Home About us Products Services Contact us Bookmark
:: wikimiki.org ::
372

372

Historio > Jarcentoj > 4-a jarcento > 372 ---- En la jaro 372 post Kristo okazis, interalie:

Eventoj

Naskiĝoj

Mortoj

----
367 | 368 | 369 | 370 | 371 | 372 | 373 | 374 | 375 | 376 | 377
3-a jarcento - 4-a jarcento - 5-a jarcento
ko:372년

Historio

La historio ampleksas ĉiujn pasintajn okazaĵojn kaj la evoluojn de homoj, popoloj, komunumoj, socioj, civilizoj, landoj, ŝtatoj,regantoj kaj regatoj. La historion studas historiologio (=scienco pri historio/historiscienco). Laŭ pri strikta difino oni nomas historion nur tion, kio estas pruvebla per dokumentoj. Tiusence, la historia tempo komenciĝis nur, kiam aperis skribitaj informoj. Tamen eblas esplori ankaŭ la antaŭan (senskriban) tempon; ĝin oni nomas antaŭhistorio aŭ prahistorio. (NB! Por historioj de lingvo, teatro ktp., vd. je la koncernataj artikoloj.) ---- :Historio de Scienco kaj Teknologio

Historiistoj (kaj historiaj pensintoj)

:Diodoro Sicila - Norbert ELIAS - Gibbon - Hamilton - Hekateo el Mileso - Herodoto - Josephus - ibn-Khaldun - Keynes - Lev GUMILEV - Makiavelo - Markso - Montesquieu - Plutarko - Rousseau - Adam SMITH - Suetonio - Tacito - Toqueville - Toynbee - Gloraj virinoj

Historiistaj sciencoj kaj teĥnikoj

: datadmetodoj - arĥivoj

Antikvaj Gentoj kaj Ŝtatoj

:Akadanoj - Akado - Amoreoj - Arameoj - Asirianoj - Asirio - Aztekoj - Babilonio- Elamo - Etruskoj - Frankoj - Gaŭloj - Galatoj - Gepidoj - Geto-dakoj - Ĝermanoj - Habiroj -Hunoj - Iberia - Induso-civilizo - Judoj - Kasitoj - Karirioj-Kuŝanoj kvadoj- Maŭroj - Ostrogotoj - Panonio - Parthoj - Peĉenegoj - Piktoj - Seleŭkio - Sumeranoj - Sumero - Urartu - Vandaloj - Visigotoj -

Bataloj Famaj

Civilizoj

:Feŭdismo - Historio de Pratempo - Historio de Antikveco - Historio de Mezepoko - Kavaliro

Dinastioj

vidu ankaŭ:
- Listo de papoj
- Listoj de Imperiestroj
- Francaj reĝoj
- Reĝoj de Hispanio
- (por caroj de Rusio aŭ aliaj imperiestroj, vidu ene de Listoj de Imperiestroj)

Historio de kontinentoj


- Historio de Afriko
- Historio de Ameriko
- Historio de Aŭstralio
- Historio de Azio
- Historio de Eŭropo

Kalendaroj

:Gregoria Kalendaro - Luna Kalendaro - Hebrea Kalendaro - Hinda Kalendaro - Islama Kalendaro - Persa Kalendaro - Julia Kalendaro - Franca Respublika Kalendaro - Bahaa Kalendaro

Imperioj

Imperiestro (inkluzive ĉiuj la listoj) :Araba Imperio - Bizanca Imperio - Brita Imperio - Ĉina Imperio - Kolĥeti - Grandmoravia regno - Meŝika imperio - Otomana imperio - Romio - Rusa Imperio - Sovetunio - Tria Regno - Usona Imperio

Mapoj

:[http://www.roman-emperors.org/Index.htm Eŭropo kaj Mediteraneo politike, en la jaroj 1-1500] Vidu ankaŭ: Geografio Diaoyu-Insularo De Ĉinio

Militoj

vidu la Listo de militoj

Nuntempaj Etnoj: Vidu ĉe Etnoj

Nuntempaj Ŝtatoj: Vidu ĉe Nacioj

Okazintaĵoj

:Hipia revolto - Franca Revolucio - R.M.S. Titanic - 11-a de septembro 2001 - Koloniigado - Krucmilito - Sorĉopersekuto

