Editing Optical telescope (section)

===Observable world===
The observable world is 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.