Optical telescope
Many types of telescope fold or divert the optical path with secondary or tertiary mirrors. These may be integral part of the optical design (Newtonian telescope, Cassegrain reflector or similar types), or may simply be used to place the eyepiece or detector at a more convenient position. Telescope designs may also use specially designed additional lenses or mirrors to improve image quality over a larger field of view. 
Design specifications relate to the characteristics of the telescope and how it performs optically. Several properties of the specifications may change with the equipment or accessories used with the telescope; such as Barlow lenses, star diagonals and eyepieces. These interchangeable accessories don't alter the specifications of the telescope, however they alter the way the telescopes properties function, typically magnification, apparent field of view and FOV.
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.