Atmospheric Effects And Their Reduction

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The Earth's atmosphere has four deleterious effects on telescope images that prevent us from obtaining images of exoplanets.

First, it absorbs radiation from space thus reducing the quantity reaching the ground. Most of the absorption is due to atmospheric gases. The atmosphere is fairly transparent at visible wavelengths, but it is completely opaque in much of the UV, due largely to O2 and ozone (O3). It is transparent in the infrared only over certain wavelength ranges; over other ranges it is less transparent, even opaque, due largely to absorption by water vapor in the lower atmosphere. It becomes transparent again at radio wavelengths, except at very long wavelengths where, high in the atmosphere, in what is called the ionosphere, most radiation from space is reflected back to space. Figure 8.3 shows the wavelength regions in the UV, visible, and infrared, in which the Earth's atmosphere is fairly transparent to electromagnetic radiation from space, and those where it is much less transparent, even opaque (100% absorption).

Second, the atmosphere emits radiation. Most of the emission arises from the thermal motions of the atmospheric gases. The wavelength range of this thermal emission depends on the temperature of the atmosphere, and for the Earth it is largely in the mid-infrared, particularly in the range 6-100 micrometers with a

FIGURE 8.3 The wavelength regions showing absorption in the Earth's atmosphere of radiation from space, at UV, visible, and infrared wavelengths.

peak around 10 micrometers. At these wavelengths the atmosphere seems to glow, night and day. This emission makes it more difficult to discern faint celestial objects. By placing infrared telescopes at high altitudes the problems of emission and absorption in the mid-infrared can be reduced, but only by going into space can they be completely overcome.

The third deleterious effect is the scattering of radiation. This is where radiation is redirected rather than absorbed. The molecules in the atmosphere are responsible for some of the scattering, the most obvious example being the blue sky, which is the result of solar radiation scattered off atmospheric molecules. It is blue because the intensity scattered increases as wavelength decreases. There is also scattering from aerosols. An aerosol is a suspension of tiny liquid or solid particles, and though aerosols supplied in cans are familiar to you, most aerosols in the atmosphere are of natural origin, and include water droplets, ice crystals, dust, and organic particles. An increasingly troublesome artificial aerosol is contrails (made of ice crystals) from high flying aircraft.

The Sun is such a bright source that scattering of solar radiation from air and aerosols obscures the stars at visible and infrared wavelengths. Scattering of the Moon's radiation also produces obscuration. Other sources of atmospheric illumination at night include ground level artificial lighting, which causes radiation pollution.

As well as degrading images by producing extraneous radiation, scattering also degrades images by scattering back to space some of the radiation from a celestial object, thus reducing its apparent brightness. By siting a telescope away from artificial lighting and at high altitude, these problems can be reduced, but again can only be eliminated by going into space.

The fourth deleterious effect of the Earth's atmosphere is caused by distortion of the waves reaching us from any object in space. This arises from point to point variations in the optical properties of the Earth's atmosphere, and their rapid changes with time due to winds and turbulence. This effect is measured by what is called the atmospheric "seeing". In good seeing the stars shine much more steadily than in poor seeing, when they twinkle a lot - very pretty but not good for astronomy. In poor seeing, in telescopes with D less than about 100 mm the visible image jumps around a lot, whereas in much larger telescopes fine detail is blurred out.

You might despair of being able to do anything at ground level to improve seeing. You would have been right to do so until the last few decades, since when the powerful technique of adaptive optics has been available.

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