page_title: Light pollution
section_title: Effect on astronomy
hierarchical_section_title: Light pollution / Consequences / Effect on astronomy
caption_attribution_description: English: In the outskirts of the Atacama Desert, far from the light-polluted cities of northern Chile, the skies are pitch-black after sunset. Such dark skies allow some of the best astronomical observing to take place — and at an altitude of 2400 metres, ESO’s La Silla Observatory has an incredibly clear view of the night sky. However, even such a remote, high, and dry location cannot always escape the weather that sometimes comes with the winter months, when blankets of snow can cover the mountain peak and its telescope domes. This image shows a wintry La Silla sitting beneath a spray of stars from our Milky Way, the plane of which slants across the frame. Visible (from right to left) are the ESO 3.6-metre telescope, the 3.58-metre New Technology Telescope (NTT), the ESO 1-metre Schmidt telescope, and the MPG/ESO 2.2-metre telescope, which has snow on its dome. The small dome of the decommissioned Coudé Auxiliary Telescope can be seen adjacent to that of the ESO 3.6-metre telescope, and between it and the NTT are the water tanks of the observatory. While the sight of snow at La Silla may initially be surprising, the high altitude ESO sites can experience both hot and cold temperatures through the year, and occasionally be subject to harsh conditions. This photograph was taken by José Francisco Salgado, an ESO Photo Ambassador. Español: En las afueras del Desierto de Atacama, lejos de la contaminación lumínica de las ciudades del norte de Chile, tras la puesta de Sol, los cielos son totalmente negros. Estos cielos tan oscuros permiten que allí tengan lugar algunas de las mejores observaciones astronómicas — a una altitud de 2.400 metros, el Observatorio La Silla de ESO tiene unas vistas del cielo nocturno increíblemente despejadas. Sin embargo, pese a ser un lugar remoto, alto y seco, no siempre puede escapar a las condiciones meteorológicas que, a veces, acompañan a los meses de invierno, cuando la nieve llega a cubrir el pico de la montaña y las cúpulas de sus telescopios. Esta imagen muestra una La Silla invernal bajo un chorro de estrellas de nuestra Vía Láctea, cuyo plano se ve oblicuo en este fotograma. A la vista (de derecha a izquierda) el telescopio de 3,6 metros de ESO, el telescopio de 3,58 metros NTT (New Technology Telescope), el telescopio Schmidt de un metro de ESO, y el telescopio de 2,2 metros MPG/ESO, con nieve sobre su cúpula. Junto al telescopio de 3,6 metros de ESO, puede verse la pequeña cúpula del Coudé Auxiliary Telescope, fuera de servicio, y entre este y el NTT se encuentran los tanques de agua del observatorio. Pese a que ver un paisaje nevado en La Silla pueda parecer sorprendente, los lugares de observación de ESO están a tal altitud que pueden experimentar, a lo largo del año, temperaturas que pueden ir de lo más frío a lo más caluroso y, ocasionalmente, pueden padecer condiciones muy duras. Esta fotografía fue tomada por José Francisco Salgado, un Fotógrafo Embajador de ESO. Русский: На окраине пустыни Атакама, вдали от светового загрязнения городов на севере Чили, после захода Солнца небо угольно-черное. Такое темное небо позволяет проводить самые лучшие астрономические наблюдения, и на высоте 2400 метров, в обсерватории Ла Силья, ESO, небо невероятно четкое и ясное. Тем не менее, даже в такие отдаленные, высокие и сухие места может прийти иногда зимняя непогода, и снежное одеяло может покрыть горный пик и купол телескопа. На этом фото -- Ла Силья зимой, под брызгами звезд нашего Млечного Пути, наклонной полосой пересекающего всю площадь кадра. Справа налево – телескопы: 3,6-метровый, 3,58-метровый NTT, метровый «Шмидт» и 2,2-метровый телескоп MPG/ESO, купол которого засыпан снегом. Рядом с 3,6-метровым телескопом виден небольшой купол списанного вспомогательного телескопа кудэ; между ним и NTT виднеются резервуары для воды. Вид заснеженной обсерватории Ла Силья сначала может показаться странным. Но в высокогорных обсерваториях ESO в течение года может быть и жарко, и холодно, а иногда погодные условия могут быть довольно суровыми. Этот снимок был сделан Хосе Франсиско Сальгадо, фото-посланником ESO.
context_page_description: Light pollution is the presence of anthropogenic and artificial light in the night environment. It is exacerbated by excessive, misdirected or obtrusive use of light, but even carefully used light fundamentally alters natural conditions. As a major side-effect of urbanization, it is blamed for compromising health, disrupting ecosystems and spoiling aesthetic environments.
context_section_description: 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. Skyglow (the scattering of light in the atmosphere at night) reduces the contrast between stars and galaxies and the sky itself, making it much harder to see fainter objects. This is one factor that has caused newer 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 times in astrophotography. By means of software the stray light can be reduced, but at the same time object detail is getting lost in the images. The following picture of the area around the Pinwheel Galaxy (Messier 101) with the apparent magnitude of 7.5ᵐ with all stars down to an apparent magnitude of 10ᵐ was taken in Berlin in 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:
  Some astronomers use narrow-band "nebula filters", which allow only specific wavelengths of light commonly seen in nebulae, or broad-band "light pollution filters", which are designed to reduce (but not eliminate) the effects of light pollution by filtering out spectral lines commonly emitted by sodium- and mercury-vapor lamps, thus enhancing contrast and improving the view of dim objects such as galaxies and nebulae. 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 purposes.
  Light pollution affects the visibility of diffuse sky objects like nebulae and galaxies more than stars, due to their low surface brightness. 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.
  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. 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 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 to the dark.
caption: The Atacama Desert in northern Chile is far from any cities, and the night sky there is pitch-black. Photo by José Francisco Salgado.[84]
exif: {"Image ImageDescription": "In the outskirts of the Atacama Desert, far from the light-polluted cities of northern Chile, the skies are pitch-black after sunset. Such dark skies allow some of the best astronomical observing to take place \u2014 and at an altitude of 2400 metres, ESO\u2019s La Silla Observatory has an incredibly clear view of the night sky. However, even such a remote, high, and dry location cannot always escape the weather that sometimes comes with the winter months, when blankets of snow can cover the mountain peak and its telescope domes. This image shows a wintry La Silla sitting beneath a spray of stars from our Milky Way, the plane of which slants across the frame. Visible (from right to left) are the ESO 3.6-metre telescope, the 3.58-metre New Technology Telescope (NTT), the ESO 1-metre Schmidt telescope, and the MPG/ESO 2.2-metre telescope, which has snow on its dome. The small dome of the decommissioned Coud\u00e9 Auxiliary Telescope can be seen adjacent to that of the ESO 3.6-metre telescope, and between it and the NTT are the water tanks of the observatory. While the sight of snow at La Silla may initially be surprising, the high altitude ESO sites can experience both hot and cold temperatures through the year, and occasionally be subject to harsh conditions. This photograph was taken by Jos\u00e9 Francisco Salgado, an ESO Photo Ambassador.", "Image Artist": "Jos\u00e9 Francisco Salgado PhD", "Image Copyright": "\u00a9 Jos\u00e9 Francisco Salgado, PhD"}