Editing Astrophotography (section)

==Amateur astrophotography{{anchor|Amateur}}==
{{broader|Amateur astronomy}}
[[File:Comet-Hale-Bopp-29-03-1997.jpeg|thumb|2 minute time exposure of the comet Hale-Bopp imaged using a camera on a fixed tripod. The tree in the foreground was illuminated using a small flashlight.]]

Astrophotography is a popular hobby among photographers and amateur astronomers. Techniques ranges from basic film and digital cameras on tripods up to methods and equipment geared toward advanced imaging. Amateur astronomers and [[amateur telescope maker]]s also use homemade equipment and modified devices.

=== Media ===
Images are recorded on many types of media and imaging devices including [[single-lens reflex camera]]s, [[35mm format|35 mm film]], 120 film, [[digital single-lens reflex camera]]s, simple amateur-level, and professional-level commercially manufactured astronomical CCD and CMOS cameras, [[video camera]]s, and even off-the-shelf [[webcam]]s used for [[Lucky imaging]].

The conventional over-the-counter film has long been used for astrophotography. Film exposures range from seconds to over an hour. Commercially available color film stock is subject to [[Reciprocity (photography)|reciprocity failure]] over long exposures, in which sensitivity to light of different wavelengths appears to drop off at different rates as the exposure time increases, leading to a color shift in the image and reduced sensitivity over all as a function of time. This is compensated for, or at least reduced, by cooling the film (see [[Cold camera photography]]). This can also be compensated for by using the same technique used in professional astronomy of taking photographs at different wavelengths that are then combined to create a correct color image. Since the film is much slower than digital sensors, tiny errors in tracking can be corrected without much noticeable effect on the final image. Film astrophotography is becoming less popular due to the lower ongoing costs, greater sensitivity, and the convenience of [[digital photography]].

[[File:Milky_way_-route_292_shiga_kusatsu_road-_1920x1080.webm|thumb|left|Video of night sky made with [[DSLR camera|DSLR camera's]] [[Time-lapse photography|time-lapse]] feature. The camera itself is moving in these shots on a motorized mount.]]

Since the late 1990s amateurs have been following the professional observatories in the switch from film to digital CCDs for astronomical imaging. CCDs are more sensitive than film, allowing much shorter exposure times, and have a linear response to light. Images can be captured in many short exposures to create a synthetic long exposure. Digital cameras also have minimal or no moving parts and the ability to be operated remotely via an infrared remote or computer tethering, limiting vibration. Simple digital devices such as [[webcam]]s can be modified to allow access to the focal plane and even (after the cutting of a few wires), for [[Exposure (photography)|long exposure]] photography. Digital video cameras are also used. There are many techniques and pieces of commercially manufactured equipment for attaching [[DSLR camera|digital single-lens reflex (DSLR) cameras]] and even basic [[Point-and-shoot camera|point and shoot]] cameras to telescopes. Consumer-level digital cameras suffer from [[image noise]] over long exposures, so there are many techniques for cooling the camera, including [[cryogenics|cryogenic]] cooling. Astronomical equipment companies also now offer a wide range of purpose-built astronomical CCD cameras complete with hardware and processing software. Many commercially available DSLR cameras have the ability to take long time exposures combined with sequential ([[Time-lapse photography|time-lapse]]) images allowing the photographer to create a motion picture of the night sky. CMOS cameras are increasingly replacing CCD cameras in the amateur sector.<ref>{{cite web |title=CCDs, CMOS, and the Future of Astrophotography |url=https://skyandtelescope.org/astronomy-blogs/imaging-foundations-richard-wright/ccds-cmos-and-the-future-of-astrophotography/ |website=Sky and Telescope |publisher=American Astronomical Society |access-date=1 March 2023}}</ref> Modern CMOS sensors offer higher quantum efficiency, lower thermal and read noise and faster readout speeds than commercially available CCD sensors.<ref>{{cite web |title=CMOS VS CCD data. Which one is the best?  |url=https://telescope.live/blog/cmos-vs-ccd-data-which-one-best |website=Telescope Live |publisher=Telescope Live Ltd |access-date=8 November 2024}}</ref>

====Post-processing====
[[File:Bob Star - M45 Carranza Field (by).jpg|thumb|left|The Pleiades Star Cluster photographed with a 6 megapixel DSLR connected to an 80mm refracting telescope piggybacked on a larger telescope. Made from seven 180 second images combined and processed in Photoshop with a noise reduction plugin.]]

