The telescope is the most commonly used optical instrument. It can be used alone or combined with other instruments for astronomy, military and hunting. Astronomical telescopes are used to observe the structure and state of celestial bodies. As a sight, it is assembled with a firearm to aim at a target. The telescope combines with a thermal imager to observe targets that are not easy to find. When hunting, use the telescope to observe the movement of the prey.
The optical system of the telescope is composed of an objective lens and an ocular lens. After the incident light beam passes through the ocular and the objective lens, it still maintains an optical system that emits in parallel. In addition to using double-convex lenses and planoconvex lenses, achromatic lenses and air-spaced doublets are also used in telescopes. The cemented lens corrects spherical aberration, coma and chromatic aberration. However, the correction effect is limited. It is generally used in a system with a short focal length and a small relative aperture. The positive and negative lenses of air-spaced doublets are separated by an air gap, which reduces the spherical aberration of the intermediate zone and doesn’t correct the chromatic spherical aberration, which is suitable for optical systems with enlarged diameters. The objective lens generally uses a single lens, and when the distortion does not need to be changed, the single lens is plano convex lens.
The achromatic lenses are affected by the layer force and degumming. The air-spaced doublets are not subject to this limitation. The loss of light energy is larger than that of cemented lenses. It is difficult to assemble the center of the dual-separation objective lens. It is easy to lose coaxiality during use.
The Kepler telescope is composed of two convex lenses. The imaging is inverted. An erecting system needs to be added. The erecting system includes a prism erecting system and a lens erecting system. The prism erecting system uses a single roof prism or a combination of ordinary prisms to rotate the image and divert the optical axis. The lens erecting system adds a lens behind the real image of the objective lens to make the reversed image inverted once again to become a positive image. At present, military telescopes, small astronomical telescopes and other professional telescopes all adopt the Kepler structure.
The Galileo telescope is a telescope with a convergent lens as the objective lens and a divergent lens as the ocular. The advantage is that the structure is compact, lighter, and less light energy loss. The object is in an upright image, without the need for an erecting system.