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How To Buy An Outdoor Telescope
Oct 21, 2018

For different people, the choice of outdoor telescope will be different due to the different emphasis of the requirements. There is no uniform rule. Choosing a suitable outdoor telescope according to your requirements and preferences will bring great pleasure to your outdoor activities. How to buy an outdoor telescope?

Ways to buy an outdoor telescope are as follows;

1. The meaning of the lens body logo

Take the BAIGISH10X40 telescope mirror as an example: the word "10X40" is printed on the shoulder of the left mirror. 10 indicates that the target is observed through the telescope, which is 10 times larger than the direct observation of the naked eye.

40 indicates that the diameter of the objective lens is 40 mm. The larger the diameter of the objective lens, the brighter the image, but the portability will decline. On the shoulder of the left mirror is the word "114M/1000M", which means that at a distance of 1000 meters, the scope visible through the telescope is a circle with a diameter of 114 meters, which actually means the concept of field of view Angle, which translates to 6.5 degrees.

Other telescopes are xx ft at 1000yds for the size of the field of view, ft for feet and yds for yards, all in English. The Zei 7x50 B/GA telescope, for example, has a field of view of 390ft at 1000yds.

2. BAK4 and BK7 materials

BAK4 and BK7 are the Numbers of two different kinds of optical glass. The refractive index of these two kinds of glass is different. BAK4 has a high refractive index. For the porro prism made of BAK4, the incident light can be reflected 100% completely without considering the absorption, while BK7 can only reflect about 83% of the incident light. The simplest way to determine the prism's material is to look at the shape of the exit pupil behind the eyepiece.

For roof prism, due to its special structure, there is no difference between the two materials, and the shape of exit pupil cannot distinguish the prism material. Many roof prism telescopes also claim to adopt BAK4 prism, which is basically for commercial consideration and technically meaningless.

3. Significance of lens coating

When light in the lens surface, some light is reflected off, a typical telescope has more than 10 reflective surface, even though each reflector only reflects the light of a few percentage points, plus lens inside to absorb light, finally into the eyes of the light may lost more than 50%, not only imaging, dark and reflected light multiple reflections inside the telescope, section through the eyepiece into eyes, form a glare, interfere with the observation, would also reduce the image contrast.

When a 1/4 wavelength film is coated on the surface of the lens, the transmittance of this wavelength light can be improved. The common blue film is the simplest permeation film.

There are a number of red-film telescopes on the market, and these mirrors are generally only suitable for high-reflecting environments such as snow. Daily use will cause problems such as dim and off-color images. In order to reduce the cost, some mirrors only coated part of the lenses (usually the outer surface of the objective lens and the eyepiece). Good mirrors coated all the lenses. The simplest way to judge the coating quality of a mirror is to observe the reflection of the lens.

4. Nitrogen-filled waterproof and all-metal structure

Many telescopes will highlight the features of nitrogen-filled waterproofing, which is typical of high-quality telescopes. It means that water vapor and dust can't get into the inside of the telescope. Protected by dry nitrogen, the inside lens will not be foggy or moldy, which ensures the long service life of the telescope and is suitable for extremely harsh environments. So general military telescopes, like the type 95, are designed to be nitrogen-filled and waterproof. Some civil mirrors also adopt this design. Don't think it is a very expensive measure, only the 6X18 with a price of only 100 yuan is also adopted.

It is also important to note that this design does not directly lead to optical improvements, the only improvement is durability. At the moment, both Russian and domestic conventional telescopes are made of all-metal structures (typically aluminum, and older ones tend to be made of brass), which have the advantage of being very durable, without the drawback that plastic ages and deforms easily. Although it can not directly bring the improvement of optics, it can guarantee that the optical parts are always in a precise position, thus bringing long-term stability of optics. Because of the high cost of the metal structure, only military telescopes and advanced civilian mirrors use this structure, while ordinary civilian telescopes often use a mixture of metal and plastic.

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

The image seen through a typical Kepler telescope is reversed from top to bottom. In order to meet the needs of ground observation, a set of prisms must be added to the objective lens (to make the distant object into a real inverted image) to transform the inverted image into a real one. There are two kinds of prisms, porro prism and roof prism.

