Optical telescope
"Observable world" describes what can be seen using a telescope, when viewing an object or range the observer may use many different techniques. Understanding what can be viewed and how to view it depends on the field of view. Viewing an object at a size that fits entirely in the field of view is measured using the two telescope properties—focal length and aperture, with the inclusion of an ocular eyepiece with suitable focal length (or diameter). Comparing the observable world and the angular diameter of an object shows how much of the object we see. However, the relationship with the optical system may not result in high surface brightness. Celestial objects are often dim because of their vast distance, and detail may be limited by diffraction or unsuitable optical properties.
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.