As a beginner in astronomy, how to choose an astronomical telescope?

This paper introduces some general views and suggestions on the selection of astronomical observation equipment to astronomy enthusiasts, hoping to help them make their own choices calmly and objectively in the rapidly developing and mixed astronomical equipment market in China and buy the most suitable astronomical equipment.

I. Problems and suggestions

As a student, or a salary enthusiast, or even a few astronomy enthusiasts who "get rich first", they will be dazzled when choosing a telescope, especially when they face the terms aperture, focal length and coating for the first time. At this time, the best way is to join the local astronomy enthusiast organization first, so that you have the opportunity to actually use the telescope in other people's hands before making a decision according to your own needs. Before buying a telescope, you should also carefully consider the following questions:

1. How much energy and time are you going to spend getting familiar with the sky? If you are familiar with the night sky and the celestial bodies to be observed, and don't find it hard to look for the stars in the star map, then you can choose a telescope that is cheaper, more portable, lighter and better to use. On the contrary, those telescopes with precise coordinate mechanisms or even computer-controlled automatic star searching will be the best choice. It should be noted that with the development of electronic industry and the advantages of large-scale production, the prices of fully automatic telescopes of major telescope manufacturers in the world are becoming more and more reasonable, which is by no means unattainable.

2. How far is your observation place? How to carry your telescope and how much labor are you willing to pay when carrying it? The answer to this question not only determines the aperture of the telescope, but also relates to the optical structure of the telescope. Please remember a conclusion that countless astronomy enthusiasts have paid a lot of money, that is, the frequency of telescope use is inversely proportional to its weight. We think that a telescope that is often taken out for observation is far better than those that are too heavy to stay at home.

3. What accessories do you want? Most modern telescopes have diverse functions and countless accessories, but only a handful are used in actual observation (at least often used). We found that most enthusiasts, including the author, like the "basic" telescope, and the benefits brought by too many functions and accessories are far less than the economic burden they bring you.

4. Are you going to take celestial photography or CCD imaging? Both "astrophotography" and "CCD" are very expensive, and beginners usually need several telescopes and several years to construct satisfactory equipment and obtain satisfactory results.

The most important parameter that determines the performance of a telescope is the aperture. The larger the aperture, the darker the celestial body you can see and the more details you can distinguish. But caliber is not everything. A telescope with poor workmanship can't achieve its performance or even work. Fortunately, the caliber used by enthusiasts, whether optical or mechanical, is not difficult to process. As long as the general manufacturers take it seriously, they can produce satisfactory products. But occasionally there will be some unqualified products, but now> has been deeply rooted in people's hearts. As long as we find it in time and insist strongly when necessary, replacement or return is not a problem.

Different optical structures make the telescope have different optical properties. Schmidt-Ka seglin, Newton reflector and various refractors all have their own advantages and disadvantages, which lead to the difference of optical performance, but this difference is secondary to the difference of aperture. We can think that the optical performance of telescopes with the same caliber and the same technology level should be close, but the quality difference caused by different processing levels is really huge.

In addition, the astronomical atmosphere will affect the telescope's ability to distinguish details, and the brightness of the sky background will affect the telescope's ability to observe dark objects. Visual objects have a great influence on large-aperture telescopes. If you observe in a place with bright sky background and poor visibility, such as a big city, there is no need to move out a large telescope. If you keep observing in such a place, you don't need to buy a large telescope.

Generally speaking, modern high-quality refractors have the best optical performance per aperture, but they are also the most expensive compared with other types of telescopes. In addition, when the aperture exceeds 10 cm, the lens barrels are usually too long, which makes them very portable (of course, APO refractors are not included). Schmidt-Ka seglin and Maksutov-Ka seglin folding reflecting telescope have the best portability. However, this kind of telescope is still expensive. Considering the optical performance alone, Newton-type mirrors, especially Dobson-type Newton-type mirrors, have the highest cost performance (non-optical quality). Their portability is better than that of refractive mirrors at the same aperture (because their relative aperture can be made very large), but it is obviously not as good as that of folding mirrors.

Two. About telescopes

For beginners, ordinary refractometers with small aperture (less than 10 cm) are the best choice. They are very cheap (compared with catadioptric mirrors and APO refractors) and simple to operate and maintain. It is suggested that if economic conditions permit, you should try to buy regular astronomical telescopes. "Conventional" refers to the standard eyepiece interface with excellent optical quality (the most commonly used interface at present is 1.25 inch). At present, the low-grade refractors (mostly 50-60 mm in diameter) on the market are uneven in quality, and most of them have defects in design and manufacture. It is best to ask an expert telescope to help the staff when purchasing.

