Telescope

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a quick stub for telescopes

Telescope

From various Wikipedia sources, the free encyclopedia.

A telescope is perhaps the most important astronomical tool; such technology gathers (and focuses) electromagnetic radiation. Telescopes increase the apparent angular size of objects, as well as their apparent brightness. Galileo Galilei is credited with being the first to use a telescope for astronomical purposes. Telescopes used for non-astronomical purposes are often referred to as transits, spotting scopes, monoculars, binoculars, camera lenses, or spyglasses.

The word "telescope" usually refers to optical telescopes, but there are telescopes for most of the spectrum of electromagnetic radiation. An optical telescope is a telescope which is used to gather, and focus, light, for directly viewing a magnified image, making a photograph, etc. The term is used especially for a monocular with static mounting for observing the sky. Handheld binoculars are common for other purposes.

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.

Angular Resolution * 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.

Field Of View * The field of view of a telescope is the angular width of the patch of sky it can capture an image of at one time. Achieving a large field of view with uniformly high image quality is one of the recurring problems in all optical designs, and requires ingenious and often complex optics; early telescopes in particular often had very limited usable fields of view (a fraction of a degree). Special wide-field telescopes with fields of view up to tens of degrees are used to track satellites and asteroids, for cosmic-ray research, and for surveys of the sky.

F-Ratio * The f-ratio, or f-number of a telescope is the ratio of its focal length to its aperture. Low f-ratio (short focal length) primary optics are desirable because they make telescopes more compact, but they are more strongly curved and thus more difficult to fabricate than high f-ratio optics. Low f-ratio optical systems are also more subject to aberrations. One of the advantages of the Newtonian reflecting telescope over the Galilean refractor was that it was much easier to make large low f-number mirrors than large low f-number lenses; the reverse is true for very small optics (fast lenses are easier than fast mirrors) so even very early microscopes used sharply-curved, low-f-number lenses. * f-ratio also determines the "plate scale" of a telescope (the size that the image of a given object appears on a photographic plate or other detector); for a given aperture, a low f-number system will capture more sky on a given size photographic plate or other detector than a high f-number system. Thus many (but not all) wide-field telescopes have low f-number optics. More light in a given area also means photographic film or plates can be exposed for a shorter time, hence low f-number optics are also called "fast" optics.

Light-Gathering Power * 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.

Research Telescopes * Nearly all large research-grade astronomical telescopes are reflectors. Some reasons are:

In a lens the entire volume of material has to be free of imperfection and inhomogeneities, whereas in a mirror, only one surface has to be perfectly polished. Light of different colors travels through a medium other than vacuum at different speeds. This causes chromatic aberration.

There are technical difficulties involved in manufacturing and manipulating large-aperture lenses. One of them is that all real materials sag in gravity. A lens can only be held by its perimeter. A mirror, on the other hand, can be supported by the whole side opposite to its reflecting face.

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:

Binoculars are just two monoculars mounted side-by side with adjustments to let both be used. A major practical advantage of these telescopes is not magnification, so much as a brighter field of view at dusk and dawn. Monoculars and binoculars with built-in compasses are used by army artillery units and ships to navigate by triangulating from topographic (shore) features. Hand-held telescopes are limited by hand-shaking to about 7 power. The brightest-field, best-magnifying practical monocular is about 7x50.