Newtonian telescope
The Newton telescope is an early-formed, important main form of reflecting telescope .
history
The Newton telescope is that of Isaac Newton developed in 1668 form the reflecting telescope . The original consists of a concave main mirror made of mirror metal and a flat secondary mirror inclined at 45 ° to the central axis , which deflects the light at a right angle and directs it from the tube into the eyepiece . So you look into the telescope from the side. This original is owned by the Royal Society .
Newton still used a spherical primary mirror, but such a mirror does not collect all the light in one point, but along a catacoustic . In the case of a telescope with a small aperture ratio (mirror diameter / focal length ) from about 1: 8, the resulting image error is small. For more powerful telescopes with a larger focal ratio, the errors are sometimes very annoying.
Newton's invention was greatly improved in 1721 by the brothers John , George, and Henry Hadley by using a parabolic instead of a spherical primary mirror. While James Gregory had already pointed out the advantages of the parabolic mirror, they succeeded for the first time in making it much more difficult to manufacture. In contrast to the spherical mirror, the parabolic mirror collects incident light parallel to the optical axis in one point, the focal point.
In 1789 Wilhelm Herschel built a telescope with a mirror diameter of 122 cm and a focal length of 6 m. In the years 1842-1845 William Parsons had the " Leviathan " built. This had a mirror diameter of 1.8 m and a focal length of 18 m.
Use today in the amateur sector
The specifically lower costs of a Newtonian telescope in terms of aperture and resolving power compared to lens telescopes make "Newtons" popular devices in the hobby and amateur sector .
Newtonian telescopes with an aperture of 114 mm (4.5 inches ) have been the standard in the amateur field for many years. With a weak aperture ratio (mirror diameter / focal length ) of about 1: 8, the optical performance of these telescopes is not significantly impaired by the use of the spherical main mirror, which is particularly easy and inexpensive to manufacture . The resolution of approx. 1 to 2 arc seconds that can be achieved with the air turbulence customary in Central Europe is often achieved by the device. In addition, with focal lengths of approx. 900 mm, the tube is not too sensitive to vibrations and is easy to transport, so that it is easier to escape light pollution in urban areas. Even a 114 mm telescope enables the first deep sky observations of objects such as the Orion Nebula (M42), observations of the rings of Saturn , the Great Red Spot and the large bands of clouds in Jupiter's atmosphere or the shadows of Jupiter's moons on Jupiter's surface. Even in some globular clusters, single stars flash up in the edge area. The “light-collecting power”, which is important for the observation of faint objects, and the resolving power under very good observation conditions are, however, still very limited.
With a 114 mm aperture, however, the limit is reached up to which a spherical primary mirror is useful. The aperture ratio required for a spherical mirror to be sufficient changes with the third root of the aperture. If a 114 mm Newtonian telescope still manages with an aperture ratio of approx. 1: 8 and thus a focal length of 900 mm, you would need an aperture ratio of approx. 1: 9.7 for 200 mm and thus a focal length of 1,940 mm. Such a telescope would be almost 2 m long and too unwieldy. A parabolic mirror is therefore essential here if the advantage of the larger opening is to be fully exploited. Although a bit more difficult to grind than a spherical mirror, a larger parabolic mirror with simple hobby equipment in excellent quality can largely be made by yourself (see Amateur Telescope Making ).
Due to the mass production in the Far East , the prices for Newtonian telescopes have fallen so sharply that telescopes with a primary mirror of approx. 150–200 mm diameter (6–8 inches) are often recommended as beginner devices (see also Dobsonian telescope ). With average visibility, a larger primary mirror can be used to better observe faint objects, such as galaxies . With excellent atmospheric conditions, much more details can be resolved. In order not to impair their mechanical manageability (tube length, weight, vibration behavior) too much, Newtonian telescopes with a larger main mirror are mostly used with a relatively short focal length, i.e. H. a stronger aperture ratio (1: 6 to approx. 1: 4.5). This also has the advantage for astrophotography that the exposure times are shorter. The higher focal ratio, however, entails increasing demands on the manufacture and optical alignment of all components of the telescope (" collimation ").
Very bright Newtonian telescopes (focal ratio 1: 5 or higher) tend to have image defects even when using parabolic mirrors. Above all, the coma , a “comet” -shaped breakout of 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, cannot compensate for this and sometimes show significant edge blurring. In the middle and upper price segment, however, there are also eyepieces that have been specially designed for observation on "fast" reflector telescopes and that greatly reduce these image errors.
DIY
Up until the 1990s it was quite common for amateurs to build a Newtonian telescope themselves - often also to make their own mirror grinding , for which there are still special courses today. For equatorial mounting of small telescopes up to about 10 kg, W. Schroeder ( Praktische Astronomie , 1960) gave a well-suited, stable construction made of hardwood. For mounts made of metal that could carry up to 30 kg, building instructions u. a. by the Swiss astronomer Hans Rohr .
Especially with mirror diameters of more than 30 cm, the cost of a self-made Newtonian telescope is still below the cost of commercial goods. On the other hand, since the manufacture of high-precision equatorial mounts is expensive and requires complex equipment, simple azimuthal mounts are mostly used in the amateur field for purely visual observation . The so-called Dobsonian is particularly popular in the do-it-yourself scene .
See also
- Schmidt-Newton telescope
- Maksutov-Newton telescope , a variant with a correction plate like the Maksutov telescope
Web links
Individual evidence
- ↑ a b Amazing piece of metal (speculum) on YouTube.
- ↑ [1] Vladimir Sacek , www.telescope-optics.net, accessed on February 24, 2013.