Please add some astronomical knowledge (about the earth, moon, sun and the principles and parameters of optical telescopes)

The objective diameter of a telescope refers to the effective diameter, that is, the diameter of the objective part that is not blocked by the frame, represented by D. It is an important indicator of the telescope's light-gathering ability. The larger the diameter of the telescope, the brighter the stars it can see, and the more faint stars it can see. Due to the large diameter, the light gathering ability is greatly increased. For example, the pupil diameter of the human eye is 6mm. If observed with a 6m telescope, the increased optical flow is 106 times greater than that of the human eye [(6000mm/6mm)2=106]. However, in urban areas where light pollution is particularly serious, large apertures may not be effective. If you want to shoot celestial objects in urban areas, experienced people believe that a 15mm aperture can meet the shooting conditions.

2. Relative aperture (A)

Refers to the ratio of effective aperture D and focal length F, represented by A. That is:

Celestial objects that present a certain viewing surface in a telescope are called extended celestial bodies, such as the moon, the sun, planets, etc. The brightness of extended objects in the telescope is proportional to A2, that is, the larger the relative aperture, the brighter the extended objects, which also means the higher its ability to observe extended objects. Therefore, when doing astrophotography, you should pay attention to choosing the appropriate relative aperture (for example: the aperture number on the camera is an indication of the relative aperture).

3． Focal length (F)

Telescopes generally consist of two systems with limited focal lengths. One is the focal length of the objective lens, represented by F; the other is the focal length of the eyepiece, represented by f. The focus of the two systems coincides. Using traditional film to create images after exposure, the focal length of the objective lens is the main symbol of the film scale in astrophotography. For the same celestial body, the longer the focal length, the larger the image size of the celestial body on the focal plane. For example, when photographing Venus, its apparent diameter is 61″, and an image of 0.7mm is formed on the focal plane.

4. Magnification (G) and film scale

The magnification (G) of a visual telescope is directly proportional to the focal length of the objective lens and inversely proportional to the focal length of the eyepiece. That is, the objective lens of the telescope is fixed. As long as it is equipped with several eyepieces with different focal lengths, several different magnifications can be obtained. The photographic telescope does not require an eyepiece, and the starry sky phenomena are directly captured on the photographic film, and the angular distance on the celestial sphere becomes the linear distance on the film. The relationship between the angular distance on the celestial sphere and the linear distance on the film is generally determined by the film scale. To express, one angular minute of the celestial sphere is equivalent to how many millimeters on the film the film scale is proportional to the focal length.

5 Resolution angle (δ)

Refers to the distance that can be resolved by the telescope. The angular distance between two points on the celestial sphere is represented by δ. The reciprocal of the resolution angle is the resolution power. That is, the smaller the resolution angle, the greater the resolution power. Theoretically, according to the principle of light diffraction, the limit resolution angle of the telescope is: where λ is the wavelength of incident light, D is the effective aperture of the telescope, and λ and D are both in millimeters (mm). The pupil diameter of the human eye is between 8 and 2 mm. It is calculated that the ideal value of the resolution angle of the human eye is 18″~70″. (60″=1′); if observed with a 6m telescope, the minimum resolution angle is 0.02″, which is 1 to 3 thousand times higher than the resolution of the naked eye.

6. Field of view (ω)<. /p>

The angular diameter of the sky area that can be observed with a telescope is called the field of view, represented by ω. The field of view is inversely proportional to the magnification. The greater the magnification, the smaller the observed sky area. The size of the field of view can be constrained by the design size of the viewing angle of the objective lens and the camera film. For a catadioptric telescope or a reflecting telescope, the field of view design has a certain size due to the light blocking of the secondary mirror, while the refracting telescope is often used for imaging. Quality limitation. For example, when we use a 120mm telescope connected to a 135mm camera to shoot celestial objects, the constrained field of view is 120mm (59′). Generally speaking, the shorter the focal length of the telescope, the larger the field of view. This is also the case when the camera lens directly shoots celestial objects. .

7. Penetrating power

Using a telescope to observe the magnitude of the faintest star that can be seen near the zenith on a clear night is called the penetrating power or limiting magnitude of the telescope. . It is closely related to the aperture of the telescope. The larger the aperture, the fainter the celestial objects can be observed. If the aperture is 5cm, stars of magnitude 10 can be observed; if the aperture is 5m, stars of magnitude 21 can be observed. The definition is introduced in Chapter 6)

Because the star is too far away and the resolution of the telescope is not high enough, the image of the star in the telescope is still in the shape of a point. These objects are usually called point images in the telescope. They are point light source celestial bodies. Another type of celestial body whose surface can be distinguished by a telescope is called visible celestial bodies, including the sun, moon, planets, comets, nebulae, zodiacal light, etc. Astronomy enthusiasts take pictures of visible celestial bodies. They are quite interesting because they are not only good display and viewing materials, but more importantly, they are also part of the information for scientific research. Readers will definitely have endless fun practicing astronomical observation and astrophotography while learning to use optical telescopes.

It is worth emphasizing that early astronomical telescopes only made visual observations, and the terminal equipment only had eyepieces. Later, with the continuous development of science and technology, terminal equipment gradually added photography systems, photoelectric photometers, spectrometers, charge-coupled devices (CCD), and so on. Since the completion of the 5-meter Hale Telescope in 1948, the development of large optical telescopes has become a world trend.

Such as: Keck Telescope, European Southern Observatory's Very Large Optical Telescope, Japan's Subaru Telescope, the Gemini Telescope jointly manufactured by seven countries, and China's Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), etc.