Editing Observational astronomy (section)

==Subdivisions of observational Astronomy==
[[File:Crab Nebula in Multiple Wavelengths.png|thumb|250px|The [[Crab Nebula]] as seen in various wavelengths]]
[[File:Openstax Astronomy EM spectrum and atmosphere.jpg|thumb|upright=1.6|Overview of types of observational astronomy by observed wavelengths and their observability.]]
A traditional division of observational astronomy is based on the region of the [[electromagnetic spectrum]] observed:
* [[Radio astronomy]] detects [[radiation]] of millimetre to decametre wavelength. The receivers are similar to those used in [[radio]] broadcast transmission but much more sensitive. See also [[Radio telescope]]s.
* [[Infrared astronomy]] deals with the detection and analysis of [[infrared radiation]] (this typically refers to wavelengths longer than the detection limit of silicon solid-state detectors, about 1&nbsp;μm wavelength). The most common tool is the [[reflecting telescope]], but with a detector sensitive to infrared wavelengths. Space telescopes are used at certain wavelengths where the atmosphere is opaque, or to eliminate noise (thermal radiation from the atmosphere).
* [[Optical astronomy]] is the part of astronomy that uses [[optical instrument]]s (mirrors, lenses, and solid-state detectors) to observe [[light]] from near-[[infrared]] to near-[[ultraviolet]] wavelengths. [[Visible-light astronomy]], using [[wavelength]]s detectable with the human eyes (about 400–700&nbsp;nm), falls in the middle of [[visible spectrum|this spectrum]].
* [[High-energy astronomy]] includes [[X-ray astronomy]], [[gamma-ray astronomy]], and extreme [[UV astronomy]].
*[[Occultation astronomy]] is the observation of the instant one celestial object occults or eclipses another.  Multi-[[Chord (astronomy)|chord]] asteroid occultation observations measure the profile of the asteroid to the kilometre level.<ref>{{Cite journal |arxiv = 1611.02798|doi = 10.1051/0004-6361/201628620|title = Results from a triple chord stellar occultation and far-infrared photometry of the trans-Neptunian object (229762) 2007 UK126|journal = Astronomy & Astrophysics|volume = 600|pages = A12|year = 2017|last1 = Schindler|first1 = K.|last2 = Wolf|first2 = J.|last3 = Bardecker|first3 = J.|last4 = Olsen|first4 = A.|last5 = Müller|first5 = T.|last6 = Kiss|first6 = C.|last7 = Ortiz|first7 = J. L.|last8 = Braga-Ribas|first8 = F.|last9 = Camargo|first9 = J. I. B.|last10 = Herald|first10 = D.|last11 = Krabbe|first11 = A.|bibcode = 2017A&A...600A..12S| s2cid=48357636 }}</ref>

=== Methods ===
In addition to using electromagnetic radiation, modern astrophysicists can also make observations using [[neutrino]]s, [[cosmic ray]]s or [[gravitational wave]]s. Observing a source using multiple methods is known as [[multi-messenger astronomy]].

[[File:La Silla Poses for an Ultra HD Shoot.jpg|thumb|250px|Ultra HD photography taken at [[La Silla Observatory]]<ref>{{cite news|title=La Silla Poses for an Ultra HD Shoot|url=http://www.eso.org/public/images/potw1415a/|access-date=16 April 2014|newspaper=ESO Picture of the Week}}</ref> ]]

Optical and radio astronomy can be performed with ground-based observatories, because the atmosphere is relatively transparent at the wavelengths being detected. Observatories are usually located at high altitudes so as to minimise the absorption and distortion caused by the Earth's atmosphere. Some wavelengths of infrared light are heavily absorbed by [[water vapor]], so many infrared observatories are located in dry places at high altitude, or in space.

The atmosphere is opaque at the wavelengths used by X-ray astronomy, gamma-ray astronomy, UV astronomy and (except for a few wavelength "windows") [[far infrared astronomy]], so observations must be carried out mostly from [[balloon]]s or space observatories. Powerful gamma rays can, however be detected by the large [[air shower (physics)|air shower]]s they produce, and the study of cosmic rays is a rapidly expanding branch of astronomy.

=== Important factors ===
For much of the history of observational astronomy, almost all observation was performed in the visual spectrum with [[optical telescope]]s. While the Earth's atmosphere is relatively transparent in this portion of the [[electromagnetic spectrum]], most telescope work is still dependent on [[astronomical seeing|seeing]] conditions and air transparency, and is generally restricted to the night time. The seeing conditions depend on the turbulence and thermal variations in the air. Locations that are frequently cloudy or suffer from atmospheric turbulence limit the resolution of observations. Likewise the presence of the full [[Moon]] can brighten up the sky with scattered light, hindering observation of faint objects.

For observation purposes, the optimal location for an optical telescope is undoubtedly in [[outer space]]. There the telescope can make observations without being affected by the [[Earth's atmosphere|atmosphere]]. However, at present it remains costly to lift telescopes into [[orbit]]. Thus the next best locations are certain mountain peaks that have a high number of cloudless days and generally possess good atmospheric conditions (with good [[astronomical seeing|seeing]] conditions). The peaks of the islands of [[Mauna Kea Observatory|Mauna Kea, Hawaii]] and [[Roque de los Muchachos Observatory|La Palma]] possess these properties, as to a lesser extent do inland sites such as [[Llano de Chajnantor Observatory|Llano de Chajnantor]], [[Paranal Observatory|Paranal]], [[Cerro Tololo Inter-American Observatory|Cerro Tololo]] and [[La Silla Observatory|La Silla]] in [[Chile]]. These observatory locations have attracted an assemblage of powerful telescopes, totalling many billion US dollars of investment.

The darkness of the night sky is an important factor in optical astronomy. With the size of cities and human populated areas ever expanding, the amount of artificial light at night has also increased. These artificial lights produce a diffuse background illumination that makes observation of faint astronomical features very difficult without special filters. In a few locations such as the state of [[Arizona]] and in the [[United Kingdom]], this has led to campaigns for the reduction of [[light pollution]]. The use of hoods around street lights not only improves the amount of light directed toward the ground, but also helps reduce the light directed toward the sky.

Atmospheric effects ([[astronomical seeing]]) can severely hinder the [[Angular resolution|resolution]] of a telescope. Without some means of correcting for the blurring effect of the shifting atmosphere, telescopes larger than about 15–20&nbsp;cm in [[aperture]] can not achieve their theoretical resolution at visible wavelengths. As a result, the primary benefit of using very large telescopes has been the improved light-gathering capability, allowing very faint magnitudes to be observed. However the resolution handicap has begun to be overcome by [[adaptive optics]], [[speckle imaging]] and [[aperture synthesis|interferometric imaging]], as well as the use of [[space telescope]]s.

=== Measuring results ===
Astronomers have a number of observational tools that they can use to make measurements of the heavens. For objects that are relatively close to the Sun and Earth, direct and very precise [[astrometric|position measurements]] can be made against a more distant (and thereby nearly stationary) background. Early observations of this nature were used to develop very precise orbital models of the various planets, and to determine their respective masses and gravitational [[perturbation (astronomy)|perturbation]]s. Such measurements led to the discovery of the planets [[Uranus]], [[Neptune]], and (indirectly) [[Pluto]]. They also resulted in an erroneous assumption of a fictional planet [[Vulcan (planet)|Vulcan]] within the orbit of [[Mercury (planet)|Mercury]] (but the explanation of the [[precession]] of Mercury's orbit by [[Albert Einstein|Einstein]] is considered one of the triumphs of his [[general relativity]] theory).