Regionoj

:Anatolio - Antiloj - Mezopotamio - Palestino - ktp

Rolantoj

:Antonio - Breĵnevo - Osama Bin-Laden - Napoléon BONAPARTE - Marko Bruto - Aŭgusto Cezaro - Julio Cezaro - Charles DE GAULLE - Vo Nguyen GIAP - Gorbaĉevo - Herodo la Granda - Hipioj - Jelcino - Katono - Robert F. Kennedy - Lenino - Nasser - Pompejo - Sargono - Stalino - Vercingetorix - Usonaj Prezidentoj - Meksikaj prezidantoj

Tempordigo: Famaj aŭ klasikaj epokoj:
- Inter-du-milita epoko (Eŭropo, 20 jarcento)
- "Jarcento de Ludoviko la 14-a (Francio 17-a jarcento)
- Mezepoko
- Renesanco (Eŭropo 15-a jarcento -Italio - aŭ 16-a jarcento - aliaj landoj)

Historio enklasigata laŭ dato


- Jarcentoj
  - 19-a jarcento
  - 20-a jarcento
  - 21-a jarcento
  - ...
- Tagoj
  - januaro
    - 1, 2, 3, ... Komparu kun:

arkeologio, analo... fiu-vro:Aolugu ja:歴史 ko:역사 ms:Sejarah simple:History th:ประวัติศาสตร์ zh-min-nan:Le̍k-sú

Jarcentoj

Historio > Jarcentoj ---- Jarcento ampleksas tempon de 100 jaroj. La jarcentoj post Kristo do komenciĝas je la 1-a de januaro de jaro ..01 kaj finiĝas je la 31-a de decembro ..00. Ekzemple la 20-a jarcento daŭris de la 1-a de januaro 1901 ĝis la 31-a de decembro 2000. Tiu oficiala difino ne kongruas kun la populara difino, kiu emas festi la ŝanĝon de la jarcenta cifero en la jarnumero, ekzemple je jarŝanĝo 1999/2000. La jarcentoj antaŭ Kristo (kiam la Gregoria kalendaro ne estis konata) komenciĝis je la 1-a de januaro de iu jaro ..00 kaj daŭris ĝis la 31-a de decembro ..01. Ekzemple la unua jarcento antaŭ Kristo komenciĝis je la 1-a de januaro -100 kaj daŭris ĝis la 31-a de decembro -1. La sekvan tagon, la 1-an de januaro 1, komenciĝis la unua jarcento post Kristo. Jarcentoj antaŭ kaj post Kristo: Kategorio:Tempo ja:年表 simple:Century

4-a jarcento

Historio > Jarcentoj: 3-a jarcento - 4-a jarcento - 5-a jarcento Jaroj:
301302303304305 306307308309310
311312313314315 316317318319320
321322323324325 326327328329330
331332333334335 336337338339340
341342343344345 346347348349350
351352353354355 356357358359360
361362363364365 366367368369370
371372373374375 376377378379380
381382383384385 386387388389390
391392393394395 396397398399400
ja:4世紀 ko:4세기 th:คริสต์ศตวรรษที่ 4

368

Historio > Jarcentoj > 4-a jarcento > 368 ---- En la jaro 368 post Kristo okazis, interalie:

Eventoj

Naskiĝoj

Mortoj

----
363 | 364 | 365 | 366 | 367 | 368 | 369 | 370 | 371 | 372 | 373
3-a jarcento - 4-a jarcento - 5-a jarcento
ko:368년

370

Historio > Jarcentoj > 4-a jarcento > 370 ---- En la jaro 370 post Kristo okazis, interalie:

Eventoj

Naskiĝoj

Mortoj

----
365 | 366 | 367 | 368 | 369 | 370 | 371 | 372 | 373 | 374 | 375
3-a jarcento - 4-a jarcento - 5-a jarcento
ko:370년

373

Historio > Jarcentoj > 4-a jarcento > 373 ---- En la jaro 373 post Kristo okazis, interalie:

