Editing Light pollution (section)

===Effect on astronomy===
[[File:Light pollution It's not pretty.jpg|thumb|The constellation [[Orion (constellation)|Orion]], imaged at left from dark skies, and at right from within the Provo/Orem, Utah metropolitan area.]][[Astronomy]] is very sensitive to light pollution. The [[night sky]] viewed from a city bears no resemblance to what can be seen from dark skies.<ref>{{cite web |url=http://ngm.nationalgeographic.com/2008/11/light-pollution/klinkenborg-text |archive-url=https://web.archive.org/web/20081018060653/http://ngm.nationalgeographic.com/2008/11/light-pollution/klinkenborg-text |url-status=dead |archive-date=October 18, 2008 |work=[[National Geographic]] |publisher=[[National Geographic Society]] |date=November 2008 |access-date=2011-12-03|title=National Geographic Magazine }}</ref> [[Skyglow]] (the [[Light scattering by particles|scattering of light]] in the atmosphere at night) reduces the contrast between stars and [[Galaxy|galaxies]] and the sky itself, making it much harder to see fainter [[Astronomical object|objects]].<ref>{{Cite web |title=Skyglow |url=https://www.johncbarentine.com/skyglow.html |access-date=2024-04-23 |website=John C. Barentine's Personal Website |language=en |archive-date=2024-04-23 |archive-url=https://web.archive.org/web/20240423205030/https://www.johncbarentine.com/skyglow.html |url-status=live }}</ref> This is one factor that has caused newer [[Telescope|telescopes]] to be built in increasingly remote areas.

Even at apparent clear night skies, there can be a lot of [[stray light]] that becomes visible at longer [[Exposure time|exposure times]] in [[astrophotography]]. By means of software, the stray light can be reduced, but at the same time, object detail could be lost in the image.<ref>{{Cite journal |last1=Reegen |first1=P. |last2=Kallinger |first2=T. |last3=Frast |first3=D. |last4=Gruberbauer |first4=M. |last5=Huber |first5=D. |last6=Matthews |first6=J. M. |last7=Punz |first7=D. |last8=Schraml |first8=S. |last9=Weiss |first9=W. W. |last10=Kuschnig |first10=R. |last11=Moffat |first11=A. F. J. |last12=Walker |first12=G. A. H. |last13=Guenther |first13=D. B. |last14=Rucinski |first14=S. M. |last15=Sasselov |first15=D. |date=2006-04-21 |title=Reduction of time-resolved space-based CCD photometry developed for MOST Fabry Imaging data |journal=Monthly Notices of the Royal Astronomical Society |volume=367 |issue=4 |pages=1417–1431 |doi=10.1111/j.1365-2966.2006.10082.x |doi-access=free |issn=0035-8711|arxiv=physics/0703153 }}</ref> The following picture of the area around the [[Pinwheel Galaxy]] (Messier 101) with the [[apparent magnitude]] of 7.5<sup>m</sup> with all stars down to an apparent magnitude of 10<sup>m</sup> was taken in [[Berlin]] in a direction close to the [[zenith]] with a fast lens (f-number 1.2) and an exposure time of five seconds at an [[exposure index]] of ISO 12800:{{clear}}
<gallery widths="540" heights="120" perrow="2" class="center" mode="packed" caption="Pinwheel Galaxy in stray light">
File:Streulichtfilterung.0.P1023258.jpg|Original shot: lower edge [[Eta Ursae Majoris|Alkaid]], right of center the double star [[Mizar]] with [[Alcor (star)|Alcor]] and right edge [[Epsilon Ursae Majoris|Alioth]]; the [[Pinwheel Galaxy]] is a small diffuse dot in the center of the image.
File:Streulichtfilterung.1.P1023258.jpg|Black level compensation: the darkest point in the digital picture was set to zero luminance, in order to reduce the visible stray light. However, blue light caused by [[Rayleigh scattering]] is visible in the center of the image.
File:Streulichtfilterung.2.P1023258.jpg|50 percent of stray light removed: the darker half of the stray light was set to zero luminance. The darker part of the blue light caused by Rayleigh scattering is still visible in the center of the image.
File:Streulichtfilterung.3.P1023258.jpg|Complete elimination of stray light: all pixels showing stray light have been set to zero luminance, the faint and two-dimensional Pinwheel Galaxy is no longer visible, too.
</gallery>

