How Do I Choose the Best Used Telescope?

When people buy telescopes, they often have several Arabic numbers on their price list, so what technical parameters do these numbers indicate? Here is an example to illustrate it. ^[1] For example, it is marked with 10 × 50mm5 °, which means that the diameter of the objective lens is 50mm and the field of view is 5 degrees (that is, at 1000). Some people may think that the larger the number of technical parameters, the better, but this is not true. The magnification is inversely proportional to the width of the field of view, that is, the larger the magnification, the smaller the width of the field of view, which is not conducive to searching. The larger the diameter of the objective lens, the greater the amount of incoming light, and the stronger the resolving power when the light is insufficient, but this will inevitably lead to an increase in the volume of the telescope which is not conducive to carrying. Having said that, you may feel confused, but as long as you can learn from each other, you can also buy a desirable telescope.

Telescope parameters

When people buy telescopes, they often have several Arabic numbers on their price list, so what technical parameters do these numbers indicate? Let's try an example to illustrate. ^[1]

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Choosing a telescope of your choice is an important issue for users, because it determines whether the telescope can meet their own needs. But beginners often do not have the identification of telescopes and related optical knowledge, especially in the reality that there are a lot of inferior telescopes on the market. How should beginners choose telescopes? In response to my friends' confusion, I have combined this article with my many years of telescope production and sales experience and understanding of the domestic telescope market, and I have compiled this article, hoping to be helpful to everyone, especially rookie friends:

1. Decide on the optical system structure and magnification range of the telescope according to its own purpose

The ultimate purpose of buying a telescope is to use it comfortably, so determining the main use is crucial. It is the first step in the purchase and it determines the next steps. It can be imagined that if the main purpose is to watch sports games, then buying a large telescope with a PCF structure will feel heavy and clumsy in actual use, and the wrist will be sour for a long time. Where can I watch the game in a good mood?

In addition, the magnification is not as large as possible, because it enlarges not only your field of view angle, but also the shaking of your hands. Generally speaking, the most suitable magnification for human eyes on land is 6-12. Times, and the best is 6-8 times. After more than 12 times, due to the small field angle and entrance pupil diameter, the shaking of the human hand is also magnified, making it difficult to observe by hand. This is why military telescopes are mostly in this range (some marine telescopes can reach 15-20 times).

Second, pay attention to the difference between nominal parameters and real parameters

You may notice that at least two parameters similar to this are usually printed on the telescope: 8X21 131M / 1000M, (the latter may be printed as parameters such as 393FT / 1000YDS, and sometimes also printed as FILE = 7 °; Parameters, but this printing is rare). These are the most critical parameters of a telescope. The former represents the magnification and clear aperture of the telescope. For example, 8X21 represents 8 times the magnification and the clear aperture is 21mm. By analogy, if the printed parameter is 10X50, it means that the magnification is 10 times, and the clear aperture is 50mm. Then knowing the meaning of this parameter, we can easily determine whether the parameters printed by a telescope are true. For ordinary fixed magnification telescopes commonly used in the market, if the printing of magnification exceeds 12 times, we should be skeptical (change 15x, 20x, 30x or higher magnification, such as the common specifications of 7-15X35, 8-24X50, 10-30X60, etc., must be carefully evaluated whether it is true in combination with other parameters. If the magnification is printed Beyond 20 times, we can basically determine that this is a false magnification. The determination of the clear aperture is relatively simple. Generally speaking, it is approximately equivalent to the diameter of the objective lens of the telescope (actually smaller than 1-2mm, but negligible). For example, the diameter of the objective lens of the clear telescope is only 20mm, which is about 2 cm But it is marked with parameters similar to 980x30, 20x50 and even larger, so it is definitely a false magnification.

The other parameter, which is similar to 131M / 1000M, represents the field angle of the telescope. The principle is more complicated. There is also a special article. Here are common field angles of telescope. Products from different manufacturers may be slightly different. But the change is not big. If you see an exaggerated parameter like 9999M / 10000M printed on a certain telescope, it must be a work from a private workshop.