Eventoj

Naskiĝoj

Mortoj

----
368 | 369 | 370 | 371 | 372 | 373 | 374 | 375 | 376 | 377 | 378
3-a jarcento - 4-a jarcento - 5-a jarcento
ko:373년

374

Historio > Jarcentoj > 4-a jarcento > 374 ---- En la jaro 374 post Kristo okazis, interalie:

Eventoj

Naskiĝoj

Mortoj

----
369 | 370 | 371 | 372 | 373 | 374 | 375 | 376 | 377 | 378 | 379
3-a jarcento - 4-a jarcento - 5-a jarcento
ko:374년

376

Historio > Jarcentoj > 4-a jarcento > 376 ---- En la jaro 376 post Kristo okazis, interalie:

Eventoj

Naskiĝoj

Mortoj

----
371 | 372 | 373 | 374 | 375 | 376 | 377 | 378 | 379 | 380 | 381
3-a jarcento - 4-a jarcento - 5-a jarcento
ko:376년

377

Historio > Jarcentoj > 4-a jarcento > 377 ---- En la jaro 377 post Kristo okazis, interalie:

Eventoj

Naskiĝoj

Mortoj


- Sankta Juliano Sabaso
- Sankta Basilio la Granda ----
372 | 373 | 374 | 375 | 376 | 377 | 378 | 379 | 380 | 381 | 382
3-a jarcento - 4-a jarcento - 5-a jarcento
ko:377년

3-a jarcento

Historio > Jarcentoj: 2-a jarcento - 3-a jarcento - 4-a jarcento Jaroj:
201202203204205 206207208209210
211212213214215 216217218219220
221222223224225 226227228229230
231232233234235 236237238239240
241242243244245 246247248249250
251252253254255 256257258259260
261262263264265 266267268269270
271272273274275 276277278279280
281282283284285 286287288289290
291292293294295 296297298299300
ja:3世紀 ko:3세기

4-a jarcento

Historio > Jarcentoj: 3-a jarcento - 4-a jarcento - 5-a jarcento Jaroj:
301302303304305 306307308309310
311312313314315 316317318319320
321322323324325 326327328329330
331332333334335 336337338339340
341342343344345 346347348349350
351352353354355 356357358359360
361362363364365 366367368369370
371372373374375 376377378379380
381382383384385 386387388389390
391392393394395 396397398399400
ja:4世紀 ko:4세기 th:คริสต์ศตวรรษที่ 4

5-a jarcento

Historio > Jarcentoj: 4-a jarcento - 5-a jarcento - 6-a jarcento Jaroj:
401402403404405 406407408409410
411412413414415 416417418419420
421422423424425 426427428429430
431432433434435 436437438439440
441442443444445 446447448449450
451452453454455 456457458459460
461462463464465 466467468469470
471472473474475 476477478479480
481482483484485 486487488489490
491492493494495 496497498499500
ja:5世紀 ko:5세기 th:คริสต์ศตวรรษที่ 5

Seeing disk

Astronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by the Earth's atmosphere. The astronomical seeing conditions on a given night at a given location describe how much the Earth's atmosphere perturbs the images of stars as seen through a telescope. The most common seeing measurement is the diameter (technically Full Width Half Maximum) of the seeing disc. The seeing disc diameter ("seeing") is a reference to the best possible angular resolution which can be achieved by an optical telescope in a long photographic exposure, and corresponds to the diameter of the fuzzy blob seen when observing a point-like star through the atmosphere. The size of the seeing disc is determined by the astronomical seeing conditions at the time of the observation. The best conditions give a seeing disc diameter of ~0.4 arcseconds and are found at high-altitude observatories on small islands such as Mauna Kea or La Palma. A detailed description of the seeing disc can be found in the FWHM of the seeing disc subsection of the following article. Seeing is one of the biggest problems for Earth-based astronomy: while the big telescopes have theoretically milli-arcsecond resolution, the real image will never be better than the average seeing disc during the observation. This can easily mean a factor of 100 between the potential and practical resolution.

