Similar in function to the human eye, telescopes work by gathering and focusing light. But, unlike the eye, which is limited by its available space and size, telescopes can be modified to take in more and more light, thereby increasing what they can "see."
There are two main types of optical telescopes: the refractor and the reflector. Refractor telescopes use a specially designed glass lens to bend light toward a central point. From this point, the light reaches the eyepiece and is magnified and processed by the human eye. Reflector telescopes are similar, but instead of glass, use curved mirrors to reflect an image of a distant object to the eyepiece. Historically, reflectors were invented to combat fuzzy images and color aberrations caused by refractors. A third type of telescope, the Catadioptic, that is a hybrid between reflectors and refractors.
The telescope has a simple design. Generally it consists of an eyepiece, a mirror or lens, and a metal or plastic tube. The tube ensures the clearest possible image by keeping the eyepiece and the light-gathering lens or mirror at an ideal distance apart. It also keeps dust and moisture from accumulating on the lenses and ruining the image.
Why Can Some Telescopes "See" Farther Than Others?
While all telescopes start with the same basic components, their strengths are modified by the size of the objective lens (refractors) or primary mirror (reflectors). The diameter of the objective lens determines how much light the telescope can gather, and is known as the aperture. The larger the aperture, the more light it can collect. Aperture is measured in inches or millimeters.
Another aspect of a telescope that determines its strength is the focal length. When an image hits the objective lens, it is bent or reflected, and re-formed a specific distance from the lens at what is called the focal point. The length from the objective lens to the focal point is called the focal length. Along with the focal length of the eyepiece being used, this determines the magnifying power of the telescope.
A Solid Foundation
A telescope is usually mounted on an immobile base. This is important, as even small movements can result in difficulty finding and focusing on such distant objects as stars and galaxies. Historically, telescopes were mounted so that they could be moved up and down and side to side. This was known as an altitude-azimuth (alt-az) mount. When astrophotography became popular, developers introduced the equatorial mount. The equatorial mount places the telescope in line with the Earth's equatorial plane and then moves it around to mimic the rotation of the Earth. For long-exposure viewing or photography, this means not having to manually move the telescope as the stars move across the sky.