Dobsonian telescope
A Dobsonian telescope is a device for astronomical observations that is practically only used for hobby purposes . In professional circles it is also referred to as "Dobsonian" or "Dob" for short ( English dobsonian telescope ). It was developed by John Dobson in the mid-1950s as a particularly inexpensive amateur telescope .
Basics of the construction
The "classic" Dobsonian telescope is a Newtonian telescope with the following characteristic properties (deviations from conventional construction):
- thin main mirror
- above-average mirror diameter
- below average focal length
- The main mirror is floating - centering by means of a belt
- robust azimuth mount in a simple design and with Teflon plain bearings (see below)
The thin main mirror (strength 1/20–1 / 10 of its diameter) allows a considerably larger diameter than other types of construction due to its lower weight. So that the eyepiece position is not too high above the floor, a shorter focal length is often chosen ( aperture ratio around 1: 5, but also up to 1: 3.8).
The Dobsonian mount consists of a base plate on which a box (called rocker box or cradle) is rotatably mounted on three widely spaced points. The height wheels, which in turn carry the tube with the optics, rest in two recesses in the side walls of the box. Ball bearings etc. are not required because the sliding surfaces are made of a plastic laminate that slides on Teflon discs. If the tube is neither head nor tail heavy, no clamps or couplings are required. The support points, which are far apart in both directions of rotation, ensure a very stable position. The essential elements (mirror, tube and cradle) are held in place solely by gravity. This particularly simple design allows even less experienced do-it-yourselfers to quickly achieve success with simple tools when building their own.
The disadvantage of the azimuth mount is that you cannot follow the arched movement of the celestial objects by rotating them in only one axis (see polar mount or equatorial mount ), but rather “stepped” in small steps. Classical sky recordings are hardly possible because tracking with a computer control is quite complicated. On the other hand, the Dobsonian construction (laminate Teflon) allows easy tracking of the objects with visual observation by "pushing" or "pushing" the tube with the least amount of force.
John Dobson's goal was a mirror telescope with a large aperture that was easy to build and easy to use. For his first devices he is said to have used materials from bulky waste . His concept was ignored for a long time in professional circles - it was rather mocked. The breakthrough came in 1979. Today almost all well-known manufacturers offer various Dobsonians, from eight-inch models (approx. 15 kg) to 14-inch models (35 cm mirror diameter with about 30 + 30 kg weight) that can still be transported in the car.
Recent developments
The many amateur telescope builders who then got down to work developed the concept further: Today, most large Dobsonian telescopes have a grid construction that allows weight and space-saving transport. High-quality materials ( multiplex panels , aluminum , foam composite , carbon, etc.) are also increasingly being used . Occasionally, Dobsonian telescopes are mounted on an equatorial or Poncet platform , others have a stepper motor drive in at least a horizontal direction or are controlled by a PC . The cost advantage originally sought by John Dobson is then just as unattainable as the ease of manufacture.
Until the end of the 1970s, homemade amateur telescopes were mostly carried by equatorial mounts , with an opening usually less than 30 cm. Since the beginning of the 1990s, Dobsonian telescopes have been taking an ever larger market share - and often with openings over 30 cm. In both self-made and pre-purchased Dobsonian telescopes, however, devices with a smaller aperture are numerically more represented, because they offer a price-performance ratio that conventional telescopes cannot offer.
Since the mid-1990s, a faction of Dobsonian ATMs has been vying to see who can build the most slimmed-down, lightest telescope - so-called minimalist Dobsonian telescopes .
In addition to the self-made Dobsonian telescopes, industrially and mostly in China made commercial Dobsonian telescopes are enjoying ever greater popularity, especially among newcomers to the amateur astronomy hobby . While the mount often costs more than the optics, especially for telescopes with a larger aperture, the opposite is true for Dobsonian telescopes. The rocker box, which can be manufactured with relatively little financial outlay, makes it possible to spend a large part of the production costs on the optics, which can also be regarded as useful with inexpensive Dobsonian telescopes. For example, a complete 8 ″ Dobsonian telescope costs roughly the same as a sufficiently stable equatorial mount for optics of the same size.
Due to the ever-falling entry-level prices, such purchase devices are particularly suitable for entry into the hobby, as the operation and handling is quite simple and you get a large amount of light-collecting power for a manageable amount of money.
In the past, telescopes for sale in the lower price segment were mainly delivered with a full tube. Recently, somewhat more expensive devices in a tubular design have also been offered.
photography
The use of webcams or special planetary cameras derived therefrom has made it possible - provided that suitable post-processing is required - to produce high-resolution images of brighter celestial objects, especially objects of the solar system , even with Dobsonian telescopes . Videos or series of images are recorded with individual exposure times in the range of fractions of a second and then superimposed to form a single image. Due to the short individual exposure times, the tracking of a Dobsonian mount, which is actually inadequate for astronomical photography, does not have a noticeable effect on the quality of the images. This recording technology also minimizes negative effects such as image noise from electronic cameras and interference from seeing .
Occasionally, Dobsonian telescopes are also used for long-term exposure , usually with the help of an equatorial platform or az / alt tracking in both axes, but this then requires a device for image field rotation.
Double telescope
A further development of the conventional Dobson telescopes, whether or Gitterrohr- Full tube construction, provide the so-called double telescopes Bino-Dobson or Dobson-Bino represents. The essential feature is that two identical optical systems are combined in Dobson-Bino. In contrast to the Dobsonian with monocular view, the Dobsonian binoculars look binocular, i.e. through two eyepieces. The most important goal is to design the construction in such a way that the individual impressions of the two optics can be combined into one image by manual adjustment. However, this leads to a significantly higher workload when building it yourself. This is not only rewarded by the higher overall performance of this system compared to a corresponding individual lens. The increased light output results from the doubled area, comparable to an increase in the diameter by the root of 2 or z. B. 2 × area (12 ") = area (12" × 1.4) = area (17 "). Some hobby astronomers see disadvantages in the fact that the view from the front with the back to the sky takes getting used to and that the body's own warmth also rises the telescope should have a negative impact on seeing.
Transportation
While Dobsonian telescopes in truss construction can be dismantled and transported relatively easily, conventional Dobsonian telescopes, which are usually equipped with a full tube, are difficult to move to the respective observation location from an opening of 12 "at the latest. These can usually no longer be transported in a normal vehicle; a station wagon is a basic requirement for transport here.
In addition to the telescope tube, the size of the rocker box also plays a role here.
literature
- David Kriege, Richard Berry: The Dobsonian Telescop. A practical manual for building large aperture telescopes . Willmann-Bell, Richmond VA 1997, ISBN 0-943396-55-7 (English, in this book the self-construction of Dobsonian telescopes with a large aperture is described).
