page_title: Observational astronomy
section_title: Telescopes
hierarchical_section_title: Observational astronomy / Observation tools / Telescopes
caption_attribution_description: English: Two amateur telescopes joint on a mount. The one on the left hand side is for tracking and calibrating the mount via a PC, the right one is for photography. Deutsch: Zwei Amateurteleskope auf einer gemeinsamen Montierung. Das linke Teleskop ist für die Nachführung der Montierung über einen PC, mit dem rechten wird fotografiert.
context_page_description: Observational astronomy is a division of astronomy that is concerned with recording data about the observable universe, in contrast with theoretical astronomy, which is mainly concerned with calculating the measurable implications of physical models. It is the practice and study of observing celestial objects with the use of telescopes and other astronomical instruments.
  As a science, the study of astronomy is somewhat hindered in that direct experiments with the properties of the distant universe are not possible. However, this is partly compensated by the fact that astronomers have a vast number of visible examples of stellar phenomena that can be examined. This allows for observational data to be plotted on graphs, and general trends recorded. Nearby examples of specific phenomena, such as variable stars, can then be used to infer the behavior of more distant representatives. Those distant yardsticks can then be employed to measure other phenomena in that neighborhood, including the distance to a galaxy.
  Galileo Galilei turned a telescope to the heavens and recorded what he saw.
context_section_description: 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 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 , and the 100 m diameter Overwhelmingly Large Telescope.
  Amateur astronomers use such instruments as the Newtonian reflector, the Refractor and the increasingly popular  Maksutov telescope.
caption: An amateur astrophotography setup with an automated guide system connected to a laptop.
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