A telescope consists of an objective (a lens in a refracting
telescope or a curved mirror in a reflecting telescope) and an eyepiece
(also called an ocular). The objective forms a real image, which the
eyepiece magnifies. The objective lens and eyepiece normally consist of
more than one lens element.
A telescope's aperture is simply the diameter of the objective. For amateur telescopes this is up to six inches for a refractor, and up to sixteen inches for a reflector. The aperture determines the amount of light the telescope is able to collect. It is the most important optical parameter - the greater the aperture the more the instrument can resolve.
The telescope's focal length determines the image size. If the instrument is a refractor, then the focal length is just the distance from the lens to the image. With more complex telescopes like catadioptrics, the focal length is appreciably more than the length of the instrument, usually up to three times the telescope's physical length.
The magnification or power of a telescope is completely dependent on the focal length of the eyepiece being used. The magnification is the ratio of the telescope's focal length and the eyepiece focal length. For example, a refractor of one metre focal length in combination with an eyepiece of 25 mm focal length magnifies 40x because 1000 divided by 25 equals 40. This is why telescopes are not rated 20x or 100x the way binoculars are. By changing the eyepiece any power can be obtained, but what can be seen will be limited by the telescope's resolution.
Telescopic images are normally upside down or reversed right to left. This is because the objective flips the image over with no re-flipping carried out by the eyepiece. Binoculars and terrestrial telescopes have erect images because prisms or other lenses are purposely used in the design. But for astronomical instruments there is no need to have an erect image, which means more light gets through to the eyepiece.
Most modern telescopes are equipped with a diagonal, so that the observer does not have to crane his or her neck when viewing the night sky. The diagonal simply flips the image in one direction only, which means that what is seen is the right way up, but is reversed left to right. Diagonals can be made from a prism or from a plane mirror, but both transmit light just as effectively.
A telescope's aperture is simply the diameter of the objective. For amateur telescopes this is up to six inches for a refractor, and up to sixteen inches for a reflector. The aperture determines the amount of light the telescope is able to collect. It is the most important optical parameter - the greater the aperture the more the instrument can resolve.
The telescope's focal length determines the image size. If the instrument is a refractor, then the focal length is just the distance from the lens to the image. With more complex telescopes like catadioptrics, the focal length is appreciably more than the length of the instrument, usually up to three times the telescope's physical length.
The magnification or power of a telescope is completely dependent on the focal length of the eyepiece being used. The magnification is the ratio of the telescope's focal length and the eyepiece focal length. For example, a refractor of one metre focal length in combination with an eyepiece of 25 mm focal length magnifies 40x because 1000 divided by 25 equals 40. This is why telescopes are not rated 20x or 100x the way binoculars are. By changing the eyepiece any power can be obtained, but what can be seen will be limited by the telescope's resolution.
Telescopic images are normally upside down or reversed right to left. This is because the objective flips the image over with no re-flipping carried out by the eyepiece. Binoculars and terrestrial telescopes have erect images because prisms or other lenses are purposely used in the design. But for astronomical instruments there is no need to have an erect image, which means more light gets through to the eyepiece.
Most modern telescopes are equipped with a diagonal, so that the observer does not have to crane his or her neck when viewing the night sky. The diagonal simply flips the image in one direction only, which means that what is seen is the right way up, but is reversed left to right. Diagonals can be made from a prism or from a plane mirror, but both transmit light just as effectively.
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