Editing Observational astronomy (section)

== Observation tools ==
[[Image:Observatórium Skalnaté pleso 1.jpg|thumb|right|250px|[[Skalnaté pleso observatory]], [[Slovakia]]]]

[[File:Quito Observatory 04.JPG|thumb|One of the Oldest Observatories in [[South America]] is the [[Quito Astronomical Observatory]], founded in 1873 and located 12 minutes south of the [[Equator]] in Quito, Ecuador. The Quito Astronomical Observatory is the National Observatory of Ecuador and is located in the Historic Center of Quito and is managed by the [[National Polytechnic School]].<ref>[http://oaq.epn.edu.ec/ The Quito Astronomical Observatory is managed by National Polytechnic School, EPN, official website.]</ref>]]

=== Telescopes ===
[[File:Starwatching.jpg|thumb|200px|An amateur astrophotography setup with an automated guide system connected to a laptop]]
The key instrument of nearly all modern observational astronomy is the [[telescope]]. This serves the dual purposes of gathering more light so that very faint objects can be observed, and magnifying the image so that small and distant objects can be observed. Optical astronomy requires telescopes that use optical components of great precision. Typical requirements for grinding and polishing a curved mirror, for example, require the surface to be within a fraction of a wavelength of light of a particular [[cone (geometry)|conic]] shape. Many modern "telescopes" actually consist of arrays of telescopes working together to provide higher resolution through [[aperture synthesis]].

Large telescopes are housed in domes, both to protect them from the weather and to stabilize the environmental conditions.  For example, if the temperature is different from one side of the telescope to the other, the shape of the structure changes, due to [[thermal expansion]] pushing optical elements out of position. This can affect the image.  For this reason, the domes are usually bright white ([[titanium dioxide]]) or unpainted metal.  Domes are often opened around sunset, long before observing can begin, so that air can circulate and bring the entire telescope to the same temperature as the surroundings.  To prevent wind-buffet or other vibrations affecting observations, it is standard practice to mount the telescope on a concrete pier whose foundations are entirely separate from those of the surrounding dome and building.

To do almost any scientific work requires that telescopes track objects as they wheel across the visible sky.  In other words, they must smoothly compensate for the rotation of the Earth.  Until the advent of [[computer]] controlled drive mechanisms, the standard solution was some form of [[equatorial mount]], and for small telescopes this is still the norm.   However, this is a structurally poor design and becomes more and more cumbersome as the diameter and weight of the telescope increases.  The world's largest equatorial mounted telescope is the 200 inch (5.1 m) [[Hale Telescope]], whereas recent 8–10 m telescopes use the structurally better [[altazimuth mount]], and are actually physically ''smaller'' than the Hale, despite the larger mirrors.  As of 2006, there are design projects underway for gigantic alt-az telescopes: the Thirty Metre Telescope [https://web.archive.org/web/20150130044644/http://lot.astro.utoronto.ca/], and the 100 m diameter [[Overwhelmingly Large Telescope]].<ref>[http://www.eso.org/projects/owl The ESO 100-m OWL optical telescope concept]</ref>

Amateur astronomers use such instruments as the [[Newtonian telescope|Newtonian reflector]], the [[Refracting telescope|Refractor]] and the increasingly popular  [[Maksutov telescope]].

=== Photography ===
The [[photograph]] has served a critical role in observational astronomy for over a century, but in the last 30 years it has been largely replaced for imaging applications by digital sensors such as [[charge-coupled device|CCD]]s and [[CMOS]] chips. Specialist areas of astronomy such as photometry and interferometry have utilised electronic detectors for a much longer period of time. [[Astrophotography]] uses specialised [[photographic film]] (or usually a glass plate coated with photographic [[emulsion]]), but there are a number of drawbacks, particularly a low [[quantum efficiency]], of the order of 3%, whereas CCDs can be tuned for a QE >90% in a narrow band.  Almost all modern telescope instruments are electronic arrays, and older telescopes have been either been retrofitted with these instruments or closed down.  Glass plates are still used in some applications, such as surveying,{{Citation needed|date=July 2009}} because the resolution possible with a chemical film is much higher than any electronic detector yet constructed.

