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.
Robert E. Cox
It is generally conceded that anyone who mounts a mirror or lens as a telescope for viewing celestial objects has become a telescope "nut" regardless of how the optics were obtained. There is only one important requirement - that the optics be of first quality, capable of giving satisfactory views of the moon, the sun, double stars, cluster, nebulae, and the planets.