Understanding astronomical telescopes

1. What is an astronomical telescope?  

Astronomical telescopes are the “eyes” for human observation and understanding of the universe. They are the most used and iconic astronomical observation equipment.  It is also a necessary observation tool for the masses of astronomy popularization workers and astronomy lovers and is used to help users observe the details in the sky that are hard to discernible by the naked eye, and search for farther and fainter celestial bodies.  

2. Optical properties of astronomical telescopes  

Magnification: when you look at the landscape or the moon through an astronomical telescope, objects seem to get closer. At the same time, you can also see many holes on the surface of the moon. This is because the telescope has the function of magnification.  

Eyepiece focal length: In magnification calculation, the focal length of the objective lens is usually fixed, and different eyepieces can be used to observe the star seasons with a variety of magnification.  The larger the magnification, the smaller the view.  

Light collecting power: Another important feature of a telescope is light collecting power.  Light collecting power is the ability of a telescope to collect light.  The amount of concentrating power depends on the diameter of the telescope, the bigger the diameter, the darker the stars can be seen.  

Objective lens: The larger the diameter of the objective lens, the darker the magnitude can be seen. The smaller the diameter of the objective lens is suitable for observing planets. For different stars, different telescopes should be used.  

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3. The meaning of the numbers in the telescope models  

Unlike binoculars, there is no magnification in the astronomical telescope model, but instead the focal length of the objective lens.  For example, 70076 indicates that the focal length and aperture of the telescope objective lens are 700mm and 76mm respectively.  1800150 indicates that the focal length and aperture of the objective lens of the telescope are 1800mm and 150mm respectively.  Also, put the aperture in front of the focal length to indicate.  But in any case, the larger number is the focal length, and the smaller number is the objective aperture, which is not easy to mistake.  

4. Focal ratio of astronomical telescope  

Focal ratio: The ratio of the focal length of an objective lens to its aperture, equivalent to the aperture on the camera lens.  If the aperture is unchanged, the longer the focal length of the objective lens, the larger the focal ratio, and the higher the multiplier is easy to get;  The shorter the focal length of the objective lens, the smaller the focal ratio, it is not easy to get higher magnification, but the image is brighter, the field of view is larger.  

* Short focal length lens (small focal ratio, focal ratio <=6): suitable for observing nebulae and searching for comets;  

* Long focal length lens (large focal ratio, focal ratio >15): suitable for observing the moon and planets;  

* Medium focal length lens (medium focal ratio, 6< focal ratio <=15): suitable for observing double stars, clustered stars, variable stars, and star clusters, but also can take care of both, very suitable for beginners.  

5. The role of an astronomical telescope eyepiece  

The eyepiece of an astronomical telescope has two functions. One is to make the parallel light incident into the objective lens remain the parallel light when it exits the eyepiece.  Another function is to magnify the image produced by the objective lens, which is very important for observing celestial bodies with visible surfaces and close binaries.  

6. Star finder and guide mirror for astronomical telescopes  

The primary mirror of an astronomical telescope is responsible for the observations.  However, many astronomical observations can not be made with the primary mirror alone.  It also needs an assistant, which is a star finder or guide.  

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In order to quickly search for the celestial bodies to be observed, there is often a small astronomical telescope attached to the primary mirror, which is called the star finder mirror.  Starfinder is a refracting astronomical telescope.  Its optical axis is parallel to that of the primary mirror so that it is aligned with the target of the primary mirror.  The diameter of the objective lens is generally about 5 ~ 10 cm, the field of view is about 30 ~ 50, the magnification is about 7 ~ 20 times, and the focal plane is equipped with a demarcation plate for calibration.  When you make an observation, you use the star finder to find the object you want to observe, and you move that object to the center of your field of view.  At this point, the object is naturally in the center of the primary mirror field of view.  

In order to correct the tracking error in time when the primary mirror is observing for a long time, a watching mirror is set beside the primary mirror, which is called the star guide mirror.  The telescope used for astronomy was replaced by a guide mirror.  

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