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.
Finderscope
A finderscope is an aiming device used in astronomy, typically a small auxiliary telescope mounted on the main astronomical telescope along the same line of sight. The finderscope usually has a smaller magnification than the main telescope, providing a much larger field of view, useful for manually aiming (also called "slewing") a telescope and locating a desired astronomical object. Some finderscopes have crosshairs to aid in accurately pointing the telescope system at a target.