A less hospitable climate meant that open-air observatories were impractical. The astronomer and the instruments needed a roof over their heads. Initially, these roofs were constructed with sliding panels or doors that could be pulled back to open the building to the night sky. Since the 19th century most telescopes are housed in dome-shaped buildings whose roofs slide back at night to expose the telescopes to the stars. The earliest domes were made of papier mâché, the only substance known to be sufficiently light and strong. Now most domes are made of fibre glass or high-resistance plastics and computers control telescopes, therefore they can point to any part of the sky.
(above) One of the newest and finest observatories of the world located at Mauna Kea volcano on the island of Hawaii where it has the thinner air of high altitudes and the temperate climate of Pacific.
The stars appear to move through the heavens because the Earth spins on its axis. So, in order to follow a particular star for any length of time, the telescope must be driven. This is most simply done if the telescope has an equatorial mounting. This had one axis of movement parallel with the Earth's axis. The telescope is then rotated round this axis at the same speed as the Earth (and the star) is moving.
AN EQUATORIAL MOUNT
The equatorial mount used to be the favoured mount, and is still preferred by amateur astronomers. One axis of the telescope is lined up with the Pole Star, which appears stationary. In Southern Hemisphere, another bright star is used. The telescope can swing around the axis, automatically following the tracks of stars in the sky as they circle around the Pole Star.
AN ALTAZIMUTH MOUNT
The altazimuth mount is like a gun turret. The telescope can track a star by moving up and down as well as turning around. It was not a versatile mount for following stars across the sky because stars travel on tilted paths (because of the tilt of the Earth's axis). Today, computers can make continual adjustments when tracking. Astronomers prefer this mount because it is stable for big telescopes.