The Effects of Astronomical Seeing

Image:Zeta_bootis_short_exposure.png|Typical short-exposure negative image of a binary star (Zeta Bootis in this case) as seen through atmospheric seeing. 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 (one pattern above left, the other below right). The speckles are a little difficult to make out in this image due to the coarse pixel size on the camera used (see the simulated images below for a clearer example). The speckles move around rapidly, so that each star appears as a single fuzzy blob in long exposure images (called a seeing disc). The telescope used had a diameter of about 7r0 (see definition of r0 below, and example simulated image through a 7r0 telescope).
Astronomical seeing has several effects: # It causes the images of point-sources (e.g. stars) to break up into speckle patterns, which change very rapidly with time (the resulting speckled images can be processed using speckle imaging) # 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 to move rapidly # The distribution of atmospheric seeing through the atmosphere (the CN2 profile described below) causes the image quality in adaptive optics 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. 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 devices, 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 infra-red wavebands. At large telescopes the long exposure image resolution is generally slightly higher at longer wavelengths, and the timescale (t0 - see below) for the changes in the dancing speckle patterns is substantially lower.

Measures of Astronomical Seeing

There are three common descriptions of the astronomical seeing conditions at an observatory: # The FWHM of the seeing disc # r0 and t0 # The CN2 profile These are described in the sub-sections below:

The FWHM of the seeing disc

Without an atmosphere, a small star would have an apparent size in a telescope image determined by diffraction and would be inversely proportional to the diameter of the telescope. However when light enters the Earth's atmosphere, 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 modelled as rotating cells of air moving turbulently. At most observatories the turbulence is only significant on scales larger than r0 (see below -- the seeing parameter r0 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 of seconds or even minutes, the different distortions average out as a filled disc called the seeing disc. The diameter of the seeing disc (technically the Full Width at Half Maximum intensity (FWHM)) is a common 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 what you see through a telescope when you look at a star at high magnification (negative images). The telescope used had a diameter of about 7r0 (see definition of r0 below, and example simulated image through a 7r0 telescope). Notice how the star breaks up into multiple blobs (speckles) -- entirely an atmospheric effect. Some telescope vibration is also noticible.
The FWHM of the seeing disc (or just Seeing) is usually measured in arcseconds, 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) degrading the seeing as does wind and clouds. At the best high-altitude mountaintop observatories the wind brings in stable air which has not previously been in contact with the ground, sometimes providing seeing as good as 0.4".

r0 and t0

The astronomical seeing conditions at an observatory can be well described by the parameters r0 and t0. For telescopes with diameters smaller than r0, the resolution of long-exposure images is inversely proportional to the telescope diameter. For telescopes with diameters larger than r0, the image resolution is independent of telescope diameter, remaining constant at the value given by a telescope of diameter equal to r0. r0 also corresponds to the length-scale over which the turbulence becomes significant (10-20 cm at visible wavelengths at good observatories), and t0 corresponds to the time-scale over which the changes in the turbulence become significant. r0 determines the spacing of the actuators needed in an adaptive optics system, and t0 determines the correction speed required to compensate for the effects of the atmosphere. r0 and t0 vary with the wavelength used for the astronomical imaging, allowing slightly higher resolution imaging at longer wavelengths using large telescopes. r0 is often known as the Fried parameter (pronounced freed), named after David L. Fried.

