Video lecture for this chapter
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 atmospheric distortion is poor seeing, reddening, extinction and the adding of absorption lines to stellar spectra.
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 near-infrared astronomy can be done (near-infrared is the part of the infrared band that is nearest the visible light band, so it is slightly longer wavelengths than red). Kitt Peak's elevation of 2070 meters is too low for infrared telescopes. Telescopes used for near-infrared astronomy need to be at least 2750 meters elevation to get above enough of the water vapor to make the investment worthwhile.
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last updated: January 20, 2022