With the help of two refractors, one from Televue and another from Celestron, this video de-mystifies the “f-numbers”, focal ratio and focal length.
The focal length is the distance it takes the lens or mirror to focus star light. The focal ratio is simply the focal length divided by the diameter of the objective lens or mirror. Each number is written on each telescope.
Focal length governs the length of the tube and also the magnification you get with each eyepiece. The magnification is simply the focal length of the telescope divided by the focal length of the eyepiece
You’ll learn why short focal ratio telescopes (less than F/7 or so) aren’t the best choice for high magnification. Though they do give you wonderful wide-field images of the stars and daytime scenes.