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There are two types: Galileo telescope with concave lens as eyepiece; Kepler telescope with convex lens as eyepiece. The basic principle of Kepler telescopes is that the light from a distance enters the convex lens of the objective lens, and the real image becomes inverted and reduced for the first time, which is equivalent to a camera; then this real image enters the convex lens of the eyepiece, and the virtual image becomes upright and enlarged for the second time. , Which is equivalent to a magnifying glass.
Because the chromatic aberration and spherical aberration of the single-lens objective lens are quite serious, modern refracting telescopes often use two or more lens groups as crop mirrors. Among them, the two-lens objective lens (ordinary achromatic telescope) is the most popular. It consists of a convex lens made of a crown glass and a concave lens made of flint glass that are very close to each other. The positional chromatic aberration is completely eliminated for two specific wavelengths, and the positional chromatic aberration of the remaining wavelengths can be reduced accordingly.
When certain design conditions are met, part of spherical aberration and coma can be eliminated. Due to the influence of residual chromatic aberration and other aberrations, the relative aperture of the double-lens objective lens is small, generally 1/15-1/20, rarely greater than 1/7, and the usable field of view is not large. A double-lens objective lens with an aperture of less than 8 cm can cement two lenses together, called a double-coated lens, and a double-separated objective lens with a certain gap that is not glued. In order to increase the relative aperture and field of view, a multi-lens objective lens group can be used. For the Galileo telescope, the structure is very simple and the light energy loss is small. The lens barrel is short and very light. And it is upright, but the field of view is small with small multiples, and it is generally used for drama mirrors and toy telescopes. For Kepler telescopes, it is necessary to add a prism group or lens group behind the objective lens to rotate the image so that the eye can observe a positive image. The general refracting telescope adopts the Kepler structure. Because the imaging quality of refracting telescopes is better than reflecting telescopes under the same caliber, the field of view is large, it is convenient to use, and easy to maintain. Small and medium astronomical telescopes and many special instruments mostly use refracting systems, but large-scale refracting telescopes are much more difficult to manufacture than reflecting telescopes. Because it is very difficult to smelt high-quality large-aperture lenses, and there is a problem of glass absorption of light, and the main mirror lens will be deformed due to gravity, resulting in poor optical quality, so large-aperture telescopes are all reflective
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