09-02-2013, 02:43 PM
ATMOSPHERIC REFRACTION
[attachment=50364]
INTRODUCTION
Photograph of the full Moon partly obscured by Earth's atmosphere. Note the deviation from circular in the Moon's lower edge, caused by refraction.
Atmospheric refraction is the deviation of light or other electromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of altitude. Atmospheric refraction near the ground produces mirages and can make distant objects appear to shimmer or ripple, elevated or lowered, stretched or shortened
Atmospheric refraction causes astronomical objects to appear higher in the sky than they are in reality. It affects not only lightrays but all electromagnetic radiation, although in varying degrees (see dispersion in optics). For example in visible light, blue is more affected than red. This may cause astronomical objects to be spread out into a spectrum in high-resolution images.
VALUES
Atmospheric refraction is zero in the zenith, less than 1′ (one arcminute) at 45° apparent altitude, and still only 5.3′ at 10° altitude; it quickly increases as altitude decreases, reaching 9.9′ at 5° altitude, 18.4′ at 2° altitude, and 35.4′ at the horizon (Allen 1976, 125); all values are for 10 °C and 101.3 kPa in the visible part of the spectrum.
RANDOM RAFRACTION EFFECTS
Turbulence in the atmosphere magnifies and de-magnifies star images, making them appear brighter and fainter on a time-scale of milliseconds. The slowest components of these fluctuations are visible as twinkling (also called "scintillation").
Turbulence also causes small random motions of the star image, and produces rapid changes in its structure. These effects are not visible to the naked eye, but are easily seen even in small telescopes. They are called "seeing" by astronomers.
[attachment=50364]
INTRODUCTION
Photograph of the full Moon partly obscured by Earth's atmosphere. Note the deviation from circular in the Moon's lower edge, caused by refraction.
Atmospheric refraction is the deviation of light or other electromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of altitude. Atmospheric refraction near the ground produces mirages and can make distant objects appear to shimmer or ripple, elevated or lowered, stretched or shortened
Atmospheric refraction causes astronomical objects to appear higher in the sky than they are in reality. It affects not only lightrays but all electromagnetic radiation, although in varying degrees (see dispersion in optics). For example in visible light, blue is more affected than red. This may cause astronomical objects to be spread out into a spectrum in high-resolution images.
VALUES
Atmospheric refraction is zero in the zenith, less than 1′ (one arcminute) at 45° apparent altitude, and still only 5.3′ at 10° altitude; it quickly increases as altitude decreases, reaching 9.9′ at 5° altitude, 18.4′ at 2° altitude, and 35.4′ at the horizon (Allen 1976, 125); all values are for 10 °C and 101.3 kPa in the visible part of the spectrum.
RANDOM RAFRACTION EFFECTS
Turbulence in the atmosphere magnifies and de-magnifies star images, making them appear brighter and fainter on a time-scale of milliseconds. The slowest components of these fluctuations are visible as twinkling (also called "scintillation").
Turbulence also causes small random motions of the star image, and produces rapid changes in its structure. These effects are not visible to the naked eye, but are easily seen even in small telescopes. They are called "seeing" by astronomers.