Editing Astrophotography (section)

====Equipment setups====
;Fixed or tripod
The most basic types of astronomical photographs are made with standard cameras and photographic lenses mounted in a fixed position or on a tripod. Foreground objects or landscapes are sometimes composed in the shot. Objects imaged are [[constellation]]s, interesting planetary configurations, meteors, and bright comets. Exposure times must be short (under a minute) to avoid having the stars point image become an elongated line due to the Earth's rotation. Camera lens focal lengths are usually short, as longer lenses will show image trailing in a matter of seconds. A [[rule of thumb]] called the ''500 rule'' states that, to keep stars point-like,
:Maximum [[shutter speed|exposure time]] in seconds = {{sfrac|500|[[Focal length]] in mm × [[Crop factor]]}}
regardless of [[aperture]] or [[film speed|ISO setting]].<ref>{{cite book |first=Alan |last=Dyer |year=2014 |url=https://books.google.com/books?id=25bqBQAAQBAJ&pg=PA185 |title=How to Photograph & Process Nightscapes and Time-Lapses |isbn=0993958907}}</ref> For example, with a 35 mm lens on an [[APS-C]] sensor, the maximum time is {{sfrac|500|35 × 1.5}} ≈ 9.5 s. A more accurate calculation takes into account [[pixel pitch]] and [[declination]].<ref>{{Cite web |url=http://astrobackyard.com/the-500-rule |title=Why You Should Still Use the 500 Rule for Astrophotography}}</ref>

Allowing the stars to intentionally become elongated lines in exposures lasting several minutes or even hours, called “[[star trail]]s”, is an artistic technique sometimes used.

; Tracking mounts
[[File:Starwatching.jpg|thumb|upright=0.8|An astrophotography set up with an automated guide system connected to a laptop.]]
[[Telescope mount]]s that compensate for the Earth's rotation are used for longer exposures without objects being blurred. They include commercial equatorial mounts and homemade equatorial devices such as [[barn door tracker]]s and [[equatorial platform]]s. Mounts can suffer from inaccuracies due to backlash in the gears, wind, and imperfect balance, and so a technique called [[Autoguider|auto guiding]] is used as a closed feedback system to correct for these inaccuracies.<ref>{{Cite web |title=What is an autoguider? |website=BBC Sky at Night Magazine |url=https://www.skyatnightmagazine.com/advice/what-is-an-autoguider/|access-date=2022-01-09|language=en}}</ref>

Tracking mounts can come in two forms; single axis and dual axis. Single axis mounts are often known as star trackers. Star trackers have a single motor which drives the [[right ascension]] axis. This allows the mount to compensate for the Earth's rotation. Star trackers rely on the user ensuring the mount is polar aligned with high accuracy, as it is unable correct in the secondary declination axis, limiting exposure times. 

Dual axis mounts use two motors to drive both the right ascension and the declination axis together. This mount will compensate for the Earth's rotation by driving the right ascension axis, similar to a star tracker. However using an auto-guiding system, the secondary declination axis can also be driven, compensating for errors in polar alignment, allowing for significantly longer exposure times.<ref>{{Cite book |last=Ballard |first=Jim |title=The Handbook for Star Trackers |publisher=Sky Publishing Corporation |year=1988 |isbn=0933346476 |language=English}}</ref>

; "Piggyback" photography
Piggyback astronomical photography is a method where a camera/lens is mounted on an equatorially mounted astronomical telescope. The telescope is used as a guide scope to keep the field of view centered during the exposure. This allows the camera to use a longer exposure and/or a longer focal length lens or even be attached to some form of photographic telescope co-axial with the main telescope.

; Telescope focal plane photography
In this type of photography, the telescope itself is used as the "lens" collecting light for the film or CCD of the camera. Although this allows for the magnification and light-gathering power of the telescope to be used, it is one of the most difficult astrophotography methods.<ref>[http://www.prescottastronomyclub.org/prime_focus.html Prime focus astrophotography – Prescott Astronomy Club] {{webarchive |url=https://web.archive.org/web/20100731184636/http://www.prescottastronomyclub.org/prime_focus.html |date=July 31, 2010 }}.</ref> This is because of the difficulties in centering and focusing sometimes very dim objects in the narrow field of view, contending with magnified vibration and tracking errors, and the added expense of equipment (such as sufficiently sturdy telescope mounts, camera mounts, camera couplers, off-axis guiders, guide scopes, illuminated cross-hairs, or auto-guiders mounted on primary telescope or the guide-scope.) There are several different ways cameras (with removable lenses) are attached to amateur astronomical telescopes including:<ref>{{cite book|author=Michael A. Covington|title=Astrophotography for the Amateur|url=https://books.google.com/books?id=tzXv4WrvZ-EC|year=1999|publisher=Cambridge University Press|isbn=978-0-521-62740-5|page=69}}</ref><ref>[http://home.comcast.net/~astrokeith/methods/methods.htm Keith Mackay, Keith's Astrophotography and Astronomy site, Methods of Astrophotography] {{webarchive |url=https://web.archive.org/web/20090831081806/http://home.comcast.net/~astrokeith/methods/methods.htm |date=August 31, 2009 }}</ref>
* '''Prime focus''' – In this method the image produced by the telescope falls directly on the film or CCD with no intervening optics or telescope eyepiece.
* '''Positive projection''' – A method in which the telescope [[eyepiece]] (''eyepiece projection'') or a positive lens (placed after the [[focal plane]] of the telescope objective) is used to project a much more magnified image directly onto the film or CCD. Since the image is magnified with a narrow field of view this method is generally used for lunar and planetary photography.
* '''Negative projection''' – This method, like positive projection, produces a magnified image. A negative lens, usually a [[Barlow lens|Barlow]] or a photographic [[teleconverter]], is placed in the light cone before the focal plane of the telescope objective.
* '''Compression''' – Compression uses a positive lens (also called a ''focal reducer''), placed in the converging cone of light before the focal plane of the telescope objective, to reduce overall image magnification. It is used on very long focal length telescopes, such as [[Maksutov telescope|Maksutov]]s and [[Schmidt–Cassegrain telescope|Schmidt–Cassegrain]]s, to obtain a wider field of view, or to reduce the focal ratio of the setup thereby increasing the [[Lens speed|speed of the system]].<ref>{{cite web |last1=Wright |first1=Richard |title=How Focal Ratio Affects Your Astro Images |url=https://skyandtelescope.org/astronomy-blogs/imaging-foundations-richard-wright/how-focal-ratio-affects-your-astro-images/ |website=Sky & Telescope |access-date=March 15, 2021}}</ref>
When the camera lens is not removed (or cannot be removed) a common method used is [[afocal photography]], also called ''afocal projection''. In this method, both the camera lens and the telescope eyepiece are attached. When both are focused at infinity the light path between them is parallel ([[Afocal system|afocal]]), allowing the camera to basically photograph anything the observer can see. This method works well for capturing images of the moon and brighter planets, as well as narrow field images of stars and nebulae. Afocal photography was common with early 20th-century consumer-level cameras since many models had non-removable lenses. It has grown in popularity with the introduction of [[Point-and-shoot camera|point and shoot]] digital cameras since most models also have non-removable lenses.