The performance of the telescope is mainly reflected by the following aspects:

1. Effective aperture and relative aperture

---The distance from the center of the objective lens to the focus is called the focal length of the objective lens, which is represented by the symbol F. The diameter of the objective lens that is not blocked by the frame and aperture is called the effective aperture of the objective lens, which is indicated by the symbol D. The performance of astronomical telescope is mainly marked by these two data. The ratio of effective aperture to focal length is called relative aperture, which is indicated by the symbol a. That is: a = D / F, where D and F are in mm.

2. Magnification

---The ratio of the objective focal length (f) to the eyepiece focal length (f) of a telescope is called the magnification of the telescope, which is represented by the symbol M. An astronomical telescope is usually equipped with several eyepieces with different focal lengths, so that several different magnification can be obtained. For example, when the focal length of the objective lens of a telescope is 840 mm and the focal length of the eyepiece is 10 mm, the magnification is 84 times. If the focal length of another eyepiece is 20 mm, the magnification of the telescope is 42 times.

---However, there is a certain limit to the improvement of telescope magnification, which is the effective magnification of the telescope. If the magnification is too high, the image will be darkened and the observation effect will not be good. For ordinary astronomical telescopes, the maximum effective magnification is about twice the aperture. For example, when the aperture of a telescope is 80 mm, the maximum effective magnification is about 160 times; when the aperture is 100 mm, the maximum effective magnification is about 200 times.

3. Light collecting power

---Light enters the human eye through the pupil, and the human eye can only collect light equivalent to the pupil area. In the dark, the diameter of the pupil is about 7 mm. Therefore, the multiple of the effective area of the telescope objective relative to the pupil area is called the light collecting force. That is: light collecting force = (d * d) / (7 * 7), where D is in mm.

4. Resolution

---Due to the diffraction of light, the image of the celestial body in the telescope will become a small disk. If two stars are so close that they can just be distinguished, their minimum angular distance is called resolution, expressed by the symbol θ, in angular seconds. The relationship between telescope resolution and effective aperture of objective lens can be roughly expressed as: θ = 140 / D, where D is in mm. For astronomical observation, resolution is often more important than magnification.

5. Limiting magnitude

---On a clear and moonless night, the darkest magnitude that can be seen with a telescope is called the telescope's ultimate magnitude, which is represented by the symbol me. There is an empirical formula for reference: me = 7.1 + 51gd, where D is in centimeter. Of course, this is the theoretical value. Due to the influence of the atmosphere and the observer's vision, the actual value is not necessarily consistent with the calculated value.

6. Field of view

---The angular diameter of the area of the sky that can be seen in a telescope is called the field of view, denoted by the symbol ω. For a telescope, the field of view is related to the focal length of the eyepiece. The shorter the focal length of the eyepiece, the smaller the field of view of the telescope. The larger the magnification of the telescope, the smaller the field of view.

7. Relative aperture

---The larger the relative aperture of an astronomical telescope is, the stronger the ability of concentrating light is, and the brighter the celestial bodies can be seen through the telescope. In addition, the larger the effective aperture is, the larger the relative aperture is. Obviously, the larger the effective aperture, the better the performance of the telescope. In other words, the performance of an astronomical telescope is mainly determined by its aperture.