The F number on a telescope refers to the focal ratio. This is the ratio between the aperture of the telescope and its focal length. Where people get confused is the use of inches in relationship to the focal length - to get the ratio, we need to divide the focal length by the aperture, both measure in millimetres. So if you have a focal length of 1200mm, and an aperture of 100mm, you'd end up with a focal ratio of 12 (represented as f/12).