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.
Petoscope
A Petoscope is an optoelectronic device for detecting small, distant objects such as flying aircraft. The design, as described in 1936, consisted of an instrument with two parallel light paths. In each path was a collimating objective lens, a screen marked with many small, alternating opaque and transparent squares in a chequerboard pattern, and a second concentrating lens focused on a photocell. The two screens were inverted with respect to each other. This caused a small object in the instrument's field of view to produce differing signals in the two photocells, while a large object affected both light paths equally. The difference between the two signals was amplified and used to raise an alarm. At the beginning of World War II, the device was adapted for use in proximity fuses for bombs.