The primary purpose of a telescope is to collect light over a large surface area and secondarily to produce a magnification of the image of the objects under study. The site at which one or more telescopes have been constructed is known as an observatory.
A refracting telescope, or refractor, uses the property of refraction to bend the paths of light entering at every position on the objective lens to bring it to a common focus (see Figure 1). The basic function is similar to that of the eye of a camera. The largest existing refractor is the 1.02‐meter‐diameter Yerkes telescope in Wisconsin. Refractors have gone out of favor for astronomical work due to the enormous size of the glass lenses (the Yerkes objective lens weighs 2 tons), the bending of the long tube of the telescope due to its weight, their awkwardness because of the length of the tube, and the cost of the large enclosure needed to shelter such a telescope. Also, as objective lenses are made larger, they become thicker, causing progressively significant loss of light by absorption in the glass.
A refracting telescope.
A reflecting telescope, or reflector, uses a curved mirror as its light gathering surface ( primary mirror) and other mirrors and optical elements to bring all incident light to a common focus (see Figure 2). Modern reflectors have a number of advantages over refractors in that they can be constructed with larger light gathering surfaces (they can be structurally supported on the back of the mirror), are mechanically more stable, are physically smaller instruments (multiple reflections allow the light path to be “folded over”), require a smaller and less expensive observatory building, and are much easier to use. These reflectors do have the disadvantage that they require more critical alignment of the optical elements (the mirrors); but with modern computers and laser technology, alignment can be easily corrected. The most recent large telescopes are also able, under computer control, to modify the surface configuration of their primary mirrors ( active optics) to correct for the visually blurring effects of the atmosphere.
A reflecting telescope.
Although the original detector of light was the human eye, astronomers now rely upon a wide variety technological devices for the recording of electromagnetic radiation. These devices are not only far more sensitive than the eye (a fully dark‐adapted eye detects only 2 to 3 percent of the incident photons, whereas a modern electronic light detector records 90 percent or more of the photons), but often can add (integrate) the energy received over long periods of time and in wavelength regions other than visible light.
The first device to replace the human eye for astronomical observation was the photographic plate. Modern devices, such as the charge‐coupled device (CCD), are far more efficient than photography (only 1 to 5 percent of the light is actually absorbed to produce a photographic image). A CCD is an electronic device using silicon chips to absorb the photons received over a vast array of picture elements (pixels) to produce free electrons that can then be counted to form a digital picture, which can be manipulated by a computer program. (Commercial digital cameras use the same technology.) Photometers are used where the intensity of light is to be measured. Other specialized devices, often carried by artificial satellites, have been developed to explore other regions of the spectrum, from extremely short gamma‐ and X‐radiation to long wavelength radio radiation.