Binoculars, also known as "binoculars". A telescope consisting of two monoculars side by side.

The distance between the two eyepieces can be adjusted so that both eyes can observe at the same time, thus obtaining a three-dimensional sense. If two Galilean telescopes are used, they are called "opera glasses". It has a shorter barrel and a smaller field of view and magnification. If two Kepler telescopes are used, the mirrors are relatively long and inconvenient to carry; Therefore, a pair of total reflection prisms are often installed between the objective lens and the eyepiece, so that the incident light is fully reflected in the tube for many times to reduce the length of the tube, and the inverted image formed by the objective lens can be reversed to become a positive.