The advantages of porro prism are simple structure, high light transmittance and good imaging quality, but the size of telescope is large.

In order to overcome this shortcoming, the reverse porro prism can be used for image rotation, but it brings a new problem. The objective lens is too small to be used in low-illumination environment.

The biggest advantage of roof prism is that it can minimize the size of the telescope and the weight of the telescope. However, this prism is complex in structure and its light transmittance is 5% lower than that of porro prism. It needs to be coated with phase film.

6. Exit pupil diameter and distance

The diameter of the exit pupil is the size of the spot formed behind the eyepiece by the image through the telescope.

The telescope an important indicator, it represents the telescope can reach into - like - light - degree, typically an exit pupil between 2.5 mm to 4 mm telescope, suit to use in the day, and 4 mm to 7 mm between the telescope, and low illumination environment can still be observed in the day, and while the telescope under 2.5 mm, even during the day, the image brightness is low, and the human eye pupil alignment difficulties, observation of comfort is poor. The simple formula for calculating the exit pupil is that the objective lens diameter/multiple = the exit pupil diameter. For example, with the 7X50 telescope, the exit pupil diameter =50/7= 7.14mm. Pupil distance is the farthest distance from the eyepiece that the eye can see from the whole field of view.

This is also an important parameter of the telescope. If the exit pupil distance is too short, the eye must be close to the eyepiece to see the entire field of view. The eye will be very tired. Telescopes with pupil distance are suitable for the wearer. Some extended-pupil binoculars, such as the 8X42 telescope, are designed to be retractable for people with normal vision as well as for those with eyes.

7. How many times to enlarge is suitable

Generally speaking, the size of the ordinary handheld telescope should not be more than 10 times, because the hand holding the telescope will be slightly jittery, and when the telescope is observed, it will be magnified accordingly. The higher the dithering is, the bigger the dithering will be. That's why military telescopes are typically only 6, 7 and 8 times larger. Even a telescope on the tripod observation, multiples is also limited, because the atmosphere is affected by the ground buildings and other factors, the density is not homogeneous, when the distance is observing the reflection of content (or transmission, emission) uneven density of light through the atmosphere, can produce light refraction, cause the image blur. And the higher the telescope's magnification, the clearer the blur. Generally speaking, in low-altitude cities, the telescope's multiple should be no more than 40 times. In addition, if the amplification factor is large, the actual field of view is small. In this way, it is not conducive to finding and finding targets, especially for the observation of frequently changing targets. Even if the target is found, tripod a little bit shake can easily lose the target. For those without automatic tracking devices, manual adjustments are often required to keep the target in view. In addition, to increase the multiple, the aperture of the objective lens must be improved accordingly to ensure sufficient brightness of the image. The human eye cannot see clearly without brightness. As a result, the size, weight, and price of the telescope would increase dramatically. So, should not be one-sided pursuit of multiples, as for the elevation of many times the telescope on market, multiples usually have exaggerated, there are a few multiples, appear this kind of situation the reason is very simple, the primary aim of factories and businesses is the pursuit of profit, rather than popularize knowledge of correct telescope, since the market need high power telescopes, and guide the correct consumption view, might as well to cater to this kind of consumption to the simple.

How far can a telescope see

This is a favorite question of friends who are new to telescopes. The answer is infinity. The real intention of this question should be to ask the telescope what its resolution is. This involves three factors:

1. How to observe the environment, including light intensity and direction, contrast, atmospheric stability and transparency, etc.

2. The quality of the telescope itself, including specifications, categories, precision, etc.

3. The visual condition of the observer after correction and the degree of proficiency in using the telescope.

These three aspects are basically uncertain. If the discrimination ability of the telescope itself is only considered, the recognition ability of the telescope with a larger size and a higher multiple is generally higher. The resolution of the porro prism telescope is higher than that of the roof prism telescope, and the resolution of the coated mirror is higher. It needs to be emphasized that the telescope multiple is only one of the many related factors that affect resolution, and the blind pursuit of large multiple is not desirable.

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