Corresponding field angles of common telescope models (this parameter is for reference only, there may be a slight gap between different manufacturers):

8x21 131m / 1000m

10x25 96m / 1000m

12x25 80m / 1000m

7x35 147m / 1000m

7x50 147m / 1000m

10x50 122m / 1000m

12x50 97m / 1000m

3. Whether the appearance is beautiful and fine, whether the structural design is reasonable, whether the rotating parts of the mechanism are flexible and comfortable, and whether the quality of the assembly (especially the focusing of the optical axis) passes.

Regular telescopes have high requirements for appearance when they leave the factory, and control the factors that affect the appearance from all aspects of raw materials and assembly. Pay attention to whether the appearance of the telescope is beautiful and fine, such as whether the skin is smooth and trimmed neatly ( The works in the workshop generally use inferior decorative skins, mostly manual trimming methods. Due to the limitations of workers' technical level and equipment, there are disadvantages such as unevenness and unevenness of the decorative skins. Is there any damping grease residue or leakage on the outer surface of the telescope? On the basis, pay attention to whether the structural design is reasonable, such as whether the position of the focusing handwheel is reasonable, whether the hand feels flexible, smooth, and comfortable, and whether the visual compensation of the eyepiece part (generally the right eyepiece) is flexible to rotate.

Another point that needs special attention is whether the adjustment of the optical axis of the telescope is accurate. The parallelism of the optical axis of the right and left lens barrels of regular telescopes is calibrated to a certain angle by special instruments before delivery (divergence 35 ', convergence 60'). When the parallelism of the optical axis exceeds this range, it is called the optical axis deviation, and it will be judged as a serious unqualified product in a regular factory. Telescopes with poor optical axis will have dizziness, eye pain and other adverse reactions when used. Long-term use will cause severe damage to vision. Therefore, the adjustment of the optical axis is very strict in a regular factory. The workers responsible for the adjustment are also the best workers among the assembly workers. After a long period of special training, it takes a long time to adjust a telescope. Works in the workshop are restricted by product quality (comprehensive factors such as inferior components and assembly quality may determine that such telescopes are difficult to adjust to the standard optical axis), and equipment constraints (the price of the adjuster is higher, and due to the low productivity, a large number of Equipment), on the other hand, in order to increase production and reduce costs (because inferior telescopes are not worth the cost if they are debugged in accordance with a regular calibration method), they often use methods that lower inspection standards and even subjective adjustments (I have seen some Underground factories are adjusted by adjusting workers to face a white wall, without any equipment, and based on workers' subjective feelings to determine whether they are dizzy or not.) The quality of the products is extremely insecure.

Fourth, whether the optical element is a genuine optical element

The telescope market is mixed, and the development of telescope materials is changing with each passing day. Before 2000, inferior telescope manufacturers basically used waste optical components to recharge, and after 2001, there were newcomers with resin lenses, and the cost was even lower. (It is reported that 0.2 Yuan / PCS, the price of normal eyepiece lenses, even the lowest price of the most common small size DCF telescope is 4-5 Yuan / PCS), and the optical performance is further reduced. The prism of optical element identification method, because of the large volume of the telescope prism, it is more difficult to make it with resin, so it is basically the second best method. It is also easier to identify. Turn the telescope upside down and look from the end of the objective lens to a bright and open place. If you find a black broken line inside (as opposed to a DCF telescope using a roof prism) or a black stain, then this means that This telescope was assembled with scrap products. The black broken lines and black spots are actually the broken ridges and broken points of the prism. Lens appraisal, carefully observe the telescope's eyepiece. If it is not light blue but white (actually colorless), it means that the eyepiece is not coated, why is it not coated? Because it is pressed with plastic resin, it is put into the coating machine and it is baked or deformed or melted at high temperature during the pre-heating phase of the coating material before the coating is started. However, the eyepiece system of a general telescope has several lenses. Some of the works in the workshop use the outermost lens with optical elements, and the resin lens in the inside. Ha ha! There are always people who better than you. It turns out that this kind of resin lens has a congenital deficiency, which is that the image quality is poor, especially the chromatic aberration and distortion are extremely serious. It may be better to pick up the telescope and look at the distant rod-shaped object, preferably a light color, such as a telephone pole. The image of the rod is covered with a rainbow-like coat, and the field of view of the edge of the telescope is very blurred, which indicates that it is a fake.

