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.
Amateur Telescope Making
Telescope making is a scientific hobby and it appeals doubtless because it exacts intelligence; requires patience and sometimes dogged persistence in order to whip the knotty but fascinating problems which arise; demands hard work -- is not dead easy; and compels the exercise of a fair amount of handiness -- enough to exclude the born bungler but no more than is possessed by the average man who can "tinker" his car or the household plumbing...The hobby also appeals because the worker derives something of a thrill while shaping the refined curve of the glass as he realizes that, with scarecly any special tools...he is able to work to within almost a millionth of an inch of absolute perfection.