Background (astronomy)  From Wikipedia, the free encyclopedia
Jump to: navigation, search  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 telescopes. The Hubble Space
Telescope does not suffer from this problem.  In infrared
astronomy, the problem can be much worse: due to the longer
wavelengths 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(fr), where a
mirror rapidly oscillates between the object of in