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MAGNIFICATION F/Number The f/number of a telescope is the focal ratio. You find the focal ratio or f/number by dividing its focal length by its aperture. A telescope’s size is equal to its aperture. Aperture is the diameter of the mirror or objective lens. Focal length is simply the distance between the mirror or objective lens and where the image focuses. For example: If your telescope has a focal length of 1800mm and the aperture is 406mm, the focal ratio would be f/4. 1800 divided by 406 = 4   Magnification The magnification of a telescope is calculated by dividing the focal length of the eyepiece into the focal length of the telescope. For Example: If your telescope has a focal length of 1800mm and you‘re using a 25mm eyepiece, the magnification would be 72X. 1800 divided by 25 = 72 