Background (astronomy)
Encyclopedia
In astronomy
, 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 from the atmosphere
(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
per square-arcsecond: a very low level, but anyway well within the limits of the current generation of telescope
s. The Hubble Space Telescope
does not suffer from this problem.
In infrared astronomy
, the problem can be much worse: due to the longer wavelength
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
), in the case of ground based instruments. Even if we able to minimize the effect of instrumental and atmospherical components (e.g. using a spacecraft), there are still several astrophysical 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
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.
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
, background commonly refers to the incoming light from an apparently empty part of the night sky
Sky
The sky is the part of the atmosphere or outer space visible from the surface of any astronomical object. It is difficult to define precisely for several reasons. During daylight, the sky of Earth has the appearance of a pale blue surface because the air scatters the sunlight. The sky is sometimes...
.
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 from the atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
(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
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
per square-arcsecond: a very low level, but anyway well within the limits of the current generation of telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
s. The Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
does not suffer from this problem.
In infrared astronomy
Infrared astronomy
Infrared astronomy is the branch of astronomy and astrophysics that studies astronomical objects visible in infrared radiation. The wavelength of infrared light ranges from 0.75 to 300 micrometers...
, the problem can be much worse: due to the longer wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
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
Airglow is the very weak emission of light by a planetary atmosphere. In the case of Earth's atmosphere, this optical phenomenon causes the night sky to never be completely dark .-Development:The airglow phenomenon was first identified in 1868 by Swedish scientist...
), in the case of ground based instruments. Even if we able to minimize the effect of instrumental and atmospherical components (e.g. using a spacecraft), there are still several astrophysical 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
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
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.