Seeing
Astronomical seeing refers to the ability to view celestial objects through the obscurations of the earth's atmosphere. These obscurations include opacity, scattering, turbulence, atmospheric and thermal emission, and ionization.
Opacity refers to the fact that Earth's atmosphere is transparent only to relatively narrow wavelength ranges of light. These include visual light, the near infrared, microwaves and radio waves with wavelengths between about 0.35 mm and 1 m. The atmosphere is almost completely opaque to ultraviolet light, x rays, gamma rays, and radio waves with wavelengths greater than 1 m. The need to observe heavenly bodies outside of these narrow wavelength windows, along with the desirability to avoid the degrading affects of the atmosphere are among the main reasons for the development of space-based telescopes.
Ultraviolet photons are absorbed by electron transitions in oxygen and ozone atoms in the upper atmosphere. Because the amount of ozone varies greatly with location and seasonal time of year (e.g., the hole in the ozone layer over Antarctica during the winter) so does the amount of ultraviolet light reaching the surface of Earth. Ultraviolet light is mainly absorbed by molecular nitrogen. Infrared light is absorbed mainly by water vapor and carbon dioxide in the earth's atmosphere. Millimeter wavelengths are absorbed by rotational bands of water and molecular oxygen, while long radio waves are absorbed by ions high in the earth's atmosphere. Because 50% of the water vapor lies within three kilometers of the earth's surface, to some degree the infrared spectrum may be observed with instruments located on high mountains, or mounted on airplanes or balloons. Wavelength regions at which the atmosphere is transparent are called atmospheric windows.
Scattering of light particles degrades seeing. The mechanism by which molecules scatter visible light is called Rayleigh scattering, and the degree to which light is scattered is inversely proportional to the fourth power of the wavelength and proportional to the density of atmosphere. Thus, blue light is scattered more strongly than red light, accounting for the blue color of the sky. Red sunsets are an optical illusion caused by the intense scattering of blue light as the light rays travel through the horizon-level thickest regions of the atmosphere.
Atmospheric turbulence resulting from thermal currents, or wind, creates small changes in the density of pockets of air that cause the direction of light rays from point sources such as stars to be changed by refraction. In effect, the position of the star seems to shift slightly, and the star appears to twinkle. On a photographic plate, turbulence results in a smearing of the stellar image. Details of planetary features are often highly obscured and degraded by turbulence. The human eye, which processes light almost instantly, often sees a much sharper image than may be obtained with photographs.
Atmospheric emission of the night sky, also called air-glow, is caused by the recombination of electrons with atoms that were ionized during the day by photochemical dissociation. This fluorescent light arises from neutral oxygen atoms and molecules, sodium, hydrogen, and hydroxide molecules, and is emitted about 100 km above the earth's surface. The total airglow over one arcsecond, roughly the apparent size of a star with a large telescope, corresponds to a visual magnitude of 22. Thus, stars dimmer than this, or galaxies with a surface area brightness less than this, are difficult to detect.
Thermal emission of the night sky is also a factor in the near infrared part of the spectrum. Any warm object in thermal equilibrium will emit black body radiation (e.g., iron heated to several hundred degrees will glow red or white). The earth's atmosphere below about 50 km emits a faint light by this mechanism, equivalent to a few Janskys (10−26 Watts/m2Hz).
Ionization in the earth's ionosphere degrades radio waves. Turbulence in this layer of the atmosphere causes small fluctuations in the density of free electrons, which alter the direction of radio waves and cause dispersion in the frequencies of the radio waves. A radio wave emitted from a star of a particular wavelength or frequency will be dispersed into a small range of frequencies by ionization.
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