Spherical Astronomy
3.1 Observing Through the AtmosphereAtmosphere
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, more seri- ous ways as well. Real problems are the weather, restlessness of the atmosphere, and, worst of all, opacity of the atmosphere in many wavelength re- gions.
The air is never quite steady, and there are lay- ers 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 re- sult. Thus, the amount of light reaching a detec- tor, 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, and variations in the light coming from dif- ferent parts partly cancel each others.
When a wavefront passes an aperture its am- plitude and phase vary with time. The amplitude variations cause scintillation and phase changes blur the image. A telescope collects light over a larger area, which evens out rapid changes and diminishes scintillation. Instead, differences in the phase do not cancel out the same way;
they smear the image and point sources are seen in telescopes as vibrating speckles. This phe- nomenon is called seeing. When the exposure time is at least a few seconds the light oscillates in all directions, and the stellar image becomes a round disk (Fig.3.3). The diameter of the seeing disk can vary from less than an arc second to even tens of arc seconds. If the size of the disk small, the seeing is good. It is only in good conditions on high mountains that the seeing is less than one arc second.
Seeing limits how small details can be seen with a telescope. Sometimes the atmosphere can calm down for a short period of time, and then it is possible to glimpse visually details that are not seen in pictures exposed for a longer time.
Some wavelength regions in the electromag- netic spectrum are strongly absorbed by the atmo- sphere (Fig.3.2). The most important transparent
© Springer-Verlag Berlin Heidelberg 2017
H. Karttunen et al. (eds.), Fundamental Astronomy, DOI10.1007/978-3-662-53045-0_3
51
52 3 Observations and Instruments 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 reach- ing 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 ultra- violet radiation. At still shorter wavelengths, the
main absorbers are O2, N2and 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 to 1.3 µm. There are some absorp- tion belts caused by water and molecular oxy- gen, but the atmosphere gets more opaque only at wavelengths of longer than 1.3 µm. At these
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)
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 ab-
sorbed by interstellar gas, as shown in the lowermost very schematic figure. The interstellar absorption also varies very much depending on the direction (Chap.15)
Fig. 3.3 Stellar images vibrate about equally often in all directions, on the average. If the picture is exposed for more than about one second the image of the star spreads into a circular seeing disc. The better the observing condi- tions are the smaller is the diameter of the disc
wavelengths, radiation reaches the lower parts of the atmosphere only in a few narrow windows.
All wavelengths between 20 µm 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 up- per parts of the atmosphere reflects all radiation (Fig.3.2). The exact upper limit of the radio win- dow 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 at- mosphere, and the radiation is attenuated. Scat- tering and absorption together are called extinc- tion. 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 in- versely 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
Fig. 3.4 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 con- siderable disturbance to astronomical observations. (Pho- tos by Ferdinand Ellerman and International Dark-Sky As- sociation)
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 back- ground sky be as dark as possible, and the atmo- sphere as transparent as possible. That is why the large observatories have been built on mountain tops far from the cities. The air above an observa- tory 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 excep- tional sites. In the 1970’s, several new major ob- servatories were founded at these sites. Among the best sites in the world are: the extinct vol- cano Mauna Kea on Hawaii, rising more than 4000 m above the sea; the dry mountains in north- ern 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 ob- servatories are severely plagued by the lights of nearby cities (Fig.3.4).
54 3 Observations and Instruments In radio astronomy atmospheric conditions are
not very critical except when observing at the shortest wavelengths, damped by atmospheric moisture. 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.