A monocular is a visual optical instrument used to observe distant objects. It can enlarge a small angle of a distant object at a certain magnification, so that it has a large opening angle in the image space, making it impossible to use the naked eye. Objects that are seen or distinguished become clear and legible. Therefore, monoculars are an indispensable tool in astronomical and ground observations. It is an optical system that keeps the incident parallel beams still in parallel through the objective lens and the eyepiece.