The telescope is a kind of visual optical instrument used for observing distant objects. It can enlarge the small Angle of distant objects by a certain magnification so that it has a large Angle in the image space so that the objects that can not be seen or distinguished by the naked eye become clearly discernible.  Therefore, the telescope is an indispensable tool in astronomy and ground observation.  

A simple working telescope requires only a pair of lenses mounted in a tube.  The front lens, called the objective lens, is used to focus the image;  The rear lens, called the eyepiece, magnifies the image.  While it may look like a crude piece of equipment, a simple telescope is a good illustration of the basic workings of more powerful astronomical instruments.  

A telescope is an optical system that enables an incident parallel beam to remain parallel through an objective lens and eyepiece.  According to the basics of telescopes, there are three kinds.  

1. Bottoms up Beer system  

Refracting telescopes fall into two types: Galilean telescopes with concave lenses as eyepieces;  By convex lens eyepiece called Kepler telescope.  A refractor telescope is a system that uses a lens as an objective to focus light.  The world’s first astronomical telescope is Galileo’s refraction telescope, which uses a convex lens as its objective lens.  Due to the different refractive index of glass to different color light, the refracting telescope will produce serious chromatic aberration, therefore, the later refracting telescope mostly uses a compound lens as the objective lens, that is, composed of more than two lenses, to eliminate chromatic aberration. 

 According to the different light paths, the refractor telescope is divided into the Galileo telescope and Kepler telescope two kinds.  Usually, refraction telescope relative aperture is small, that is, the focal length is long, the film scale is large, thus the resolution is high, more suitable for astrometry work.  

Due to the influence of residual chromatic aberration and other aberrations, the relative aperture of the two-lens objective is small, generally, 1/15-1/20, rarely greater than 1/7, and the available field of view is not large.  A double-lens objective lens with a diameter less than 8 cm can glue two lenses together and is called a double-glued objective lens.

A certain gap is not glued and is called a double-separated objective lens.  In order to increase the relative aperture and field of view, a multi-lens objective set can be used.  Refraction telescope has better imaging quality than reflection telescope, larger field of view, easy to use, easy to maintain, small and medium-sized astronomical telescope and many special instruments mostly use refraction system, but the manufacture of large refraction telescope is much more difficult than reflection telescope.  

2. Reflecting telescopes  

A telescope with a concave mirror as an objective.  It can be divided into several types: Newtonian telescope, Cassegrain telescope, Gregory telescope, and folded axis telescope.  The main advantage of a reflecting telescope is the absence of chromatic aberration and the elimination of spherical aberration when the objective is parabolic.  However, in order to reduce the influence of other aberrations, the available field of view is small.  

The materials for making mirrors require only a low coefficient of expansion, low stress, and ease of grinding.  Polished mirrors are generally coated with an aluminum film, which has a reflectivity greater than 80% in the 2000-9000 angstrom band. Therefore, in addition to the optical band, reflecting telescopes are also suitable for studying near-infrared and near-ultraviolet bands.  

The relative aperture of the reflecting telescope can be made larger, the relative aperture of the primary focus telescope is about 1/5-1/2.5 or larger, and except for the Newtonian telescope, the length of the mirror tube is much shorter than the focal length of the system, plus the primary mirror has only one surface to work on, which greatly reduces the cost and manufacturing difficulties.  Therefore, all-optical telescopes with diameters larger than 1.34 meters are reflection telescopes.  

The primary focus system (or Newtonian system), the Cassegrain system, and the fold-axis system can be obtained in a larger aperture reflecting telescope by changing different secondary mirrors.  Thus, a telescope can acquire several different relative diameters and fields of view.  Reflecting telescopes are mainly used for astrophysical work.  

3. Refraction telescope  

A telescope consists of a combination of refracting and reflecting elements.  Including the Schmidt telescope and Marksutov telescope and their derivatives, such as the super Schmidt telescope, Baker-Noon camera, etc.  In a refraction telescope, the image is formed by a mirror, which is used to correct aberrations. 

 It is characterized by a large relative aperture (even greater than 1), strong light power, a wide field of view, and excellent image quality.  Suitable for sky survey photography and observation of nebulae, comets, meteors, and other celestial bodies.  A small visual telescope with a refraction Cassegrain system can have a very short tube.