It's the same number used by photographers, and refers simply to the focal length divided by the aperture of the telescope. They also require very long exposure times for photography. Mirror Images Don't be surprised to find an upside-down or backwards view in many telescopes. Newtonians usually are upside down or sideways, depending on the viewing position. Finder scopes will usually have an inverted or reversed image as well.
Optional image erecting prisms are available for most telescopes.
They fit between the eyepiece and eyepiece holder, correcting the view and allowing straight-through viewing. The spotting scope refractor will always give you a normally oriented image, without need of an erecting prism. Many telescopes have a miniature telescope attached to the side. A finder scope is aligned perfectly with the main instrument, and has a very low power and wide field of view, so you can easily find an object and center it where it can be seen in the main eyepiece. Standardized eyepieces usually cannot be used on these scopes.