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.
Asteroid impact prediction
The Near Earth Object Survery TELescope (NEOSTEL) is an ESA funded project, starting with an initial prototype currently under construction. The telescope is of a new "fly-eye" design that combines a single reflector with multiple sets of optics and CCDs, giving a very wide field of view (around 45 square degrees). When complete it will have the widest field of view of any telescope and will be able to survey the majority of the visible sky in a single night. If the initial prototype is successful, three more telescopes are planned for installation around the globe. Because of the novel design, the size of the primary mirror is not directly comparable to more conventional telescopes, but is equivalent to a conventional 1 metre telescope.