Reflectors are mainly divided into Newton type and Ka-seglin type, and Ka-seglin type is relatively rare and expensive in the hands of enthusiasts. Because the eyepiece of Newton reflector is located at the front end of the lens barrel, it is not easy to operate and relatively complicated to maintain, such as correcting the optical axis and coating, etc., but its biggest advantage lies in its low price, and it is the easiest telescope to make, so it has always been very popular among amateur astronomers, and there are also many experts in grinding lenses and assembling Newton reflectors in China. There are many books and articles about self-made Newton mirrors at home and abroad. Those who are interested may wish to have a try. At present, the mainstream of Newton lens sold in the market is large aperture and small focal ratio. This kind of telescope has strong optical power and large field of view, which is very suitable for visual observation of deep space celestial bodies. In addition, the design level of eyepiece has been substantially improved compared with 20 or 30 years ago. The ultra-wide-angle eyepiece with a field of view greater than 80 degrees can still have satisfactory imaging quality when used in a mirror with a relative aperture greater than F/4.

Reflective telescope is the most popular telescope in amateur astronomy abroad at present. The 120 telescope produced by Nanjing Tianyi Center (formerly Nanjing Astronomical Instrument Factory) was once the champion of popular science telescope. The biggest feature of this kind of telescope is its short barrel and good portability, so it is welcomed by everyone. Large demand leads to mass production reducing costs, and low prices further stimulate consumption. Such a virtuous circle naturally benefits our lovers. In the American market, the price of Schmidt-Ka seglin telescope with a diameter of 200mm and equatorial telescope with automatic tracking fork F/ 10 is less than 1000, which can be said to be value for money. The obvious individual differences in the quality of reflective telescopes may be mainly caused by the complexity of their assembly and adjustment. A manufacturer with strict quality management should not let unqualified products leave the factory, but if you want to buy the best ones, you should also choose them yourself, and it is best to check their image quality by looking at the stars.

If you are a perfectionist, economic conditions will not hinder your pursuit of perfection. You should seriously consider fluorite or ED (ultra-low dispersion) APO (apochromatic) refractor. Due to the different design ideas and standards of different manufacturers, not all refractors that claim to be APO are the best, but the best telescopes must come from these refractors. The "best" here not only refers to the optical quality, because the chromatic aberration is effectively controlled and their relative aperture can be made larger, so it is more convenient to carry; Manufacturers are usually willing to match expensive objective lenses with a well-designed lens barrel, so the appearance is also beautiful. The price of the above advantages is very high. The price of an APO refracting telescope with a diameter of 100 mm often exceeds that of a Schmidt-Ka seglin telescope with a 200 mm equatorial telescope. ..

Let's talk about landscape/astronomical binoculars, including binoculars and a single lens (that is, a point mirror) that forms a positive image through a prism. I believe many friends have used binoculars when traveling or watching football games. Compared with traditional astronomical telescopes, they have the advantages of large field of view, bright imaging, comfortable viewing and convenient carrying, so they have become necessary equipment for astronomy enthusiasts who love observation. At present, there are many kinds and types of binoculars on the market, which can be roughly divided into two categories according to astronomical purposes; First, it is used for carrying around, idolize and large-field observation, and its volume is usually small. The most commonly used binoculars are 7X50 and 10X50, and the exit pupil diameter of 7X50 is 7. 1mm, which is roughly equivalent to the pupil diameter when young people's eyes are completely adapted to darkness. But with the increase of age, the maximum diameter of human eyes will gradually decrease. When they are over 30, they will choose 650. The other is the large-aperture coma-seeking binoculars, the caliber of which is mostly 80-150 mm. Besides coma-seeking, it is also an effective weapon for observing celestial bodies in deep space. In order to carry out astronomical observation smoothly, binoculars should be fixed on a tripod. General manufacturers provide tripod connection devices for purchase, and the price is one or two hundred yuan, so binoculars and tripods can be easily connected.

Spotting Scope is very popular abroad, and its owners are mostly nature and bird lovers called Bird. The diameter of the spotting mirror is mostly 50-80 mm, and the positive image is made by the built-in prism (group). Generally, the magnification can be changed by changing different eyepieces, but like the 35mm single-lens reflex camera, different manufacturers have different eyepiece bayonets. Almost all Spotting Scope can be easily connected to the camera tripod, and most manufacturers have also designed a camera interface with replaceable eyepiece for Spotting Scope, which can be used as a telescope head. In order to adapt to everyone's different uses and habits, Spotting Scope has two kinds of lenses: direct-looking eyepiece and 45-degree squint eyepiece, as well as two common achromatic and apochromatic objective lenses to choose from. Compared with astronomical telescope, Spotting Scope is completely upright, compact in structure and good in sealing and dustproof performance, which is suitable for use in the wild and harsh environment. However, due to the complex optical structure, the imaging quality is slightly lower than that of astronomical telescopes of the same level, especially not suitable for high-magnification observation (the maximum magnification of eyepieces provided by manufacturers is generally less than 60 times).

