Optical telescope
Many types of telescope fold or divert the optical path with secondary or tertiary mirrors. These may be integral part of the optical design (Newtonian telescope, Cassegrain reflector or similar types), or may simply be used to place the eyepiece or detector at a more convenient position. Telescope designs may also use specially designed additional lenses or mirrors to improve image quality over a larger field of view. 
Design specifications relate to the characteristics of the telescope and how it performs optically. Several properties of the specifications may change with the equipment or accessories used with the telescope; such as Barlow lenses, star diagonals and eyepieces. These interchangeable accessories don't alter the specifications of the telescope, however they alter the way the telescopes properties function, typically magnification, apparent field of view and FOV.
Telescope
An optical telescope gathers and [[Focus (optics)|focuses]] light mainly from the visible part of the [[electromagnetic spectrum]] (although some work in the [[infrared]] and [[ultraviolet]]). Optical telescopes increase the apparent [[angular size]] of distant objects as well as their apparent [[brightness]]. In order for the image to be observed, photographed, studied, and sent to a computer, telescopes work by employing one or more curved optical elements, usually made from [[glass]] [[Lens (optics)|lenses]] and/or [[mirror]]s, to gather light and other electromagnetic radiation to bring that light or radiation to a focal point. Optical telescopes are used for [[astronomy]] and in many non-astronomical instruments, including: "[[theodolite]]s" (including "transits"), "[[spotting scope]]s", "[[monocular]]s", "[[binoculars]]," "[[camera lens]]es", and "spyglasses". There are three main optical types: