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
Radio telescopes are [[Directional antenna|directional]] [[radio antennas]] used for [[radio astronomy]]. The dishes are sometimes constructed of a conductive wire mesh whose openings are smaller than the [[wavelength]] being observed. Multi-element [[Radio telescope]]s are constructed from pairs or larger groups of these dishes to synthesize large 'virtual' apertures that are similar in size to the separation between the telescopes; this process is known as [[aperture synthesis]]. As of 2005, the current record array size is many times the width of the [[Earth]]—utilizing space-based [[Very Long Baseline Interferometry]] (VLBI) telescopes such as the [[Japan]]ese [[HALCA]] (Highly Advanced Laboratory for Communications and Astronomy) VSOP (VLBI Space Observatory Program) satellite. Aperture synthesis is now also being applied to optical telescopes using [[Optical interferometry#Astronomical optical interferometry|optical interferometers]] (arrays of optical telescopes) and [[aperture masking interferometry]] at single reflecting telescopes. Radio telescopes are also used to collect [[microwave radiation]], which is used to collect radiation when any visible light is obstructed or faint, such as from [[quasar]]s. Some radio telescopes are used by programs such as [[Search for Extraterrestrial Intelligence|SETI]] and the [[Arecibo Observatory]] to search for extraterrestrial life.