Mathematical Description of r0 and t0

David L. Fried David L. Fried David L. Fried 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 r_ (defined below). r_ is a commonly used measurement of the astronomical seeing at observatories. At visible wavelengths, r_ 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 centre 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 \psi. For monochromatic plane waves arriving from a distant point source with wave-vector \mathbf: \psi_ \left(\mathbf,t\right) = A_e^ where \psi_ is the complex field at position \mathbf and time t, with real and imaginary parts corresponding to the electric and magnetic field components, \phi_ represents a phase offset, \nu is the frequency of the light determined by \nu=c\left | \mathbf \right | / \left ( 2 \pi \right ), and A_ is the amplitude of the light. The photon flux in this case is proportional to the square of the amplitude A_, and the optical phase corresponds to the complex argument of \psi_. 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 \psi_ may be related at any given instant to the original planar wavefront \psi_ \left(\mathbf\right) in the following way: \psi_ \left(\mathbf\right) = \left ( \chi_ \left(\mathbf\right) e^\right ) \psi_ \left(\mathbf\right) where \chi_ \left(\mathbf\right) represents the fractional change in wavefront amplitude and \phi_ \left(\mathbf\right) is the change in wavefront phase introduced by the atmosphere. It is important to emphasise that \chi_ \left(\mathbf\right) and \phi_ \left(\mathbf\right) 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 (1961), based partly on the studies of turbulence by the Russian mathematician Andreï Kolmogorov (see references below by Kolmogorov). This model is supported by a variety of experimental measurements (see e.g. references below by Buscher et al 1995, Nightingale and Buscher 1991, O’Byrne 1988, Colavita et al 1987) 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 \phi_ \left(\mathbf\right), 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 infra-red wavelengths the instantaneous imaging performance is dominated by the phase fluctuations \phi_ \left(\mathbf\right). The amplitude fluctuations described by \chi_ \left(\mathbf\right) have negligible effect on the structure of the images seen in the focus of a large telescope. The phase fluctuations in Tatarski's model are usually assumed to have a Gaussian random distribution with the following second order structure function: D_\left(\mathbf \right) = \left \langle \left | \phi_ \left ( \mathbf \right ) - \phi_ \left ( \mathbf + \mathbf \right ) \right | ^ \right \rangle _ where D_ \left ( \right ) is the atmospherically induced variance between the phase at two parts of the wavefront separated by a distance \mathbf in the aperture plane, and <...> represents the ensemble average. The structure function of Tatarski (1961) can be described in terms of a single parameter r_: D_ \left ( \right ) = 6.88 \left ( \frac \right ) ^ r_ 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 r_ values for I band (900 nm wavelength) observations at good sites are 20---40 cm. Fried (1965) and Noll (1976) noted that r_ also corresponds to the aperture diameter for which the variance \sigma ^ of the wavefront phase averaged over the aperture comes approximately to unity: \sigma ^=1.0299 \left ( \frac \right )^ This equation represents a commonly used definition for r_, a parameter frequently used to describe the atmospheric conditions at astronomical observatories. r_ can be determined from a measured CN2 profile (described below) as follows: r_=\left ( 16.7\lambda^( \cos \gamma )^\int_^dh C_^(h) \right )^ where the turbulence strength C_^(h) varies as a function of height h above the telescope, and \gamma is the angular distance of the astronomical source from the zenith (from directly overhead). The timescale t0 is simply proportional to r0 divided by the mean wind speed.

References

zenith Much of the above text is taken (with permission) from http://www.mrao.cam.ac.uk/telescopes/coast/theses/rnt/
- BUSCHER, D. F., ARMSTRONG, J. T., HUMMEL, C. A., QUIRRENBACH, A., MOZURKEWICH, D., JOHNSTON, K. J., DENISON, C. S., COLAVITA, M. M., & SHAO, M. 1995. [http://ukads.nottingham.ac.uk/cgi-bin/nph-bib_query?bibcode=1995ApOpt..34.1081B&db_key=INST Interferometric seeing measurements on Mt. Wilson: power spectra and outer scales]. Applied Optics, 34(Feb.), 1081-1096.
- COLAVITA, M. M., SHAO, M., & STAELIN, D. H. 1987. [http://ukads.nottingham.ac.uk/cgi-bin/nph-bib_query?bibcode=1987ApOpt..26.4106C&db_key=INST Atmospheric phase measurements with the Mark III stellar interferometer]. Applied Optics, 26(Oct.), 4106-4112.
- FRIED, D. L. 1965. [http://ukads.nottingham.ac.uk/cgi-bin/nph-bib_query?bibcode=1965OSAJ...55.1427F&db_key=AST Statistics of a Geometric Representation of Wavefront Distortion], Optical Society of America Journal, 55, 1427-1435.
- KOLMOGOROV, A. N. 1941. Dissipation of energy in the locally isotropic turbulence. Comptes rendus (Doklady) de l'Académie des Sciences de l'U.R.S.S., 32, 16-18.
- KOLMOGOROV, A. N. 1941. The local structure of turbulence in incompressible viscous fluid for very large Reynold's numbers. Comptes rendus (Doklady) de l'Académie des Sciences de l'U.R.S.S., 30, 301-305.
- NIGHTINGALE, N. S., & BUSCHER, D. F. 1991. [http://ukads.nottingham.ac.uk/cgi-bin/nph-bib_query?bibcode=1991MNRAS.251..155N&db_key=AST Interferometric seeing measurements at the La Palma Observatory]. Monthly Notices of the Royal Astronomical Society, 251(July), 155-166.
- NOLL, R. J. 1976. [http://ukads.nottingham.ac.uk/cgi-bin/nph-bib_query?bibcode=1976OSAJ...66..207N&db_key=INST Zernike polynomials and atmospheric turbulence]. Optical Society of America Journal, 66(Mar.), 207-211.
- O'BYRNE, J. W. 1988. [http://ukads.nottingham.ac.uk/cgi-bin/nph-bib_query?bibcode=1988PASP..100.1169O&db_key=AST Seeing measurements using a shearing interferometer]. Publications of the Astronomical Society of the Pacific, 100(Sept.), 1169-1177.
- TATARSKI, V. I. 1961. Wave Propagation in a Turbulent Medium. McGraw-Hill Books.

