galileo telescope
The original telescope designed by Galileo was called galileo telescope. He used a convex lens as the objective lens and a concave lens as the eyepiece. Galileo telescope's portrait is upright, but the vision is limited, with spherical and chromatic aberration, and the eye relief is not good.
Kepler telescope
Kepler telescope was invented by Kepler in 16 1 1, and he improved Galileo's design. He replaced it with a convex lens instead of the concave lens used by Galileo. The advantage of this arrangement is that the light emitted by the eyepiece is convergent, which can have a larger field of view and a larger eye distance, but the image seen is inverted. This design can achieve higher magnification. However, it requires a high focal length ratio to overcome the distortion caused only by the objective lens. (johannes hevelius built a refracting mirror with a focal length of 45 meters. This design is also used in the focal plane of a microscope (for measuring the angular distance between two observed objects).
Achromatic refractor
Achromatic refractor was invented by British lawyer Chester Moore Hall in 1733, although the patent right was given to another independent inventor, John Dollond. This design uses two pieces of glass ("Crown Glass" and "Flint Glass" with different dispersion) as the objective lens, which reduces the chromatic aberration and spherical aberration. Each side of two pieces of glass should be polished and then bonded together.
Highly achromatic refraction
The high achromatic refractor uses special materials, especially low dispersion materials to make the objective lens. Its design can make three different colors (usually red, green and blue) converge on the same focal plane, and the residual (secondary spectrum) of colors is one order of magnitude smaller than that of achromatic lens. The primary mirror of this telescope is fluorite or ultra-low dispersion (ED) glass lens. It produces very clear and achromatic images. This kind of telescope is a very valuable product in the amateur astronomical telescope market. The aperture diameter of achromatic refractor can reach 553 mm, but most of them are still between 80 ~152 mm.
Edit the relevant content of this paragraph.
The objective lens of a refractive telescope is generally composed of two glass lenses with different refractive indexes, so as to reduce the chromatic aberration and make the red and blue images converge on the same focus. This lens is called achromatic lens. Strictly speaking, there is still a very lavender halo around the image of this lens. In order to reduce the spherical aberration of the lens, coma and astigmatism can generally increase the focal ratio, so the ratio of aperture to focal length (focal ratio) of a general refractive telescope is at least between f 10 and f 16. Advanced lenses consist of three glass lenses with different refractive indices, or glass with low dispersion (ed) or even fluorite crystals. Can eliminate the color difference of red, green and blue. These lenses are called apochromatic lens. Their aperture focal ratio can reach f5. It makes the telescope shorter, lighter and easier to use, but it is very expensive. Because the refractive telescope tube can be sealed, it is more convenient to maintain and more suitable for moving to the wild. At the same time, it is not affected by the airflow in the lens barrel. Because the lens consists of at least two pieces of glass, the cost (grinding four mirrors) is more expensive than that of a reflective telescope with the same caliber. Most of the small astronomical telescopes sold in the market are refractive telescopes.