How many lenses does a telescope consist of?

Common telescopes can be simply divided into galileo telescope, Kepler and Newton telescopes. The telescope invented by Galileo plays an important role in the history of human understanding of nature. It consists of a concave lens (eyepiece) and a convex lens (objective lens). Its advantage is simple structure, which can directly form an upright image. However, since the invention of Kepler telescope, this structure has not been adopted by professional telescopes, but mostly by toy telescopes, so it is also called drama-watching mirror.

Kepler telescope: The principle consists of two convex lenses. Because there is a real image between the two, the reticle is easy to install and has excellent performance, so at present, military telescopes, small astronomical telescopes and other professional telescopes all adopt this structure. But the imaging of this structure is inverted, so an upright system should be added in the middle.

First, the refracting telescope

A telescope with a lens cutting mirror. There are two kinds: galileo telescope with concave lens as eyepiece; Kepler telescope with convex lens as eyepiece. Because the chromatic aberration and spherical aberration of Dan Toujing objective lens are quite serious, modern refractive telescopes generally use two or more lens groups. Among them, the double-lens objective lens is the most widely used. It consists of a convex lens made of crown glass and a concave lens made of flint glass, which are very close. It can completely eliminate the position chromatic aberration of two specific wavelengths and weaken the position chromatic aberration of other wavelengths. When certain design conditions are met, spherical aberration and coma can also be eliminated. Due to the influence of aberration such as residual chromatic aberration, the relative aperture of the dual-lens objective is relatively small, generally115-1/20, and rarely larger than 1/7, so the available field of view is not large. A double-lens objective with a diameter less than 8 cm can glue two lenses together, which is called a double-glued objective, and a double-separated objective with a certain gap is called a double-glued objective. In order to increase the relative aperture and field of view, a multi-lens objective group can be used. The imaging quality of refractive telescope is better than that of reflective telescope, with large field of view, convenient use and easy maintenance. Refraction systems are mostly used in small and medium-sized astronomical telescopes and many special instruments, but the manufacturing of large-scale refracting telescopes is much more difficult than that in reflecting telescope.

Second, reflecting telescope.

Telescope with concave reflector and cutting mirror. It can be divided into Newton telescope, seglin telescope, Gregory telescope and folding axis telescope. The main advantage of reflective telescope is that there is no chromatic aberration. When the objective lens is a paraboloid, spherical aberration can be eliminated. However, in order to reduce the influence of other aberrations, the available field of view is smaller. The material for making the mirror surface only requires small expansion coefficient, small stress and easy grinding. Generally, polishing mirrors are coated with aluminum film, and the reflectivity of aluminum film is more than 80% in the range of 2000-9000 angstroms, so reflective telescopes are suitable for studying near infrared and near ultraviolet bands in addition to optical bands. The relative aperture of reflecting telescope can be made larger. The relative aperture of the main focal reflecting telescope is about 1/5- 1/2.5, or even larger. Except for Newton telescope, the length of the lens barrel is much shorter than the focal length of the system. In addition, only one surface of the primary mirror needs to be machined, which greatly reduces the cost and manufacturing difficulty. So now the aperture is greater than 1.34 meters. For the reflective telescope with larger aperture, prime focusing system (or Newton system), Casselin system and folding axis system can be obtained by replacing different secondary mirrors. In this way, the telescope can obtain several different relative apertures and fields of view. Reflecting telescope is mainly used in astrophysics.

Third, the folding reflecting telescope.

A telescope consisting of refractive and reflective elements. Including Schmidt telescope and Maksutov telescope and their derivatives, such as super Schmidt telescope, Baker-Noon camera and so on. In catadioptric reflecting telescope, the image is imaged by a mirror, and the refractive mirror is used to correct the aberration. Its characteristics are relatively large aperture (even greater than 1), strong light, wide field of vision and excellent imaging quality. Suitable for sky survey photography and observation of nebulae, comets, meteors and other celestial bodies. If catadioptric card seglin system is used in small vision telescope, the lens barrel can be very short.