The CN2 profile

A more thorough description of the astronomical seeing at an observatory is given by producing a profile of the turbulence strenght as a function of altitude, called a CN2 profile. CN2 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 CN2 profile and then compared. Some of the most common methods include: # SCIDAR (imaging the shadow patterns in the scintillation of starlight) # SLODAR # RADAR mapping of turbulence # Balloon-borne thermometers to measure how quickly the air temperature is fluctuating with time due to turbulence There are also mathematical functions describing the CN2 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.

Overcoming Atmospheric Seeing

The first answer to this problem was speckle imaging, which allowed bright objects to be observed with very high resolution. Later came NASA's Hubble Space Telescope, working outside the atmosphere and thus not have 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 interferometers such as the Navy Prototype Optical Interferometer or Cambridge Optical Aperture Synthesis Telescope. Starting in the 1990s, many telescopes have begun to develop adaptive optics systems that partially solve the seeing problem, but none of the systems so far built or designed completely removes the atmosphere effect, and observations are usually limited to a small region of the sky surrounding relatively bright stars. Category:Astronomical_imaging Category:Observational astronomy

Odszkodowanie cellulitis nadwaga Calling Cards nauka










































:: RELATED NEWS ::

Nate Doggggg
Nathaniel Dawayne Hale, commonly known as Nate Dogg (born 1969) is an American singer and occasional rapper who was born in Clarksdale, Mississippi. He began singing as a child in the New Hope Trinity Baptist Church choir where his father, Daniel Lee Hale, was pastor at the time. At the age of 16 he dropped out of
Saint Lucia giant rice-rat
The Saint Lucia giant rice-rat (Megalomys luciae) is an extinct rodent that lived on the island of Saint Lucia in the Caribbean. It was the size of a small cat and had a darker belly than the Martinique giant rice-rat and had slender c

Boldo
Boldo (Peumus boldus Molina) is a plant native to the coastal region of Chile. Its leaves, which have a strong woody aroma, are used for culinary purposes, primarily in Latin America. The leaves are used in a similar manner to bay leaves. The leaves are also used as an herbal tea, primarily in Chile and Argentina but also in other Spanish-speaking nations. They are used as a form of herbal medicine, particularly to support the
Shenfield railway station
Shenfield is a major station on the Great Eastern Main Line from Liverpool Street station in the City of London to places in the East of England. It is located in Shenfield in the borough of Brentwood in Essex,
Salt marsh harvest mouse

The salt marsh harvest mouse (Reithrodontomys raviventris), also known as the red-bellied harvest mouse, is an endangered rodent endemic to the San Francisco Bay Area of California. There are two distinct subspecies, both endangered and listed together on feder
Build the Martinique Country
Build the Martinique Country (Bâtir le Pays Martinique) is a left-wing political party in the French Overseas Department of Martinique. The party has one seat in the French National Assembly. Category:Political parties in France
All Rights Reserved 2005 wikimiki.org