The air also absorbs and scatters electromagnetic radiation by an amount that varies with the wavelength. Redder (longer wavelength) light is scattered less by atmosphere molecules and dust than bluer (shorter wavelength) light. This effect is known as reddening. This effect explains why the Sun appears orange or red when it is close to the horizon. The other colors of sunlight are scattered out of your line of sight so that only the orange and red colors make it through the atmosphere to your eyes. This effect also explains why the sky is blue. Since blue light is scattered more, you will see more blue light scattered back to your eyes when you look in a direction away from the Sun.
All wavelengths of light are scattered or absorbed by some amount. This effect is called extinction. Some wavelength bands suffer more extinction than others. Some of the infrared band can be observed from mountains above 2750 meters elevation, because the telescopes are above most of the water vapor in the air that absorbs much of the infrared energy from space. Carbon dioxide also absorbs a lesser amount of the infrared energy. Gamma-rays and X-rays are absorbed by oxygen and nitrogen molecules very high above the surface, so none of this very short wavelength radiation makes it to within 100 kilometers of the surface. The ultraviolet light is absorbed by the oxygen and ozone molecules at altitudes of about 60 kilometers. The longest wavelengths of the radio band are blocked by electrons at altitudes around 200 kilometers.
The Hubble Space Telescope (HST) is able to observe in the ultraviolet, something that ground-based research telescopes cannot do. This is one advantage that HST will always have over ground-based telescopes, even those with adaptive optics. Even though HST has a smaller objective than many ground-based telescopes, its ability to observe in shorter wavelengths will keep its resolving power very competitive with the largest ground-based telescopes with the best adaptive optics.
Telescopes used to observe in the high-energy end of the electromagnetic spectrum, like the Chandra X-ray Observatory above, must be put above the atmosphere and require special arrangements of their reflecting surfaces. The extreme ultraviolet and X-rays cannot be focussed using an ordinary mirror because the high-energy photons would bury themselves into the mirror. But if they hit the reflecting surface at a very shallow angle, they will bounce off. Using a series of concentric cone-shaped metal plates, high energy ultraviolet and X-ray photons can be focussed to make an image.
Gamma rays have too high an energy to be focussed with even the shallow angle reflecting technique, so gamma ray telescopes simply point in a desired direction and count the number of photons coming from that direction. Some other examples of high-energy space observatories are shown below. Clicking on the images will take you to sites describing the telescopes in greater detail. The first picture is of the Extreme Ultraviolet Explorer spacecraft that observed in the short-wavelength end of the ultraviolet band. The third picture shows a telescope that observed the most energetic forms of electromagnetic radiation---gamma rays. It was called the Compton Gamma Ray Observatory.
The
Compton Gamma Ray Observatory was brought down from orbit in June 2000 in a
controlled burn-up. It lasted over twice as long as its original mission
specifications.
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last updated: 28 May 2001