Telescopes are sometimes referred to as "photon buckets" as they are used to "collect streams of photons". There are two primary types of optical telescope: reflectors (which use mirrors) and refractors (which use lenses).
The basic scheme is that the primary light-gathering element, called the "objective lens", focuses light to a focal plane where it forms a bright virtual image. An "eyepiece" then magnifies the virtual image. Many types of telescopes fold the optical path with secondary or tertiary mirrors, usually to make the telescope more compact and reduce the width of its field of view.
An optical telescope's angular resolution is determined primarily by the width of the objective, termed its "aperture." Recently, it has become practical to perform aperture synthesis with optical telescopes. Increasingly, high-resolution optical telescopes are actually groups of widely-spaced smaller telescopes, linked together by carefully-controlled optical paths. The sensitivity of a telescope is determined by both the area of its objective, and the sensitivity of the sensor.
The f-ratio of a telescope denotes how wide an angle the telescope can view. Low f-ratios indicate wide fields of view. Wide-field telescopes are used to track satellites and asteroids, for cosmic-ray research, and for surveys of the sky.
The light-gathering power of an optical telescope is directly related to the diameter of the objective lens or mirror. Note that the area of a circle is proportional to the square of the diameter. A telescope with a lens which has a diameter three times that of another will have nine times the light-gathering power. Larger objectives gather more light, and more sensitive imaging equipment can produce better images from less light.
Nearly all large research-grade astronomical telescopes are reflectors. Some reasons are:
The size of optical telescopes increased
steadily in the early 20th century culminating in the 200-inch telescope at Palomar Observatory which was built in 1948. From then until the 1980s, only one larger telescope was built.
In the 1980's a number of technological improvements were made which created a new generation of telescopes. These advances included the creation of multi-mirror telescopes and the invention of cheap personal computers which could control the mirrors. Another major advanced was the invention of rotating furnaces in which centrifugal force would shape a telescope mirror to close to its final shape.
Names of types: