Newtonian telescope
The Newtonian telescope is an early formed, important main form of reflecting telescope.
History
The Newtonian telescope is the 1668 developed by Isaac Newton form of the reflecting telescope. It consists of a concave primary mirror and a flat, inclined by 45 ° to the central axis of the secondary mirror, which deflects the light at a right angle and passes out of the tube into the eyepiece. So you can look from the side of the telescope inside.
Newton still used a spherical primary mirror, but such a mirror does not collect all the light in one point but along a Catacaustic. For a telescope with a small aperture ratio ( mirror diameter / focal length) from about 1:8 of the image error caused thereby is low. But the errors are sometimes very disturbing forth for lichststärkere telescopes with a larger aperture ratio.
Has been greatly improved Newton's invention in 1721 by brothers John, George and Henry Hadley by using a parabolic instead of a spherical primary mirror. While James Gregory already had pointed out the advantages of the parabolic mirror them for the first time succeeded in its unequal difficult production. In contrast to the spherical mirror, the parabolic mirror collects parallel incident light to the optical axis at a point of the focal point.
William Herschel built in 1789 a telescope with a mirror diameter of 122 cm and a focal length of 6 m. In the years 1842-1845 William Parsons had built the "Leviathan". This had a mirror diameter of 1.8 m and 18 m focal length.
Use today in amateur
The lower specific costs of a Newtonian telescope with respect to opening and resolution compared to lens telescopes make " Newton " to popular devices in the hobby and amateur.
Newton telescopes with an aperture of 114 mm ( 4.5 inches ) were in the amateur field for many years the standard. With a faint opening ratio ( mirror diameter / focal length ) of about 1:8, the optical power of these telescopes is not significantly impaired by the use of particularly simple and moderately priced spherical primary mirror. The achievable with the usual in Central Europe air turbulence resolution of about 1 to 2 seconds of arc is already achieved by the device often. In addition, the tube is at focal lengths of 900 mm are not sensitive to vibrations and easily transportable, so you can easily get away from the light pollution in metropolitan areas. Even a 114- mm telescope allows first deep-sky observations of objects such as the Orion Nebula (M42 ), observations of the rings of Saturn, the Great Red Spot and the great cloud bands of Jupiter's atmosphere, or the shadow of Jupiter's moons on Jupiter surface. Also in some globular clusters sparkle in the edge region already on individual stars. The faint for observation objects important " light gathering power " and the resolution under very good observing conditions are still very limited.
114 mm with the opening as well as the limit is reached, to which a spherical primary mirror is useful. The required opening ratio, so that a spherical mirror is sufficient, varies with the cube root of the opening. If a 114- mm Newtonian telescope even with an aperture ratio of about 1:8, and thus a focal length of 900 mm, one would need for 200 mm already have an aperture ratio of about 1:9,7 and thus a focal length of 1,940 mm. Such a telescope would be nearly 2 m long and too unwieldy. Here, then, a parabolic mirror is mandatory if the advantage of the larger opening is to be fully exploited. Although somewhat more difficult to grind than a spherical mirror, but also a larger parabolic mirror with a simple hobby equipment can be made in excellent quality largely in the DIY ( see Amateur Telescope Making).
Due to the mass production in the Far East prices for Newton telescopes have now dropped so dramatically that telescopes with about 150-200 mm diameter of the primary mirror (6-8 inches) are already often recommended as a beginner devices (see also Dobson telescope). With typical view of faint objects with larger primary mirror can be better observed as galaxies. In excellent atmospheric conditions much more detail can be resolved. To prevent too many deteriorate their mechanical handling ( tube length, weight, vibration behavior ) are Newtonian with a larger primary mirror but usually with a relatively short focal length, ie a higher aperture opening ratio, built ( 1:6 to about 1:4, 5). This has continued for astrophotography the advantage that the exposure times are lower. The higher aperture ratio brings, however, increasingly higher demands on the fabrication and optical alignment of all components of the telescope ( " collimation " ) with it.
Very bright Newton telescopes ( aperture ratio of 1:5 or higher) tend also with parabolic mirrors are used to image defects. Especially the coma, a " comet " shaped breaking bright points of light at the edge of the image field, and a strongly curved image field cause problems. Many eyepieces, especially long focal length and wide angle types, they can not compensate and show some significant edge blurring. In the middle and upper price segment, but also find eyepieces, which were specifically designed for the observation of "fast" telescopes and reduce these artifacts greatly.
Especially with mirror diameters of more than 30 cm, the cost of a self-made Newtonian telescope are still below the cost of commercial goods. Since the production of high-precision equatorial mounts, however, is expensive and requires sophisticated equipment, mostly simple ( altitudinal - azimuthal ) Mountings are used in amateur for the purely visual observation. In the DIY scene in particular the so-called Dobson is popular.