Both digital camera images and scanned film images are usually adjusted in [[Digital image processing|image processing]] software to improve the image in some way. Images can be brightened and manipulated in a computer to adjust color and increase the contrast. More sophisticated techniques involve capturing multiple images (sometimes thousands) to composite together in an additive process to sharpen images to [[Astronomical seeing#Overcoming atmospheric seeing|overcome atmospheric seeing]], negating tracking issues, bringing out faint objects with a poor [[signal-to-noise ratio]], and filtering out light pollution. 

Digital camera images may also need further processing to reduce the [[image noise]] from long exposures, including [[dark frame subtraction|subtracting a “dark frame”]] and a processing called ''image stacking'' or "''[[Shift-and-add]]''". Commercial, [[freeware]] and [[free software]] packages are available specifically for astronomical photographic image manipulation.<ref>{{cite web|author=Chan, Maria|year=2022|title=Best Astrophotography Stacking Software [Ultimate Guide]|url=https://dopeguides.com/astrophotography-stacking-software/|access-date=2022-08-14}}</ref>

"[[Lucky imaging]]" is a secondary technique that involves taking a video of an object rather than standard long exposure photos. Software can then select the highest quality images which can then be stacked. <ref>{{Cite book|last=Gladysz|first=Szymon|title=Adaptive Optics Systems |chapter=Lucky imaging and speckle discrimination for the detection of faint companions with adaptive optics |editor-first1=Norbert |editor-first2=Claire E |editor-first3=Peter L |editor-last1=Hubin |editor-last2=Max |editor-last3=Wizinowich |date=2008 |chapter-url=https://www.researchgate.net/publication/329770189 |volume=7015 |pages=70152H |doi=10.1117/12.788442 |bibcode=2008SPIE.7015E..2HG |s2cid=121543131 |series=Proceedings of SPIE}}</ref>

==== Color and brightness ====
Astronomical pictures, like [[observational astronomy]] and photography from [[space exploration]], show astronomical objects and phenomena in different colors and brightness, and often as composite images. This is done to highlight different features or reflect different conditions, and makes the note of these conditions necessary.

Images attempting to reproduce the [[False color#True color|true color]] and appearance of an astronomical object or phenomenon need to consider many factors, including how the human eye works.  Particularly under different atmospheric conditions images need to evaluate several factors to produce analyzable or representative images, like images of space missions from the surface of [[Mars]],<ref name="Don Davis Space Art t191">{{cite web |title=Martian colors |url=http://www.donaldedavis.com/PARTS/MARSCLRS.html |access-date=2024-05-04 |website=Don Davis Space Art}}</ref> [[Venus]]<ref name="Singapore 2021 g242">{{cite web |last=Singapore |first=AFP |date=2021-01-24 |title=This image has been enhanced from a photo of Venus taken by Soviet spacecraft Venera 13 |url=https://factcheck.afp.com/image-has-been-enhanced-photo-venus-taken-soviet-spacecraft-venera-13 |access-date=2024-05-04 |website=Fact Check}}</ref><ref name="Mitchell j874">{{cite web |last=Mitchell |first=Don P. |title=Images of Venus |url=http://mentallandscape.com/V_DigitalImages.htm |access-date=2024-05-04 |website=Don P. Mitchell}}</ref><ref name="Mitchell 1959 f372">{{cite web |last=Mitchell |first=Don P. |date=1959-10-07 |title=Soviet Space Cameras |url=http://mentallandscape.com/V_Cameras.htm |access-date=2024-05-05 |website=Don P. Mitchell}}</ref> or [[Titan (moon)|Titan]].
===Hardware===
Astrophotographic hardware among non-professional astronomers varies widely since the photographers themselves range from general photographers shooting some form of aesthetically pleasing images to very serious amateur astronomers collecting data for scientific research. As a hobby, astrophotography has many challenges that have to be overcome that differ from conventional photography and from what is normally encountered in professional astronomy.

