Eight-inch
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The eight-inch telescope is the most popular telescope among amateur astronomers . As a reflector telescope , for most astronomers it has the best price-performance ratio of the instruments on the market and is still easy to transport.
The name refers to the aperture (free diameter) of the main mirror (8 inches = 20.3 cm). Common designs are the Newton and Cassegrain telescopes . Even lens telescopes with 8-inch opening are called eight-inch model, but they are used less frequently and are at least twice as expensive.
The most common eight-inch models are the 8-inch f / 6 Newtons as Dobsonians . Cassegrain reflecting telescopes are also popular because they are very compact. However, these are usually much more expensive to buy than the cheap Dobsonian devices.
Construction methods of the 8-inch mirror telescopes
Compact telescopes type Cassegrain and Maksutov
An 8-inch reflector telescope (diameter of the primary or main mirror around 20 cm) has a focal length of mostly 2 meters in the compact Cassegrain design or the Schmidt-Cassegrain or Maksutov systems . However, it is shortened to a length of about 45 cm by the beam path folded by means of a secondary mirror . As a result, such instruments, which are manufactured by many companies, are still quite handy in size.
In all three types, the secondary mirror is relatively large, which reduces the contrast performance. The beam path leads through a central hole in the main mirror and is mostly deflected by a zenith eyepiece , which creates an image that is upright but reversed . Schmidt-Cassegrain and Maksutov telescopes have a finely ground correction plate that closes the tube at the front. It prevents the ingress of dust and moisture, but takes longer to adapt to the night air temperature .
The light intensity is sufficient for almost all celestial objects, and the resolution that is important for binary stars and planets is around 0.6 arc seconds . Of course, it does not often come into play because the air turbulence is usually two to three times as much. Typical eight-inch models of this compact design weigh around 6 kg as a tube. There is also a corresponding mount. The devices can still be transported relatively easily in a small car.
Inexpensive Newton and Dobsonian telescopes
8-inch Newtonian telescopes are usually built with an aperture ratio of f / 4 to f / 6 and in exceptional cases with f / 8. For 8-inch Dobsonian devices, an 8-inch f / 6 Newton is usually used. An f / 6 Newtonian shows less coma than the devices with f / 5 and f / 4. This means you can use cheaper eyepieces that are still relatively good at f / 6. The Dobsonian does not require an expensive mount and instead uses a so-called rocker box. This can either be bought cheaply or built by amateurs themselves. This makes the 8-inch f / 6 Dobsonian telescopes the cheapest 8-inch telescopes.
The rocker box is a support structure for the Newtonian telescope and is often made of wood. The Newton telescope can be adjusted by hand. 8-inch Dobsonians are offered from around 400 euros. The viewing height of the eyepiece varies depending on the elevation angle of the object to be observed. Objects close to the horizon can therefore be a little difficult to observe. A short tripod combined with a small stepladder can be helpful. For a few years now, tracking motors have also been available for around 100 €, but this contradicts the simple design.
Schiefspiegler and Brachyt
This design avoids the obstruction by the secondary mirror by directing the beam path obliquely out of the tube. It is often made by mirror grinders themselves. (More to follow)
Eight inch refractor
The eight-inch telescope is less common as a lens telescope (lens diameter also 20 cm), although it has a higher contrast performance. With the usual focal ratio of 1:10 to 1:15, it is over 2 meters long, at least two to three times as heavy as a mirror system and significantly more expensive. Therefore, the most common openings in the lens telescopes are 4 to 6 inches.
8-inch refractors can achieve the highest contrast performance with the eight-inch models with the same size Schiefspiegler, as neither have any obstruction. For the refractor, however, it is necessary that the refractor is color-pure, otherwise the defocused wavelengths reduce the contrast.
If the air is calm, the first diffraction ring can be faintly seen on medium-bright and bright stars at high magnification . The adjustment of the optical system is usually much more stable than with reflector telescopes.
Such a refractor is no longer transportable, but stationary on a column mount it offers many advantages for planet and double star observers.
Summary: balanced areas of application
Eight-inch reflector telescopes are the most common type of construction for amateur astronomers for a reason. All in all, they usually have the best price-performance ratio , because their resolution and light intensity are adapted to urban and near-urban observation conditions.
Most of the 10,000 or so "classic" double stars can be separated, and those closer than 1 "are mostly blurred by the turbulence in the air anyway . Visibility is satisfactory for nebulae and galaxies and can be improved with special filters . They are easy to transport and compact enough. Only for planetary observation , the refractors are a third more powerful, but also two to three times as expensive.
Larger telescopes (around 10 to 14 inches) are usually only really useful for amateur astronomers if there is a fixed location with very dark nights.
literature
- Detlev Block: Astronomy as a Hobby , 208 S. Bassermann, Munich 2006
- M. Löffler-Mang: Telescopes (especially 6-8 inch). Part III in "Optical sensor technology", p, 220ff, Springer-Verlag 2012
- Meade: Operating Instructions LX 90.Schmidt-Cassegrain telescope with Autostar , Meade-Europe 2008