Five, whether the accessories are complete

Any retail optical telescope generally includes the following accessories: lanyard, lens cleaning cloth, desiccant, lens bag, and a box. If these basic configurations are incomplete, then it is likely that the workshops have cut corners in order to reduce costs, especially PCF telescopes, and workshops are generally not equipped with color boxes. There are even some unscrupulous hawkers who slaughtered customers under the banner of sending backpacks for 10 yuan, and sold the original bags of telescopes to customers again.

Fifth, resist the temptation of low prices, carefully purchase products, pay attention to performance and instructions are consistent

In 1997, the market price of an 8X21 DCF telescope was at least 300 yuan. The lowest price on the market can reach more than 10 yuan. The price war is a tool that has been tried and tested in the market, and some friends are often blinded by the low prices of unscrupulous hawkers and dark lies. Although the price was very cheap, a lot of garbage was bought back. For example, the various "military telescopes" and "Russian telescopes" that are common in the market. Generally, hawkers ask for more than 200 yuan to 80 yuan, is it really cheap? Yiwu wholesale market only sells more than 20 yuan of this kind of product. The ability to reduce the cost of the room can really reach the extreme. It is based on the price of optical rough glass purchased in kilograms. So how did this telescope come into being? Quite simply, plastic with a little bit of window glass. Therefore, users and friends must carefully judge in accordance with the above principles when purchasing telescopes, pay attention to whether the performance of the product is consistent with the description, and do not listen too much to the words of small vendors to avoid being deceived.

The author believes that as long as the above principles are grasped, under normal circumstances, you can buy a satisfactory telescope.

Specifications: 10x40 Field of view: 6 ° 30 '1000 meters Vision: 114/1000 meters Exit pupil distance: 12 mm Exit pupil diameter: 4 mm Resolution: 4.7 "Specifications: 10x40 Note that the telescope's magnification or magnification is 10 times The diameter of the objective lens of the telescope is 40 mm. For a variable magnification telescope, it may be labeled: 10-40x60, then the multiple is continuously adjustable between 10-40 times, and the aperture is 60 mm. Note that the multiple is much larger than the aperture It is meaningless. Some inferior telescopes mark: 999x50, etc. is a deception. In addition, some telescopes mark the method as "10x40 times", which is also irregular and misleading. Field of view: 6 ° 30 'means telescope The viewing field of view, the larger the telescope multiple, the smaller the field of view. 1000 meters Horizon: 114/1000 meters Another way to indicate the size of the field of view, refers to the width of the range seen by the telescope at a distance of 1000 meters. The larger the parameter, the smaller the exit pupil distance: the optimal distance between the eye and the eyepiece when a 12 mm telescope is imaging on the retina. It is related to the telescope's multiples, etc. The exit pupil distance is smaller. A sufficiently large exit pupil diameter can ensure that the telescope is used when the user wears a gas mask or glasses. Exit pupil diameter: 4 mm exit pupil diameter = caliber / multiple. For example, this 10x40 telescope, exit pupil diameter It is 4 millimeters. Of course, this is the ideal state. Some telescopes may not reach this theoretical value due to optical path design and cost. Resolution: 4.7 "refers to the ability of the telescope to resolve the smallest details. Generally related to the specifications of the telescope, material workmanship and other factors.