What is the direct effect of the astronomical telescope's excessive focal ratio? For example, the focal length ratio is 23 ...

Focal length: objective aperture = focal length ratio; The focal ratio above 10 is the long focal ratio, and the focal ratio below 10 is the short focal ratio.

The influence of focal ratio on imaging is manifested in two aspects:

The first is system amplification. Under a certain aperture, a large focal ratio means a short focal length, and the system magnification is equal to the focal length of the objective divided by the focal length of the eyepiece. Short focal length eyepiece has short exit pupil, small eyepiece lens (large lens is expensive) and poor imaging quality (relatively long focal length and medium focal length). The shorter the focal length, the more difficult it is to buy. The short focal length ratio is suitable for the observation and photography of deep space objects, such as nebulae, star clusters and galaxies. Unlike other stars or planets, such celestial bodies cannot be observed with naked eyes (except for some smaller celestial bodies such as apparent stars, such as M42 and M45), which need SLR cameras and astronomical telescopes as lenses and can only be displayed after long exposure.

The second is image quality and image field size. The difficulty of designing and manufacturing an optical system is directly proportional to its focal length ratio. At the same cost, adopting large focal ratio can only be at the expense of image quality and image field size. In addition, the larger the focal length ratio, the smaller the available image field. This effect will not have much influence under the condition of visual inspection and small target angular diameter; However, in the case of large-angle celestial bodies and photographic observation, the edge image quality will not be available.