What should I pay attention to when buying astronomical telescopes? What parameters are important?

There are many parameters, all of which are general and can be subdivided. You can add questions if you are interested. The first important parameter: aperture largely determines the resolution and limit magnitude. The entry is at least 80 mm, and less than 80 mm is included in the toy list. The second important parameter: focal ratio, focal ratio and lens type affect the brightness of the field of view, as well as chromatic aberration and coma. The corresponding focal length determines the magnification of direct focus photography. (The magnification is equal to the focal length of the objective lens divided by the focal length of the eyepiece, so you don't need to ask the merchant) Additional important parameters: optical path type: refractive, reflective or catadioptric lens type: refractive: confocal, fluorite, Ed -APO coating type: refractive: single-layer coating, full-coating bracket type: equatorial telescope, theodolite, photographic bracket (equatorial telescope and theodolite are various, so I can't say it clearly in a few words, so I will study it myself slowly. Focus types: gear focus, toothless focus and two-speed focus. Good focusing is very helpful for photography, so it doesn't matter if you look at it visually. ) Eyepiece types: The most common ones are K (low-level toy), PL (ordinary telescope) and OR (advanced telescope). This is divided according to the optical path system. In addition, the eyepiece also has fluorite and ordinary, with different focal lengths. Interface size: focusing base and eyepiece 0.9 inch (pure toy), 1.25 inch (ordinary), 2 inch (high-end) brand type: brand is also very important, and can be bought in China: Gao Qiao; Virtue; Jinghua, Sirius, Guanbo; Star Village, East Star; Phoenix; I won't introduce He Jie and others one by one. Go and see for yourself. Note: Look at the lens to see if there is mildew and coating damage. Look at the instructions again to see if the accessories are consistent with those in the instructions. Finally, look at the optical quality of the mirror, including resolution and contrast. The quality of a telescope is not only related to the objective lens, but also to the eyepiece, prism, trolley and all accessories involved in the optical path. Improving the quality of accessories is helpful to improve the overall quality of the telescope. Pay attention to the supplementary terms: don't buy it in the mall, and you won't lose it if you die. Don't ask questions like "how many times" and "how far can you see" as soon as you come up. Businessmen like to hammer amateurs. It's not a bad thing to pretend to understand when you buy something, but you should know something before.