astronomy Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest includ ...

, background commonly refers to the incoming light from an apparently empty part of the night sky. Even if no visible astronomical objects are present in given part of the sky, there always is some low luminosity present, due mostly to

light diffusion Photon diffusion is a situation where photons travel through a material without being absorbed, but rather undergoing repeated scattering events which change the direction of their path. The path of any given photon is then effectively a random wal ...

from the

atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

(diffusion of both incoming light from nearby sources, and of man-made Earth sources like cities). In the visible band, luminosity level is around the 22nd

magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...

per square-

arcsecond A minute of arc, arcminute (abbreviated as arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of a degree. Since one degree is of a turn, or complete rotation, one arcminute is of a tu ...

: a very low level, but anyway well within the limits of the current generation of

telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...

s. The

Hubble Space Telescope The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...

does not suffer from this problem. In

infrared astronomy Infrared astronomy is a sub-discipline of astronomy which specializes in the astronomical observation, observation and analysis of astronomical objects using infrared (IR) radiation. The wavelength of infrared light ranges from 0.75 to 300 microm ...

, the problem can be much worse: due to the longer

wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...

s involved, the sky and the telescope themselves are a source of light. To work around this problem, infrared telescopes often use a technique called chopping, where a mirror rapidly oscillates between the object of interest and the nearby, empty sky. The two images can be subtracted, leaving hopefully only the incoming light from the source. There are several sources which contribute to the brightness of the (night) sky. Some of these are instrumental, or due to the presence of the atmosphere (like the

airglow Airglow is a faint emission of light by a planetary atmosphere. In the case of Earth's atmosphere, this optical phenomenon causes the night sky never to be completely dark, even after the effects of starlight and diffuse sky radiation, diffuse ...

), in the case of ground-based instruments. Even if we able to minimize the effect of instrumental and atmospheric components (e.g. using a spacecraft), there are still several

astrophysical Astrophysics is a science that employs the methods and principles of physics and chemistry in the study of astronomical objects and phenomena. As one of the founders of the discipline, James Keeler, said, astrophysics "seeks to ascertain the ...

components contributing to the sky background: these could be sets of point sources like faint asteroids, Galactic stars and far away galaxies, as well as diffuse sources like dust in the Solar System, in the Milky Way, and in the intergalactic space. The actual importance of a specific component depends mostly of the wavelength of the measurement. The uncertainty (or noise) of the measurements caused by the astrophysical components of the sky background is called confusion noise. In astronomical

CCD CCD may refer to: Science and technology * Charge-coupled device, an electronic light sensor used in various devices including digital cameras * .ccd, the filename extension for CloneCD's CD image file * Carbonate compensation depth, a property ...

technology, background is usually referred to the overall optical "noise" of the system, that is, the incoming light on the CCD sensor in absence of light sources. This background can originate from electronic noise in the CCD, from not-well-masked lights nearby the telescope, and so on. An exposure on an empty patch of the sky is also called a background, and is the sum of the system background level plus the sky's one. A background frame is often the first exposure in an astronomical observation with a CCD: the frame will then be subtracted from the actual observation result, leaving in theory only the incoming light from the astronomical object being observed.

References

{{DEFAULTSORT:Background (Astronomy) Observational astronomy