Some astronomers use narrow-band "[[astronomical filter|nebula filters]]", which allow only specific wavelengths of light commonly seen in [[nebula]]e, or broad-band "light pollution filters", which are designed to reduce (but not eliminate) the effects of light pollution by [[optical filter|filtering]] out [[spectral line]]s commonly emitted by [[sodium-vapor lamp|sodium]]- and [[mercury-vapor lamp]]s, thus enhancing contrast and improving the view of dim objects such as galaxies and nebulae.<ref>{{cite web |url=http://www.astronexus.com/node/4 |title=Use of light pollution filters in astronomy |publisher=Astronexus |access-date=2011-12-03 |archive-date=2011-11-12 |archive-url=https://web.archive.org/web/20111112081736/http://www.astronexus.com/node/4 }}</ref> Unfortunately, these light pollution reduction (LPR) filters are not a cure for light pollution. LPR filters reduce the brightness of the object under study and this limits the use of higher magnifications. LPR filters work by blocking light of certain wavelengths, which alters the color of the object, often creating a pronounced green cast. Furthermore, LPR filters work only on certain object types (mainly [[emission nebulae]]) and are of little use on galaxies and stars. No filter can match the effectiveness of a dark sky for visual or [[photographic filter|photographic]] purposes.
[[File:Milky Way Shines over Snowy La Silla.jpg|thumb|The [[Atacama Desert#Astronomical observatories|Atacama Desert]] in northern Chile is far from any cities, and the night sky there is pitch-black. Photo by José Francisco Salgado.<ref>{{cite web |title=Milky Way Shines over Snowy La Silla |url=http://www.eso.org/public/images/potw1319a/ |work=ESO Picture of the Week |access-date=13 May 2013}}</ref>]]

Light pollution affects the visibility of [[deep-sky object|diffuse sky objects]] like nebulae and galaxies more than stars, due to their low surface brightness. <ref>{{Cite journal |last=Varela Perez |first=Antonia M. |date=2023-06-16 |title=The increasing effects of light pollution on professional and amateur astronomy |url=https://www.science.org/doi/10.1126/science.adg0269 |journal=Science |language=en |volume=380 |issue=6650 |pages=1136–1140 |doi=10.1126/science.adg0269 |pmid=37319198 |bibcode=2023Sci...380.1136V |issn=0036-8075 |access-date=2024-04-23 |archive-date=2024-04-24 |archive-url=https://web.archive.org/web/20240424040546/https://www.science.org/doi/10.1126/science.adg0269 |url-status=live }}</ref> Most such objects are rendered invisible in heavily light-polluted skies above major cities. A simple method for estimating the darkness of a location is to look for the [[Milky Way]], which from truly dark skies appears bright enough to cast a shadow.<ref>{{cite APOD |date=23 August 2010 |title=A Milky Way Shadow at Loch Ard Gorge |access-date=2011-12-03}}</ref>

In addition to skyglow, light trespass can impact observations when artificial light directly enters the tube of the telescope and is reflected from non-optical surfaces until it eventually reaches the [[eyepiece]]. <ref>{{Cite journal |last=Stone |first=Richard |date=2010-08-27 |title=Astronomers Hope Their Prize Telescope Isn't Blinded by the Light |url=http://dx.doi.org/10.1126/science.329.5995.1002 |journal=Science |volume=329 |issue=5995 |pages=1002 |doi=10.1126/science.329.5995.1002 |pmid=20798287 |bibcode=2010Sci...329.1002S |issn=0036-8075}}</ref> This direct form of light pollution causes a glow across the [[field of view]], which reduces contrast. Light trespass also makes it hard for a visual observer to become sufficiently adapted to the dark. The usual measures to reduce this glare, if reducing the light directly is not an option, include [[flocking (texture)|flocking]] the telescope tube and accessories to reduce reflection, and putting a light shield (also usable as a [[dew shield]]) on the telescope to reduce light entering from angles other than those near the target. Under these conditions, some astronomers prefer to observe under a black cloth to ensure maximum [[adaptation (eye)|adaptation]] to the dark.