Ken Tapping, 30 October, 2013
If you are lucky enough to have a telescope which is mounted on a tripod, and have set it up on the odd evening to have a look at the Moon, a planet or two, or a double star, you will have encountered a problem. As you observe, the object you're looking at slowly moves out of the field of vision, and to keep observing, you have to keep adjusting the telescope direction. This is due to the Earth's rotation. The Earth spins eastward, turning once a day, so cosmic objects appear to rise in the east, drift westward across the sky, and then set.
Most telescopes are designed for looking at things on the Earth, such as wildlife or ships far out at sea. The most convenient way to mount a telescope for these applications is known as the "altazimuth mount", where the telescope can be tilted up and down (movement in altitude or elevation), and scanned in the horizontal plane (azimuth). Mounts like this are difficult to use in astronomy, because astronomical objects rise and fall in elevation and move westward in azimuth, and at varying rates. This makes them hard to track for any length of time.
Astronomers in the 19th Century came up with an elegant solution. If you lean the telescope northwards so that the azimuth shaft of the mount points at Polaris, the North Star (also known as the Pole Star), the telescope mount will have the same orientation as the rotating Earth. Now following an object across the sky will involve moving the telescope in one plane only, which is much easier. Moreover, the rate of movement is constant, so it is easy to use an electric motor and gearbox to move the telescope, keeping the object you're observing in the field of view for as long as you like, or until it sets. This is called the polar or equatorial method for mounting telescopes. Until very recently all the world's observatories used this type of mounting system. For astronomy the equatorial mount is a dream to use. Most backyard astronomical telescopes are mounted in this way.
Radio telescopes are generally much larger than optical telescopes, and their much greater weight poses problems for equatorial mountings. It is best to rest objects on horizontal surfaces, with their weight pushing straight down onto them. This becomes very clear if you ever sit on a sloping chair seat for any length of time. Therefore radio telescopes are usually put on altazimuth mounts. Instruments using equatorial mounts are generally small. This means large radio telescopes need their tracking problems to be solved.
Before the advent of cheap computers the solution was a sort of electromechanical computer, known as a "coordinate converter" or Co-Co (pronounced like cocoa). However, machines like this cannot easily be used to track anything other than distant cosmic sources, and in some cases the Sun and planets. In recent years these wonderful machines, monuments to engineering precision, have been replaced by computers. Moving all the information processing into software has changed everything. It's easy to change software. This makes it possible to mount telescopes in any way we like and track almost anything: the Moon, comets, even spacecraft. It's just a matter of writing the appropriate software. This is so easy that almost all modern telescopes have altazimuth mounts, which in turn means we can make them bigger and more sensitive than ever. Telescopes with mirrors having diameters as large as 30m would be impossible any other way.
Being able to correct for hardware shortcomings or setup errors rather than laboriously eliminate them has revolutionized astronomy. Many of the major instruments now in use or under construction would not be possible to implement any other way.
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