==== Advantages ====
Prior to the invention of photography, all astronomy was done with the naked eye.  However, even before films became sensitive enough, scientific astronomy moved entirely to film, because of the overwhelming advantages:
* The human eye discards what it sees from split-second to split-second, but photographic film gathers more and more light for as long as the shutter is open.
* The resulting image is permanent, so many astronomers can use the same data.
* It is possible to see objects as they change over time ([[SN 1987A]] is a spectacular example).

==== Blink comparator ====
The [[blink comparator]] is an instrument that is used to compare two nearly identical photographs made of the same section of sky at different points in time. The comparator alternates illumination of the two plates, and any changes are revealed by blinking points or streaks. This instrument has been used to find [[asteroid]]s, [[comet]]s, and [[variable star]]s.
[[File:Telescope.jpg|thumb|right|160px|50 cm refracting telescope at [[Nice Observatory]]]]

=== Micrometer ===
The position or cross-wire [[Micrometer (device)|micrometer]] is an implement that has been used to measure [[double star]]s. This consists of a pair of fine, movable lines that can be moved together or apart. The telescope lens is lined up on the pair and oriented using position wires that lie at right angles to the star separation. The movable wires are then adjusted to match the two star positions. The separation of the stars is then read off the instrument, and their true separation determined based on the magnification of the instrument.

=== Spectrograph ===
A vital instrument of observational astronomy is the [[spectroscope|spectrograph]]. The absorption of specific wavelengths of light by elements allows specific properties of distant bodies to be observed. This capability has resulted in the discovery of the element of [[helium]] in the Sun's [[emission spectrum]], and has allowed astronomers to determine a great deal of information concerning distant stars, galaxies, and other celestial bodies. [[Doppler shift]] (particularly "[[redshift]]") of spectra can also be used to determine the radial motion or distance with respect to the [[Earth]].

Early spectrographs employed banks of [[prism (optics)|prism]]s that split light into a broad spectrum. Later the [[diffraction grating|grating spectrograph]] was developed, which reduced the amount of light loss compared to prisms and provided higher spectral resolution. The spectrum can be photographed in a long exposure, allowing the spectrum of faint objects (such as distant galaxies) to be measured.

Stellar photometry came into use in 1861 as a means of measuring [[color index|stellar colors]]. This technique measured the magnitude of a star at specific frequency ranges, allowing a determination of the overall color, and therefore [[temperature]] of a star. By 1951 an internationally standardized system of UBV-[[Apparent magnitude|magnitudes]] (''U''ltraviolet-''B''lue-''V''isual) was adopted.

=== Photoelectric photometry ===
[[Photoelectric effect|Photoelectric]] [[photometry (astronomy)|photometry]] using the [[charge-coupled device|CCD]] is now frequently used to make observations through a telescope. These sensitive instruments can record the image nearly down to the level of individual [[photon]]s, and can be designed to view in parts of the spectrum that are invisible to the eye. The ability to record the arrival of small numbers of photons over a period of time can allow a degree of computer correction for atmospheric effects, sharpening up the image. Multiple digital images can also be combined to further enhance the image, often known as "stacking". When combined with the [[adaptive optics]] technology, image quality can approach the theoretical resolution capability of the telescope.

[[Filter (optics)|Filter]]s are used to view an object at particular frequencies or frequency ranges. [[Optical coating|Multilayer film]] filters can provide very precise control of the frequencies transmitted and blocked, so that, for example, objects can be viewed at a particular frequency emitted only by excited [[hydrogen]] atoms. Filters can also be used to partially compensate for the effects of [[light pollution]] by blocking out unwanted light. [[Polarization in astronomy|Polarization filters]] can also be used to determine if a source is emitting polarized light, and the orientation of the polarization.