Building a huge, high quality telescope are by no means an easy task. Huge mirrors are difficult and expensive to make which require high-precision computer-controlled tools. Furthermore, high-precision mechanical devices are required to move the heavy telescope while tracking celestial objects. Telescopes are usually housed in dome-shaped structure (observatories) to protect from elements. In order to produce highest quality images, the observatories are usually located at high-altitude and far away from human inhabitants
where the air is dry, steady and free from dusts and pollutants. On the contrary, radio telescopes are usually located at the valleys or areas that are shielded from man-made radio signals.
Large telescope lenses are very heavy. For example, the 200-inch mirror at Palomar Observatory weighs around 14.5 tons. Very large mirrors may sag under its own weight due to the Earth's gravitational pull. Even a slight sagging is enough to create a serious distortion to the image it produces. More advance mirror-casting techniques have been introduced in order to make larger and lighter mirrors.
Recently, an innovative idea has been developed to overcome the weight-size problem. Instead of making a single-large piece of mirror, several much smaller individual pieces can be made, each of which can be mounted on a support. Each mirror is controlled electronically so that the combined mirrors can aligned with one another to form a single piece of mirror. Such technique is first used to build the two Keck telescopes at Hawaii.
Both telescopes (also known as the Twin Keck) are some of the world's largest. Each telescope consists of 36 hexagonal mirror segments with a combined diameter of ten meters.
Diagram on the left shows schematic arrangement of the 36 hexagonal segments. Each segment has a diameter of 1.8 m and weighs 400 kg.
Despite great efforts, Earth-based observatories still suffer from image blurring caused by turbulent atmosphere. However, recent introduction of the so-called 'adaptive optics' improves image resolution by at least an order of magntitude. It is basically a highly deformable mirror that changes its shape 670 times per second to cancel out the atmosphere distortion. Such device has recently been installed on one of the Keck telescope (Feb. 1999) and coupled with the largest mirror diameter, images of very faint and/or far celestial objects can be obtained with stunning details.