Is the focal ratio of astronomical telescope as small as possible or as large as possible?

The large focal ratio is suitable for visual observation, because the large focal length is longer than the focal length, the objective lens images larger, the low-power eyepiece can have higher magnification, and the low-power eyepiece has a longer exit pupil distance, which is more comfortable to watch, especially for people wearing glasses. Small focal ratio is suitable for celestial photography, because the small focal ratio is shorter than the focal length, the imaging is brighter, the exposure time of the camera is shortened, and the film yield is improved.