Through the Atmosphere

With satellites and spacecraft, observations can be made outside the atmosphere. Yet, the great majority of astronomical observations are carried out from the surface of the Earth. In the preceding chapter, we discussed refraction, which changes the apparent altitudes of objects. The atmosphere affects observations in many other ways as well. The air is never quite steady, and there are layers with different temperatures and densities; this causes convection and turbulence. When the light from a star passes through the unsteady air, rapid changes in refraction in different directions result. Thus, the amount of light reaching a detector, e. g. the human eye, constantly varies; the star is said to scintillate (Fig. 3.1). Planets shine more steadily, since they are not point sources like the stars.

A telescope collects light over a larger area, which evens out rapid changes and diminishes scintillation. Instead, differences in refraction along different paths of light through the atmosphere smear the image and point sources are seen in telescopes as vibrating speckles. This phenomenon is called seeing, and the size of the seeing disc may vary from less than an arc second to several tens of arc seconds. If the size of the seeing disc is small, we speak of good seeing. Seeing and scintillation both tend to blot out small details when one looks through a telescope, for example, at a planet.

Some wavelength regions in the electromagnetic spectrum are strongly absorbed by the atmosphere. The most important transparent interval is the optical window from about 300 to 800 nm. This interval coincides with the region of sensitivity of the human eye (about 400-700 nm).

At wavelengths under 300 nm absorption by atmospheric ozone prevents radiation from reaching the ground. The ozone is concentrated in a thin layer at a height of about 20-30 km, and this layer protects the Earth from harmful ultraviolet radiation. At still shorter wavelengths, the main absorbers are O2, N2 and free atoms. Nearly all of the radiation under 300 nm is absorbed by the upper parts of the atmosphere.

At wavelengths longer than visible light, in the near-infrared region, the atmosphere is fairly transparent up

Fig. 3.1. Scintillation of Sirius during four passes across the field of view. The star was very low on the horizon. (Photo by Pekka Parviainen)

Hannu Karttunen et al. (Eds.), Observations and Instruments.

In: Hannu Karttunen et al. (Eds.), Fundamental Astronomy, 5th Edition. pp. 47-82 (2007) DOI: 11685739_3 © Springer-Verlag Berlin Heidelberg 2007

Fig. 3.1. Scintillation of Sirius during four passes across the field of view. The star was very low on the horizon. (Photo by Pekka Parviainen)

Hannu Karttunen et al. (Eds.), Observations and Instruments.

In: Hannu Karttunen et al. (Eds.), Fundamental Astronomy, 5th Edition. pp. 47-82 (2007) DOI: 11685739_3 © Springer-Verlag Berlin Heidelberg 2007

Wavelength o ö

Gamma-rays

X-rays

EUV UV Ultraviolet

Infrared

Microwaves

Radio waves

Solar radiation

Atmospheric transmission

Absorption by Absorption oxygen and nitrogen by ozone

Absorption by /\ water vapour

Scattering from ionosphere

100%

n it

Optical Infrared window window

Radio window

100%

n it

Optical Infrared window window

Radio window

Fig. 3.2. The transparency of the atmosphere at different wavelengths. 100% transmission means that all radiation reaches the surface of the Earth. The radiation is also absorbed by inter stellar gas, as shown in the lowermost very schematic figure. The interstellar absorption also varies very much depending on the direction (Chap. 15)

Fig. 3.2. The transparency of the atmosphere at different wavelengths. 100% transmission means that all radiation reaches the surface of the Earth. The radiation is also absorbed by inter stellar gas, as shown in the lowermost very schematic figure. The interstellar absorption also varies very much depending on the direction (Chap. 15)

to 1.3 ^m. There are some absorption belts caused by water and molecular oxygen, but the atmosphere gets more opaque only at wavelengths of longer than 1. 3 ^m. At these wavelengths, radiation reaches the lower parts of the atmosphere only in a few narrow windows. All wavelengths between 20 ^m and 1 mm are totally absorbed. At wavelengths longer than 1 mm, there is the radio window extending up to about 20 m. At still longer wavelengths, the ionosphere in the upper parts of the atmosphere reflects all radiation (Fig. 3.2). The exact upper limit of the radio window depends on the strength of the ionosphere, which varies during the day. (The structure of the atmosphere is described in Chap. 7.)

At optical wavelengths (300-800 nm), light is scattered by the molecules and dust in the atmosphere, and the radiation is attenuated. Scattering and absorption together are called extinction. Extinction must be taken into account when one measures the brightness of celestial bodies (Chap. 4).

In the 19th century Lord Rayleigh succeeded in explaining why the sky is blue. Scattering caused by the molecules in the atmosphere is inversely proportional to the fourth power of the wavelength. Thus, blue light is scattered more than red light. The blue light we see all over the sky is scattered sunlight. The same phenomenon colours the setting sun red, because owing to the long, oblique path through the atmosphere, all the blue light has been scattered away.

In astronomy one often has to observe very faint objects. Thus, it is important that the background sky be as dark as possible, and the atmosphere as transparent as possible. That is why the large observatories have been built on mountain tops far from the cities. The air above an observatory site must be very dry, the number of cloudy nights few, and the seeing good.

Astronomers have looked all over the Earth for optimal conditions and have found some exceptional sites. In the 1970's, several new major observatories were founded at these sites. Among the best sites in the world are: the extinguished volcano Mauna Kea on Hawaii, rising more than 4000 m above the sea; the dry mountains in northern Chile; the Sonoran desert in the U.S., near the border of Mexico; and the mountains on La Palma, in the Canary Islands. Many older observatories are severely plagued by the lights of nearby cities (Fig. 3.3).

In radio astronomy atmospheric conditions are not very critical except when observing at the shortest wave-

Fig. 3.3. Night views from the top of Mount Wilson. The upper photo was taken in 1908, the lower one in 1988. The lights of Los Angeles, Pasadena, Hollywood and more than 40 other towns are reflected in the sky, causing considerable disturbance to astronomical observations. (Photos by Ferdinand Ellerman and International Dark-Sky Association)

Fig. 3.3. Night views from the top of Mount Wilson. The upper photo was taken in 1908, the lower one in 1988. The lights of Los Angeles, Pasadena, Hollywood and more than 40 other towns are reflected in the sky, causing considerable disturbance to astronomical observations. (Photos by Ferdinand Ellerman and International Dark-Sky Association)

lengths. Constructors of radio telescopes have much greater freedom in choosing their sites than optical astronomers. Still, radio telescopes are also often constructed in uninhabited places to isolate them from disturbing radio and television broadcasts.

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