The third period: the classification of astronomical telescopes said

The radio telescope, infrared telescope, optical telescope (visible telescope), ultraviolet telescope, X-ray telescope, and γ-ray telescope.

Here it is necessary to introduce the classification of light by wavelength first.

Because of the space problem, we will mainly introduce the common optical telescopes and radio telescopes below.

Optical telescopes

Optical astronomical telescopes mainly observe the visible wavelength, specifically the wavelength of light in 380nm (nanometers) - 750nm, which is visible to our naked eye in red, orange, yellow, green, blue, and purple. In fact, there are various wavelengths of light in our living environment, only some of which are not visible to our naked eyes.

Optical telescopes can be divided according to the design of the optical path: refractive telescope (Galileo type, Kepler type), reflective telescope (Newtonian, Cassegrain type), and refractive telescope (Schmidt-Cassegrain, Maksutov-Cassegrain).

Not false, let's introduce them one by one!

Refractor telescopes

The refracting telescope is the oldest type, that is, the original telescope. The Galileo telescope we introduced in the first issue is a typical refractor telescope. The Galilean telescope has a convex lens as the objective and a concave lens as the eyepiece, but the aberration and chromatic aberration are relatively large.

Later Kepler improved it, the eyepiece also changed to a convex lens, increasing the field of view, optical performance is better, and the imaging effect is also better, but the disadvantage is that the image becomes inverted.

Refractive telescopes have two problems that limit their development, resulting in the optical path structure must be improved.

The existence of chromatic aberration in the refracted optical path.

Refractive telescope objective lenses are limited by cost and manufacturing process, and cannot be built for large aperture telescopes.

Refractor telescopes

The reflecting telescope mainly uses a parabolic reflector as the primary mirror, and the telescope focus is located in front of the primary mirror. Newton decided to use a reflector instead of a lens as the primary mirror and used a flat mirror to direct the light from the side of the mirror tube, inventing the Newtonian reflecting telescope.

Cassegrain modified the light path of the Newtonian reflecting telescope by replacing the plane mirror in the barrel with a curved mirror and directing the light path out of the barrel from behind the primary mirror, thus inventing the Cassegrain-type reflecting telescope. The Cassegrain telescope increased the focal length of the primary mirror and improved the magnification of the telescope. The most important feature of the reflector compared to the refractor is the need for coating.

The three main advantages of reflecting telescopes have made them the main type of large-aperture telescopes used in modern astronomical research.

The fact that light does not need to be refracted through the medium, effectively avoiding chromatic aberrations.

Reflecting primary mirrors can be spliced to create larger apertures.

The reflecting mirror can be adjusted with an active adjustment system to cope with mirror distortion.

Refractive Telescope

A refractor telescope is a telescope built with a correcting mirror in front of a Cassegrain reflecting telescope. Refractor telescopes can be divided into Schmidt-Cassegrain type (Schicka/S-C) and Maksutov-Cassegrain type (Maka/M-C), the main difference is the correcting mirror. Refractive telescopes combine the advantages of refractors and mirrors with low chromatic aberration and bright image presentation. The Schick type uses an aspheric lens as a correction mirror, with a larger field of view, strong light-gathering ability, and good imaging effect. The optical path through the front end is ground into a close to parallel aspheric correction lens into the mirror tube, and then by the spherical mirror located at the rear end of the main convergence to the small reflector reflected output, correction mirror can be well corrected and eliminate the main mirror caused by spherical aberration, thus making the presentation of the image more clear. For example, the Guo Shoujing Telescope, also known as LAMOST (Large SkyArea Multi-Object Fiber Spectroscopy Telescope), the largest survey telescope in China, located at Xinglong Observatory, is using this system. The main body of the telescope was developed by the Nanjing Institute of Astronomy, Chinese Academy of Sciences, and it has achieved r breakthrough in the technology of large-aperture optical telescopes in China.

The Macca type uses a curved lunar correction mirror with a relatively small field of view and weak light-gathering capability, but with a longer focal length and an optical system with a closed barrel and a full spherical mirror. The 120 telescope produced by Nanjing Astronomical Instruments Co., Ltd (formerly Nanjing Astronomical Instruments Factory) of the Chinese Academy of Sciences uses this system.

Radio Telescope

Radio telescopes originated in the 1930s, which also promoted the rapid development of radio technology. Radio telescopes mainly observe the radio waves emitted by celestial bodies. The weak shielding effect of the atmosphere on radio waves makes radio telescopes largely unaffected by weather. In addition, radio waves on the main mirror of the radio telescope material requirements are not high, making the radio telescope aperture compared to the optical telescope easier to do larger. China's 500-meter FAST is currently the world's largest single radio telescope in terms of aperture.

Although radio telescopes are less affected by weather when observing, they are subject to interference from signals in different radio bands, such as human activities. Two important indicators of radio telescopes are sensitivity and resolution. This is the main reason why FAST was built in the karst terrain of Guizhou in a large cesspit. The 110-meter fully movable radio telescope QTT at the Xinjiang Astronomical Observatory of the Chinese Academy of Sciences is also under construction.