Does the telescope look upside down?

A telescope composed of two convex lenses is inverted. A telescope consisting of a convex lens and a concave lens is an upright image. Ordinary telescopes use upright imaging, while astronomical telescopes use inverted imaging. The upside down is greater than the vertical multiple.

In fact, there is a prism inside the telescope. The principle of a prism is to turn an upside-down image into a positive image.

There are a few optical instruments without prisms, but they are also positive images, such as sights, which are a bit special. For example, some sights don't have the mechanism of becoming a positive image, but their internal optical design is very complicated, and the image is transferred through a lens.

The toy telescope is just as positive without a lens, because the Galileo structure originally adopted by the toy telescope does not need a prism, but this structure has other defects, otherwise it will not be eliminated.

Extended data:

Classification of telescopes:

Telescope is an indispensable tool in astronomy and ground observation. It is an optical system that makes the incident parallel beams pass through the objective lens and eyepiece and still exit in parallel. According to the principle of telescope, it is generally divided into three types.

Refraction telescope is a kind of telescope with lens cutting mirror. There are two kinds: galileo telescope with concave lens as eyepiece; Kepler telescope with convex lens as eyepiece. Because the chromatic aberration and spherical aberration of Dan Toujing objective lens are quite serious, modern refractive telescopes generally use two or more lens groups. Among them, the double-lens objective lens is the most widely used.

It consists of a convex lens made of crown glass and a concave lens made of flint glass. The two lenses are very close together. It can completely eliminate the position chromatic aberration of two specific wavelengths, and correspondingly weaken the position chromatic aberration of other wavelengths. When certain design conditions are met, spherical aberration and coma can be eliminated.

Due to the influence of aberration such as residual chromatic aberration, the relative aperture of the dual-lens objective is relatively small, generally115-1/20, and rarely larger than 1/7, so the available field of view is not large. A double-lens objective with a diameter less than 8 cm can glue two lenses together, which is called a double-glued objective, and a double-separated objective with a certain gap is called a double-glued objective.

In order to increase the relative aperture and field of view, a multi-lens objective group can be used. For galileo telescope, the structure is very simple and the light energy loss is less. The lens barrel is short and light. And it is still a positive image, but the magnification is small and the field of vision is narrow. It is generally used for theater mirrors and toy telescopes. For Kepler telescope, it is necessary to add a prism group or lens group behind the objective lens to rotate the image, so that the eyes can observe the positive image.

Refractive telescopes generally adopt Kepler structure. Because the imaging quality of refraction telescope is better than reflecting telescope's, the field of view is large, and it is convenient to use and maintain, most small and medium-sized astronomical telescopes and many special instruments use refraction systems.

However, it is much more difficult to manufacture large refractive telescopes than reflective telescopes. Because it is very difficult to smelt large-caliber high-quality lenses and there is a problem that glass absorbs light, large-caliber telescopes all adopt reflective telescopes.

And other types. The main advantage of reflective telescope is that there is no chromatic aberration. When the objective lens is a paraboloid, spherical aberration can be eliminated. However, in order to reduce the influence of other aberrations, the available field of view is smaller. The material for making the mirror surface only requires small expansion coefficient, small stress and easy grinding.

Generally, polishing mirrors are coated with aluminum film, and the reflectivity of aluminum film is more than 80% in the range of 2000-9000 angstroms, so reflective telescopes are suitable for studying near infrared and near ultraviolet bands in addition to optical bands. The relative aperture of reflecting telescope can be made larger, and the relative aperture of main focal reflecting telescope is about 1/5- 1/2.5.

Even larger, and except for Newton telescope, the length of the lens barrel is much shorter than the focal length of the system, and only one surface of the main mirror needs to be processed, which greatly reduces the cost and manufacturing difficulty. Therefore, at present, all optical telescopes with a diameter greater than 1.34m are reflective telescopes.

For the reflective telescope with larger aperture, prime focusing system (or Newton system), Casselin system and folding axis system can be obtained by replacing different secondary mirrors. In this way, the telescope can obtain several different relative apertures and fields of view. Reflecting telescope is mainly used in astrophysics.

Kepler binoculars generally obtain positive images by rotating an image prism, and there are two commonly used ones: PorroPrism, the most commonly used prism.

Binoculars with prisms are wider, and the distance between the two objective lenses is greater than the distance between the eyepieces, so that the stereoscopic impression is strong when observing close objects. Some compact binoculars use inverted prisms, the distance between objective lenses is smaller than that between eyepieces, and the stereoscopic effect is weakened. Pro prism is easy to manufacture and cheaper than the roof prism with the same optical quality.

Roof prism is small, which can make the objective lens and eyepiece in a straight line, so it is often used in extremely compact binoculars. Compared with the general prism, the roof prism has two main disadvantages: one is the loss of light and the darkness of the image; Secondly, it requires high assembly accuracy, is difficult to manufacture and is expensive. Well-made roof prism can catch up with but not surpass ordinary prism in performance.

There are many kinds of optical glasses that can be used to make prisms. BK-7 glass is often used in cheap binoculars. Bak-4 glass is used for higher grade. Observe the exit pupil of binoculars under a bright background (such as the sky). If the image periphery is "cut off", BK-7 glass is used; Bak-4 prism can see a clear and bright circle.

3. Refractive-reflective telescope adds a refractive element on the basis of spherical reflector to correct aberration, which can avoid the difficulty of machining large aspheric surface and obtain good imaging quality.

There is a famous Schmidt telescope, and a Schmidt correction plate is placed in the center of the spherical mirror. It is an aspheric surface with one side flat and the other side slightly deformed, which makes the central part of the beam slightly converge and the peripheral part slightly diverge, just to correct spherical aberration and coma.

Another kind of Maksutov telescope can correct spherical aberration and coma at the same time by adding a meniscus lens in front of the spherical mirror and choosing appropriate parameters and positions of the meniscus lens. And the derivatives of these two kinds of telescopes, such as super Schmidt telescope, Baker-Noon camera and so on. In catadioptric reflecting telescope, the image is imaged by a mirror, and the refractive mirror is used to correct the aberration.

Its characteristics are relatively large aperture (even greater than 1), strong light, wide field of vision and excellent imaging quality. Suitable for sky survey photography and observation of nebulae, comets, meteors and other celestial bodies. If catadioptric card seglin system is used in small vision telescope, the lens barrel can be very short.

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