[[File:NGC 281, the 'Pacman Nebula'.jpg|thumb|upright=0.9|NGC281, popularly the 'Pacman Nebula', imaged from a suburban location using a 130mm amateur telescope and a DSLR camera.]]
Since most people live in [[urban area]]s, equipment often needs to be portable so that it can be taken far away from the lights of major cities or towns to avoid urban [[light pollution]]. Urban astrophotographers may use special light-pollution or narrow-band filters and advanced computer processing techniques to reduce ambient urban light in the background of their images. They may also stick to imaging bright targets like the Sun, Moon and planets. Another method used by amateurs to avoid light pollution is to set up, or rent time, on a remotely operated telescope at a dark sky location. Other challenges include setup and alignment of portable telescopes for accurate tracking, working within the limitations of “off the shelf” equipment, the endurance of monitoring equipment, and sometimes manually tracking astronomical objects over long exposures in a wide range of weather conditions.

Some camera manufacturers modify their products to be used as astrophotography cameras, such as Canon's [[EOS 60Da]], based on the EOS 60D but with a modified infrared filter and a low-noise sensor with heightened [[H-alpha|hydrogen-alpha]] sensitivity for improved capture of red hydrogen emission nebulae.<ref>{{cite web |url=http://www.expertreviews.co.uk/digital-cameras/1291546/canoneos-60da-astrophotography-camera-announced |title=CanonEOS 60Da astrophotography camera announced |date=24 January 2011 |access-date=April 30, 2012}}</ref>

There are also cameras specifically designed for amateur astrophotography based on commercially available imaging sensors. They may also allow the sensor to be cooled to reduce thermal noise in long exposures, provide raw image readout, and to be controlled from a computer for automated imaging. Raw image readout allows later better image processing by retaining all the original image data which along with stacking can assist in imaging faint deep sky objects.

With very low light capability, a few specific models of [[webcam]]s are popular for solar, lunar, and planetary imaging. Mostly, these are manually focused cameras containing a CCD sensor instead of the more common CMOS. The lenses of these cameras are removed and then these are attached to telescopes to record images, videos, or both. In newer techniques, videos of very faint objects are taken and the sharpest frames of the video are 'stacked' together to obtain a still image of respectable contrast. The Philips PCVC 740K and SPC 900 are among the few webcams liked by astrophotographers. Any [[smartphone]] that allows long exposures can be used for this purpose, but some phones have a specific mode for astrophotography that will stitch together multiple exposures.

====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.

==== Filters ====

: Filters can be categorised into two classes; broadband and narrowband. Broadband filters allow a wide range of wavelengths to pass through, removing small amounts of light pollution. Narrowband filters only allow light from very specific wavelengths to pass through, blocking out the vast majority of the spectrum.
: Astronomical filters usually come as sets and are manufactured to specific standards, in order to allow different observatories to make observations at the same standard. A common filter standard in the astronomy community is the Johnson Morgan UVB, designed to match a CCD’s color response to that of photographic film. However there are over 200 standards available.<ref>{{Cite book |last=Gallaway |first=Mark |title=An Introduction to Observational Astrophysics |publisher=Springer Nature Switzerland AG |year=2020 |isbn=978-3-030-43551-6 |edition=2nd |pages=17–19 |language=English}}</ref>

; Remote Telescope
{{urs|date=December 2021}}
Fast [[Internet access]] in the last part of the 20th century, and advances in computer-controlled telescope mounts and CCD cameras, allows use of 'Remote Telescopes' for amateur astronomers not aligned with major telescope facilities to partake in research and deep-sky imaging. This enables the imager to control a telescope far away in a dark location. The observers can image through the telescopes using CCD cameras.

Imaging can be done regardless of the location of the user or the telescopes they wish to use. The digital data collected by the telescope is then transmitted and displayed to the user by means of the Internet. An example of a digital remote telescope operation for public use via the Internet is [[Bareket observatory|The Bareket Observatory]].