Many people believe that astronomers want to build telescopes on tall mountains or put them in space, so they can be ``closer'' to the objects they are observing. This is INcorrect! The nearest star is over 41,500,000,000,000 kilometers (26 trillion miles) away. If you ignore the 300-million kilometer variation in the distances due to the Earth's motion around the Sun and the 12,756-kilometer variation due to the Earth's rotation, being 4 kilometers closer on a tall mountain amounts to a difference of at most 1 × 10-11 percent. Telescopes in space get up to 1 × 10-9 percent closer (again ignoring the much larger variations of the Earth's orbit around the Sun and the telescope's orbit around the Earth). These are extremely small differences---the distances to the even the nearest stars are around 100,000's times greater than the distances between the planets in our solar system. The reason large telescopes are built on tall mountains or put in space is to get away from the distortion of starlight due to the atmosphere.
The famous observing site
at the Kitt Peak National
Observatory has many large telescopes including the 4-meter Mayall telescope
(top right) and the McMath Solar Telescope (triangular one at the lower right).
Although it is over 60 kilometers from Tucson, AZ, light pollution from the increasing
population of that city has stopped the construction of any more telescopes on the
mountain.
The Mauna Kea Observatory is probably the best observing site in the world. Many very large telescopes are at the 4177-meter summit of the extinct volcano. Because of the elevation, the telescopes are above most of the water vapor in the atmosphere, so infrared astronomy can be done. Kitt Peak's elevation of 2070 meters is too low for infrared telescopes.
Good seeing is when the air is stable (little turbulence) and the twinkling is small. Details as small as 0.5 arc seconds can be seen when the seeing is good (still much larger than the theoretical resolving power of large research telescopes). Poor seeing happens when the air is turbulent so the images dance about and details smaller than 2 to 3 arc seconds cannot be seen. The more atmosphere there is above a telescope, the greater is the turbulent motion and the poorer is the seeing. This is one reason why research telescopes are located on very high mountains.
Speckle interferometry can get rid of atmospheric distortion by taking many fast exposures of an object. Each fraction-of-a-second exposure freezes the motion of the object. Extensive computer processing then shifts the images to a common center and removes other noise and distortions caused by the atmosphere, telescope, and electronics to build up a distortion-free image. See Chris Koresko's thesis for further exploration of speckle interferometry. Another technique called adaptive optics makes quick changes in the light path of the optics to compensate for the atmospheric turbulence. Before the focussed light from the objective reaches the camera, it bounces off a thin deformable mirror that can be adjusted thousands of times a second to reposition the multiple images back to the center. An excellent site to explore this topic further is the Center for Astronomical Adaptive Optics web site. A comparison of speckle interferometry and adaptive optics is at Koresko's Starfire infrared imaging site.
Telescopes in orbit like the Hubble Space Telescope are above the turbulent effect of the atmosphere and can achieve their theoretical resolving power. The Hubble Space Telescope has a 2.4-meter objective, making it the largest telescope ever put in orbit. One major drawback to satellite observatories is the large cost to build and maintain them. Advances in adaptive optics may soon remove the seeing effects and enable the huge research telescopes to take even sharper pictures than those from the Hubble Space Telescope.
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. 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 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 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 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 observes in the short-wavelength end of the ultraviolet band. The Roentgen Satellite (ROSAT) is in the second picture. It observes in the X-ray band. The third picture shows a telescope that observes the most energetic forms of electromagnetic radiation---gamma rays. It is called the Compton Gamma Ray Observatory.
The Roentgen Satellite (ROSAT)
The Compton Gamma Ray Observatory
| adaptive optics | extinction | reddening |
|---|---|---|
| seeing | speckle interferometry |
Go to
effects of our atmosphere section
last updated 07 February 1999
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Physical Science Dept.
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