov (or Von Karman) spectrum, Re[] represents taking the real part, and FT[] represents a discrete Fourier transform of the resulting two-dimensional square array (typically an FFT).
Turbulent intermittency
The assumption that the phase fluctuations in Tatarski's model have a Gaussian random distribution is usually unrealistic. In reality, turbulence exhibits intermittency.
These fluctuations in the turbulence strength can be straightforwardly simulated as follows:
where__is_a_two-dimensional_array_which_represents_the_spectrum_of_intermittency,_with_the_same_dimensions_as_,_and_where_
_represents_convolution._The_intermittency_is_described_in_terms_of_fluctuations_in_the_turbulence_strength_
._It_can_be_seen_that_the_equation_for_the_Gaussian_random_case_above_is_just_the_special_case_from_this_equation_with:
I_(k)_=_\delta_(.html" ;"title="R(\mathbf)\otimes I(\mathbf))K(\mathbf)">mbox[(R(\mathbf)\otimes I(\mathbf))K(\mathbf)
where is a two-dimensional array which represents the spectrum of intermittency, with the same dimensions as , and where represents convolution. The intermittency is described in terms of fluctuations in the turbulence strength . It can be seen that the equation for the Gaussian random case above is just the special case from this equation with:
where is the Dirac delta function.
The profile
A more thorough description of the astronomical seeing at an observatory is given by producing a profile of the turbulence strength as a function of altitude, called a profile. profiles are generally performed when deciding on the type of adaptive optics system which will be needed at a particular telescope, or in deciding whether or not a particular location would be a good site for setting up a new astronomical observatory. Typically, several methods are used simultaneously for measuring the profile and then compared. Some of the most common methods include:
# SCIDAR (imaging the ''shadow patterns'' in the scintillation of starlight)
# LOLAS (a small-aperture variant of SCIDAR designed for low-altitude profiling)
# SLODAR
# MASS
# MooSci (11-channel lunar scintillometer for ground level profiling)
# RADAR mapping of turbulence
# Balloon-borne thermometers to measure how quickly the air temperature is fluctuating with time due to turbulence
# V2 Precision Data Collection Hub (PDCH) with differential temperature sensors use to measure atmospheric turbulence
There are also mathematical functions describing the profile. Some are empirical fits from measured data and others attempt to incorporate elements of theory. One common model for continental land masses is known as Hufnagel-Valley after two workers in this subject.
Mitigation
The first answer to this problem was speckle imaging
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. They can be divided into the shift-and-add ("'' ...
, which allowed bright objects with simple morphology to be observed with diffraction-limited angular resolution. Later came NASA's Hubble Space Telescope, working outside the atmosphere and thus not having any seeing problems and allowing observations of faint targets for the first time (although with poorer resolution than speckle observations of bright sources from ground-based telescopes because of Hubble's smaller telescope diameter). The highest resolution visible and infrared images currently come from imaging optical interferometer
Interferometry is a technique which uses the ''interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber op ...
s such as the Navy Prototype Optical Interferometer
The Navy Precision Optical Interferometer (NPOI) is an American astronomical interferometer, with the world's largest baselines, operated by the Naval Observatory Flagstaff Station (NOFS) in collaboration with the Naval Research Laboratory (NRL) ...
or Cambridge Optical Aperture Synthesis Telescope
COAST, the Cambridge Optical Aperture Synthesis Telescope, is a multi-element optical astronomical interferometer with baselines of up to 100 metres, which uses aperture synthesis to observe stars with angular resolution as high as one thousandt ...
, but those can only be used on very bright stars.
Starting in the 1990s, many telescopes have developed adaptive optics
Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effect of incoming wavefront distortions by deforming a mirror in order to compensate for the distortion. It is used in astronomical tele ...
systems that partially solve the seeing problem. The best systems so far built, such as SPHERE
A sphere () is a Geometry, geometrical object that is a solid geometry, three-dimensional analogue to a two-dimensional circle. A sphere is the Locus (mathematics), set of points that are all at the same distance from a given point in three ...
on the ESO VLT and GPI on the Gemini telescope, achieve a Strehl ratio The Strehl ratio is a measure of the quality of optical image formation, originally proposed by Karl Strehl, after whom the term is named. Used variously in situations where optical resolution is compromised due to lens aberrations or due to imag ...
of 90% at a wavelength of 2.2 micrometers, but only within a very small region of the sky at a time.
A wider field of view can be obtained by using multiple deformable mirrors conjugated to several atmospheric heights and measuring the vertical structure of the turbulence, in a technique known as Multiconjugate Adaptive Optics.
Another cheaper technique, lucky imaging
Lucky imaging (also called lucky exposures) is one form of speckle imaging used for astrophotography. Speckle imaging techniques use a high-speed camera with exposure times short enough (100 ms or less) so that the changes in the Earth's atm ...
, has had good results on smaller telescopes. This idea dates back to pre-war naked-eye observations of moments of good seeing, which were followed by observations of the planets on cine film after World War II
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
. The technique relies on the fact that every so often the effects of the atmosphere will be negligible, and hence by recording large numbers of images in real-time, a 'lucky' excellent image can be picked out. This happens more often when the number of r0-size patches over the telescope pupil is not too large, and the technique consequently breaks down for very large telescopes. It can nonetheless outperform adaptive optics in some cases and is accessible to amateurs. It does require very much longer observation times than adaptive optics
Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effect of incoming wavefront distortions by deforming a mirror in order to compensate for the distortion. It is used in astronomical tele ...
for imaging faint targets, and is limited in its maximum resolution.
See also
* Atmosphere and Telescope Simulator, a simulator of atmospheric turbulence
* Clear Sky Chart Clear Sky Charts (called clocks until February 29, 2008) are web graphics which deliver weather forecasts designed specifically for astronomers.
They forecast the cloud cover, transparency and astronomical seeing, parameters which are not forecast ...
, web charts that include weather forecast
Weather forecasting is the application of science and technology to predict the conditions of the atmosphere for a given location and time. People have attempted to predict the weather informally for millennia and formally since the 19th centu ...
s for astronomical seeing
* Mirage
A mirage is a naturally-occurring optical phenomenon in which light rays bend via refraction to produce a displaced image of distant objects or the sky. The word comes to English via the French ''(se) mirer'', from the Latin ''mirari'', meanin ...
, Heat haze
A mirage is a naturally-occurring optical phenomenon in which light rays bend via refraction to produce a displaced image of distant objects or the sky. The word comes to English via the French ''(se) mirer'', from the Latin ''mirari'', meanin ...
* Planetary boundary layer
In meteorology, the planetary boundary layer (PBL), also known as the atmospheric boundary layer (ABL) or peplosphere, is the lowest part of the atmosphere and its behaviour is directly influenced by its contact with a planetary surface. On Ear ...
* Transient lunar phenomenon
A transient lunar phenomenon (TLP) or lunar transient phenomenon (LTP) is a short-lived light, color or change in appearance on the surface of the Moon. The term was created by Patrick Moore in his co-authorship of NASA Technical Report R-277 ''Ch ...
References
Much of the above text is taken (with permission) from
Lucky Exposures: Diffraction limited astronomical imaging through the atmosphere
', by Robert Nigel Tubbs.
External links
Free 72-hour seeing prediction for every location on earth (Click on 'Charts&Tools' and then 'Astronomy Seeing')
Example: San Pedro de Atacama (Chile)
* Th
Includes animated illustrations of effects of seeing.
Seeing forecasts for Mauna Kea, Hawaii
{{DEFAULTSORT:Astronomical Seeing
Articles containing video clips
Astronomical imaging
Observational astronomy
Lunar observation
Speckle imaging