Telescope imaging is upside down. Why is it upright?

Because the eyepiece of the telescope is equivalent to a magnifying glass, it becomes a vertical magnified virtual image, so the light enters the eyepiece from a focal length after entering the objective lens. The objective lens of a telescope is equivalent to a camera, and it becomes an inverted and reduced real image. Because the distance of light entering the light source becomes narrower and the viewing angle becomes larger after entering the objective lens, it becomes an enlarged image.

Not all telescope images are inverted, such as galileo telescope, which has a simple structure, and Kepler telescope, which has a complex structure, in order to eliminate projective aberration, or it can be an upright image. Unfortunately, it is impossible to add a crosshair for object measurement.

The first function of the telescope is to enlarge the opening angle of distant objects, so that the human eye can see the details with smaller angular distance. The second function of the telescope is to send the light beam collected by the objective lens, which is much thicker than the pupil diameter (up to 8 mm), to the human eye, so that the observer can see dim objects that he could not see before.

Extended data:

The objective lens is a convergent lens and the eyepiece is a telescope with a divergent lens. The real image formed by the refraction of light through the objective lens is in the focus behind the eyepiece (near the back of the human objective lens). This image is a virtual image to the eyepiece, so it is refracted by the eyepiece to form an enlarged upright virtual image. The magnification of galileo telescope is equal to the ratio of the focal length of the objective lens to the focal length of the eyepiece. Its advantage is that the lens barrel is short and can stand upright, but the field of vision is relatively small.

Because there is a real image between them, the reticle is easy to install and has excellent performance, so military telescopes, small astronomical telescopes and other professional telescopes all adopt this structure. But the imaging of this structure is inverted, so an upright system should be added in the middle.

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.

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