What Is an Astronomical Telescope?

Astronomy is the science of viewing and analyzing space objects, and telescopes are an important tool. There are three main types, including refractors, reflectors, and catadioptrics. An astronomical telescope is a device made with mirrors, lenses or a combination of both that is used to view objects in space outside of Earth's atmosphere.

Refractor telescopes were the earliest designs, invented in the early 1600s. They use a lens to collect light at one end, called the objective, and create a visible image at the other end, called the eyepiece. Refractors can collect more light by using lenses of larger diameter, and increasing the length can increase magnification, but there are practical limits to the size of refractor designs. The largest objective lens made was 40 inches (1.02 meters) in diameter, and the longest length was 68 feet (21 meters).

A second type of astronomical telescope uses mirrors to collect, focus and direct an image to an eyepiece, and are known as reflectors. An objective curved mirror reflects the light to the center of the barrel where a second mirror reflects the light sideways to an eyepiece. The objective mirror can be a single ground glass mirror, a combination of smaller mirror segments, or non-metallic materials with reflective coatings.

Reflectors can collect more light without increasing their length by changing the curvature of the objective mirror. This makes them useful for amateur astronomers who need portable equipment. Many reflectors with good image resolution can fit in an automobile.

The third type of astronomical telescope is a catadioptric unit. This is a combination of a reflector and refractor design. They appear similar to reflectors in shape and use a mirror as the objective. Refracting lenses sharpen the image and minimize distortion that can occur from mirror imperfections.

Any astronomical telescope suffers from imperfections in the lenses or mirrors, which causes viewed images to be unfocused or have differences in color. Light striking the edges of a mirror or lens may reflect or refract in a slightly different direction than the rest of the light. This is known as spherical aberration, and causes blurry images.

Another defect is chromatic aberration, which is the effect of different wavelengths of light passing through a lens at different angles. This can result in incorrect colors of the viewed image, or colors that vary from the center to the edges of the viewing field. Both types of aberration can cause image distortions that affect scientific investigation of new astronomical findings.

Design of an astronomical telescope involves determining the desired objective lens or mirror size, magnification power and materials of construction. Starting in the 20th century, reflectors were built using groups of smaller mirrors, some with electronic controls that permitted individual mirror segments to move independently. All of these designs were attempting to improve image resolution by minimizing the aberration effects of larger fixed mirrors.

Wavelengths outside the range of visible light can also be explored, primarily with reflector designs. Infrared light, or light below the range of red light, and ultraviolet, or above the range of blue, can be observed. Even radio waves can be collected from space with radio telescopes using electronic image sensors rather than eyepieces.

Very large array (VLA) systems are groups of radio telescopes separated by distance. Each unit acts as an individual mirror of a very large virtual unit. The signals from the VLA are processed in computers to create an image equivalent to a telescope many miles or kilometers in size.

Earth-based telescopes also suffer from distortion caused by the atmosphere. Many are placed at the tops of mountains or extinct volcanoes to attempt to minimize this distortion. The first design deployed in space was the Hubble space telescope placed in Earth orbit in April, 1990. Hubble, despite some mirror defects that were corrected in later repair missions, has returned a vast number of astronomical images free of atmospheric distortion.