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How To Choose Outdoor Telescopes
Jul 22, 2018

For different people, the choice of outdoor telescopes will be different because of different demands, and there is no uniform rule. Choosing a suitable outdoor telescope based on your requirements and preferences will bring great pleasure to your outdoor activities. How to choose outdoor telescopes?

The choice of outdoor telescopes is as follows.

1. The meaning of the mirror body logo

Take the BAIGISH10X40 telescope as an example: "10X40" is printed on the shoulder of the left mirror, and 10 means to observe the target through a telescope, which is 10 times larger than the naked eye and 1000 meters away, equivalent to a distance of only 100 meters.

40 indicates that the diameter of the objective lens is 40 mm. The larger the aperture of the objective lens is, the brighter the imaging is, but the portability will decrease. There is a word "114M/1000M" on the shoulder of the left mirror. It is 1000 meters away. The scope of the telescope is a circle of 114 meters in diameter. In fact, it is a concept of a field angle, which is converted into an angle of 6.5 degrees.

The telescope also uses XX ft at 1000yds to represent the size of the field of view, FT stands for feet, and YDS represents yards, all of which are British units. Take the Zei 7x50 B/GA telescope as an example, its field of view is 390ft at 1000yds.

2, BAK4 and BK7 material

BAK4 and BK7 are the numbers of two different kinds of optical glass. The refractive index of these two kinds of glass is different, the refractive index of BAK4 is high. For the Porro prism of BAK4 material, the incident light can be reflected completely without absorption, while BK7 can only reflect about 83% of the incident light, so the efficiency of BAK4 is high, but its price is also expensive, and the military telescope and some high quality civil mirrors use BAK4 prism. The mirror. The simplest way to judge the material of a prism is to see the shape of the pupil behind the eyepiece.

For the roof prism, because of its special structure, the two materials have no difference, from the shape of the pupil can not distinguish the prism material, and many roof prism telescopes also claim that they are using the BAK4 prism, which is basically for commercial consideration and is technically meaningless.

3. The significance of lens coating

When the light is on the surface of the lens, some of the light is reflected, and a typical telescope has more than 10 reflectors. Even if each reflector reflects only a few percentage points of light, plus the absorption of the light inside the lens, the light that finally enters the eye may lose more than 50%. Not only is the imaging dark, but also the reflection is in the hope of the light. The internal reflection of the telescopic mirror enters the eyes through the eyepiece, forming glare, interfering with the observation, and reducing the contrast of the imaging.

When a thin film of 1/4 wavelength is plated on the surface of the lens, the transmittance of this wavelength can be improved. The common blue film is the simplest antireflection film. The more high-grade mirrors will use multilayer films, which can improve the transmittance of the visible light wave - wave section.

There are many red - film telescopes sold on the market. This kind of mirror is generally only suitable for high reflective environment such as snow and so on. In addition, in order to reduce the cost, the mirror is coated only on the surface of the lens and the outer surface of the eyepiece. A good mirror will be coated on the surface of all the lenses. The better mirrors will be coated on the surface of the lens, and the best mirror will be coated with multilayer films on the surface of all the lenses. The simplest way to judge the coating quality of a mirror is to observe the reflection of lenses. The darkest reflection is, the more transparent the lens is, the better the coating is.

4, nitrogen filled waterproof and all metal structure

Many telescopes will focus on the characteristics of nitrogen filled and waterproof, because this is a typical feature of high quality telescopes. It means that water vapor and dust can not enter the interior of the telescope, coupled with the protection of dry nitrogen, the internal lenses will not be mist and long mouldy, which ensures the long service life of the telescope and is also suitable for extremely harsh environments. Therefore, the general military telescopes, such as 95%, are filled with nitrogen and water. Some civil mirrors also use this design. Do not think this is a very expensive measure. The 6X18, which sells for only 100 yuan, also adopts this design.

Also, it should be emphasized that this design does not directly bring about optical improvement, and the only improvement is durability. Russian telescopes and domestic regular military telescopes are all metal structures (usually aluminum, old military telescopes often use brass), and the benefits of this structure are very durable, and no plastic will slowly aging and easy to deform. Although optical progress can not be directly brought about, optical parts can be kept in an accurate position and bring long-term optical stability. Because of the high cost of metal structure, only military telescopes and advanced civilian mirrors are used in this structure, and ordinary civilian telescopes often use a mixed structure of metal and plastic.

5, Porro (Paul) and roof (roof) prism

The images, seen through a typical Kepler telescope, are upside down, and in order to meet the needs of ground observation, a set of prisms must be added to the object mirror (the object in the distance to a inverted image), and the solid image of the inverted stand is turned into a positive image. There are two kinds of prisms, Porro prisms and roof prisms.

The advantages of the Porro prism are simple structure, high transmittance and good imaging quality, but the telescope has a large volume.

In order to overcome this shortcoming, the reverse Porro prism image can be used, but it brings new problems. The object lens has a small aperture and is not suitable for low illumination environment.

The greatest advantage of the roof prism is to use it after the prism

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