Three. Brackets and accessories

Telescope supports are divided into two types: horizontal and equatorial. Horizontal supports are generally cheaper, lighter in weight and more convenient to carry and debug. However, when you need to automatically track celestial bodies, horizontal support is not enough. Telescopes automatically controlled by computers (such as LX200 series of Mead Company) can automatically track in the horizontal state, but the whole field of view will rotate around the center of the field of view, so it is impossible to shoot celestial bodies. Therefore, the equatorial support (also known as the equatorial telescope) is a necessary equipment for tracking celestial photography. No matter which kind of stent is chosen, its stability is the most important. The unstable horizon and equatorial support bring great trouble to observation, especially focusing and finding stars, which greatly reduces the fun of astronomical observation. A high-quality camera tripod is often easier to use than the bracket that comes with a general telescope, and its maximum load is usually given in the manual of the camera tripod. However, because the lens barrel of the telescope is much longer than that of the camera, and the moment acting on the tripod head is much larger, it is ideal to choose a tripod with a maximum load of about twice the weight of the telescope. Camera tripods for telescopes have two disadvantages. First, the price is higher, such as a set of tripod and tripod head of Manfutu, at least 1000 yuan; Second, the camera tripod has no micro-motion mechanism, so it is very inconvenient to find stars. There is a micro-motion pan/tilt in foreign markets. Adding it to the pan/tilt of tripod can solve this problem. Unfortunately, it is not easy to buy in China at present.

For astrophotography, the equatorial telescope is sometimes more important than the telescope itself, because the telescope is sometimes only used to guide the stars, and the tracking quality and stability of the equatorial telescope are directly related to the quality of the photos.

The eyepiece plays an important role in the optical performance of the telescope, which is no less important than the objective lens of the telescope in visual observation. The parameters that determine the performance of eyepiece are mainly focal length, field of view and exit pupil distance. The ratio of the focal length of the telescope objective to the focal length of the eyepiece is the magnification, so the focal length is the most important parameter to characterize the performance of the eyepiece. The field of view of the eyepiece determines the field of view of the telescope (telescope field of view = eyepiece field of view ÷ magnification). General microscope eyepiece (Huygens type, 2 pieces and 2 groups) has a field of view of only about 30 degrees. This kind of eyepiece is not only simple in structure and poor in aberration correction (especially chromatic aberration), but also has a small field of view and the feeling of looking from one end of the chimney to the other. At present, the field of view of standard eyepieces (such as Plossl and or eyepieces, 2 groups of 4 pieces) is 40-50 degrees, while the field of view of wide-angle eyepieces (generally more than 6 pieces) is above 60 degrees, and some can reach 84 degrees. It's wonderful to look at the sky with this eyepiece, especially at low magnification, just like walking in space. The exit pupil distance refers to the distance between the observer's eyes and the eyepiece of the eyepiece when the whole field of view can be seen clearly. The distance from the pupil directly determines the convenience and comfort of observation. An eyepiece with a moderate exit pupil distance (for example, 15-20 mm) will bring a lot of convenience to observation, especially for observers who wear glasses, they can see the whole field of view clearly without taking off their glasses, which is more important for those who wear astigmatism lenses, because even if they take off their glasses and refocus, they can't see a clear star image. For the same eyepiece, the exit pupil distance is generally proportional to the focal length. It is not good that the exit pupil distance is too short, and it will bring inconvenience if it is too long. When using the Plossl eyepiece with a focal length of 40-55 mm, the eyes have to move back and forth to find a suitable position, and then the problem of blindfold is solved.

A telescope should usually be equipped with multiple eyepieces in order to combine them into multiple magnifications. First of all, an eyepiece with low magnification and large field of view should be equipped to observe nebula clusters with large area and low surface brightness. At the same time, when using high-power eyepiece, you can find the target first, which will be the most used eyepiece. The magnification of this eyepiece should be 2-3 times the number of centimeters of the telescope aperture. For a telescope with a relatively small aperture, the Plossl eyepiece with a focal length of 40-55 mm (the field of view angle is about 40 degrees) can be competent, but when the relative aperture is large, it is best to choose a wide-angle eyepiece with a slightly shorter focal length (the field of view angle >: 60 degrees). The power eyepiece is mainly used to observe deep space objects such as nebulae and star clusters. The typical medium magnification is 5- 10 times the aperture of the objective lens. High magnification is mainly used to observe planets, binary stars and dense nebulae. A high-quality objective lens (such as 10cm APO refractor) should be allowed to use a magnification of 25 times its aperture cm without significantly reducing the imaging quality, but blindly pursuing a high magnification is often counterproductive because there are few atmospheric conditions suitable for using a magnification of more than 500 times. As mentioned above, the design level of eyepiece has been greatly improved in recent years. In foreign markets, it is not difficult to see ultra-wide-angle eyepieces with an effective field of view exceeding 80 degrees and high-power eyepieces with long pupil distance.