Types of telescopes and mounts

The oldest type of telescope is a refractor - it was first used by Galileo. It has a lens at the front of a tube to gather light and bend it (refract it) to a focus at the back. The reflector was invented by Sir Isaac Newton; it uses a shaped mirror at the back to collect light and focus it.

Although all the telescopes on Kitt Peak (with the exception of the HAT-1) are reflectors, there are different kinds of reflecting telescopes. Most often, these are identified by the location of the focal point of the light. Many of the telescopes at Kitt Peak (even the radio telescopes) are of the cassegrain design. The focal point is behind the primary mirror, so the instruments can be conveniently mounted on the solid support

As the Earth rotates on its axis, the stars appear to move in the sky. This image shows star trails taken over a period of several hours; the trails are centered on the North Celestial Pole. Without a mount that compensates for the Earth's rotation, photographs taken through a telescope would always have trails. Photo courtesy of Terry Oswalt and SARA.

structure of the telescope, making maintenance easy. It also keeps the complicated electronics out of the light path through the telescope, so that heat generated by the instruments does not disturb the air inside the telescope, thereby affecting the seeing.

The Earth rotates on its axis once per day, which means that objects appear to move across the sky over time; this is most obvious with the Sun, but the same applies to all other objects in the sky. If you point a small telescope at (say) the Moon and lock its position, in only a few minutes the Moon will drift out of the field of view. An astronomer has to correct for this motion in order to take photographs with exposures that last for more than a second, or the stars in the image would make streaks. Therefore, a mount is important, as it is the mount that moves the telescope counter to the Earth's rotation. There are two basic types of telescope mounts: equatorial and altitude-azimuth (commonly known as alt-az). An equatorial mount is aligned with the Earth's axis of rotation. Its advantage is that it is driven by a motor on only

An alt-az mount rotates in a plane that is parallel to the Earth's surface (like moving your head side to side), and on the other axis it rotates up and down. The photograph shows the alt-az mount of the WIYN 3.5-meter telescope. Although the mount is much lighter and more compact than an equatorial mount, a computer must continuously calculate where to drive the telescope on both axes to counter the Earth's rotation. Photo courtesy of National Optical Astronomy Observatory/Association of Universities for Research in Astronomy/ National Science Foundation.

one axis. Equatorial mounts are very heavy, though, while alt-az mounts can be quite light. Instead of being aligned with the Earth's rotational axis, the alt-az mount is driven by a computer that continuously adjusts the position of the telescope along both axes. This is now the preferred mount for large telescopes, because computers are so cheap and powerful. Equatorial mounts are used now only for relatively small (and usually) amateur telescopes; the Mayall 4-meter telescope was one of the last large optical telescopes built with one (see page 54).

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