Astronomical Telescopes (Figure 1) are also known as Kepler telescopes called because their construction to the famous astronomer Johannes Kepler (1571-1630) back.
The structure and the beam path in such an astronomical telescope shown in Figure 2. An astronomical telescope consists of a the object (stars, planets, comets) facing lens and a eyepiece through which the image of the object is viewed. In a Keplerian telescope made objective and eyepiece of lens systems, which act as a total collection lenses.
Through the lens is formed from a distant object in image one as intermediate image denoted. It is a scaled-down, inverted, seitenvertauschtes and real intermediate image.
The eyepiece is arranged so that this intermediate image is this lens system within the simple focal length. The eyepiece thus acts like a magnifying glass and generated by the intermediate image a magnified, upright, true and virtual image.
Thus one sees in an astronomical (Kepler’s law) telescope total a scaled, inverted, seitenvertauschtes and virtual image of the object.
Looking at the center beam incident from the subject through the lens, so it falls under the angle in the telescope and leaves it at the angle , This is illustrated in Figure 3. Since the image length of the lens for an infinitely distant object, the focal lengththe lens and the object distance of the eyepiece at maximum magnification of magnifier is obtained.
Field of view at an astronomical telescope
Considering the retinal images at a visual angle or. formed (Figure 4), then the size is just determined by the tangent of the angle.
Thus is the magnification of the retinal image by an astronomical telescope
Resolving power of the telescope
When the telescope, the resolving power is determined by the lens diameter as the eye. It is the greater, the larger the front lens diameter. Therefore, to build telescopes with the largest possible lens diameters. Nevertheless, the theoretical maximum resolving power of the largest telescopes can not be achieved on Earth by the density fluctuations in the atmosphere. Therefore, one has now with the Hubble Space Telescope brought a telescope outside the earth in position, its resolution is only limited by the diameter of the lens.
The Galilean telescope
When Galilean telescope – it is also called Dutch telescope – the magnifying glass is replaced by a diverging lens (Figure 5). Your eye facing focal plane coincides with the focal plane of the lens. The focused by the lens light is refracted by the negative lens to a parallel beam of light and leaves the eyepiece at a greater angle. This gives a larger retinal image. The advantage of this telescope is the smaller overall length and an upright laterally correct image. Therefore, it was used mainly by sailors for observation on Earth.
The reflecting telescope by NEWTON
Since the objects to be observed are often very faint in astronomy, one needs large lens apertures. These are used instead of a lens a concave mirror as the objective (Figure 6). The intermediate image is located in front of the concave mirror and is reflected by a mirror to the side, where it can be viewed with a magnifying glass. This design significantly larger telescopes are possible because with lens telescopes, the weight and the dispersion of the huge lens would cause major problems. The construction of such a reflecting telescope goes back to ISAAC NEWTON (1643-1727). It is therefore also known as Newtonian reflecting telescope called.