Because:

1, the diameter of the objective lens of the telescope is relatively large, in order to enlarge the viewing angle and make the objects you see clearer or brighter.

2. The telescope will magnify the object, and the eyepiece is equivalent to a magnifying glass, which will gather light and increase the temperature of the irradiated object. Direct observation of the sun or other strong light sources may hurt your eyes.

Telescope is an optical instrument, which can help to expand the viewing angle and improve the ability of farsightedness. But if you watch it for a long time, it will also cause symptoms of visual fatigue.

In addition, the telescope is harmful to eyes, which is obviously related to the quality of the telescope. For example, inferior telescopes, such as those coated with red film, falsely claim that they have infrared night vision function, and the coating will harm users' eyes, and long-term viewing will reduce or even damage their vision. Inferior telescopes do not correct the parallelism of the optical axes of the two lens barrels, which will cause dizziness and decreased vision when used.

Extended data:

According to the principle of telescope, it is generally divided into three types:

1. An instrument for observing electromagnetic radiation of distant objects by collecting electromagnetic waves is called a radio telescope. In daily life, telescopes mainly refer to optical telescopes, but in modern astronomy, astronomical telescopes include radio telescopes, infrared telescopes, X-ray telescopes and gamma-ray telescopes.

The concept of astronomical telescope has been further extended to gravitational waves, cosmic rays, dark matter and other fields.

2. Optical telescopes in daily life are also called "thousand-mile mirrors". It mainly includes amateur astronomical telescopes, theater telescopes and military binoculars.

The commonly used binoculars need to add a prism system in order to reduce the volume and flip the inverted image. Prism system can be divided into RoofPrism (Schmidt prism) according to its shape and type. The principles and applications of these two systems are similar.

3. Small hand-held telescope for personal use should not use too large magnification, which is generally 3 ~ 12 times. If the magnification is too large, the image definition will become worse and the jitter will be serious. Telescopes larger than 12 times are generally fixed by tripod.

Choice of telescope:

1, optical quality and optical appearance are often contradictory. If you want both, you need to increase the budget greatly.

2. Every kind of telescope has a specific environment suitable for its use, so as to achieve perfect results. No telescope can do everything.

3. The volume of the roof prism telescope is the smallest in the same size, but the optical quality is often not as good as that of the Polo prism telescope.

4. The price of telescope depends on many external factors such as cost, profit and marketing strategy, and has little to do with the multiple of telescope.

5. The imaging effect of telescope depends on many factors, and the multiple is only one of them. It is not advisable to blindly pursue multiples.

6. There is a great possibility of counterfeiting military telescopes. Regular military telescopes are basically black and expensive.

7. Don't buy binoculars with large zoom. This kind of telescope has many problems, such as small field of view, serious imaging distortion, easy deviation of optical axis and so on.

8. To know the telescope with the same specifications and parameters, the actual effect may be far from each other, and of course the price will be far from each other.

9. Try not to buy a red film telescope. Only suitable for high reflection environment such as ice and snow. Generally speaking, the imaging is dim and the color cast is serious.

10, there has never been any infrared night vision telescope, but some specifications of the telescope, such as 7X50, are also very effective in low light environment!

1 1. Try to refer to third-party websites and evaluation experience articles when choosing a telescope, which can maximize the advantages and disadvantages of the telescope.

Baidu encyclopedia-telescope