Dual-barrel telescope

The dual-tube telescope is a useful astronomical observation tool. You can use it to watch a game, concert, or birds in the sky. You can also use it to enjoy the galaxy, the crater on the moon, several satellites around Jupiter and countless stars 2 million light years away. Many people mistakenly think that the dual-tube telescope is not used for astronomical observation. In fact, it is a favorite tool of many senior astronomical observers. For beginners, It is the entrance to astronomical observation. The dual-tube telescope is not expensive. You only need to spend hundreds of dollars to buy a pair of good dual-tube telescopes.

Basic knowledge
Before purchasing a dual-tube telescope, you should first understand its features and specifications. What you need to pay the most attention to when purchasing a pair of telescope for astronomy is the "caliber 」. The aperture refers to the diameter of the telescope lens (front lenses. The larger the caliber, the brighter the imaging. For an astronomical dual-barrel telescope, the lens diameter should be at least 40mm. The tiny 20mm-30mm dual-tube telescope is suitable for daytime views, but it is not suitable for astronomical purposes because it cannot gather enough light. How can we know the diameter of the dual-tube telescope? Very simple.
Each pair of dual telescope is labeled with a set of numbers such as 7x50. The first digit "7" on the dual telescope specification is the "magnification", and the second digit "50" is the diameter of the lens. The seven-fold model is a best-selling model that brings every item you watch closer to seven times. You can also choose 10x and 16x. You may think that the high rate is necessary for astronomical purposes. The 7X dual-tube telescope is enough, and we will discuss the advantages of 7X over most high-rate models.

field of view)
you can see a set of data, such as "367 feet @ 1000 yards" or "120 m @ 1000 m", in almost every small dual-barrel telescope manual. This string of numbers represents how wide a landscape is 1,000 yards (or 1,000 meters) in the eye view. This is one of the ways to measure the field of view size. It is not appropriate to use the "hundreds of feet at 1,000 yards" method to measure the horizon of the sky. Astronomy uses degrees to measure the field of view. Twice the diameter of the full moon. Seven Degrees equals the diameter of 14 full moon, and is also a typical field of view of the dual-tube telescope. The sky shown by the high-speed model is small (3 to 5 degrees), and the wide-angle model is much more visible (8 to 10 degrees ), divide the feet data in the "feet @ 1000 yards" type by 52. 5 can be converted into degrees. The "meters @ 1000 meters" type is divided by the number of meters by 17. For example, a dual telescope with a 367 feet @ 1000 yards field of view has a 367/52. 5 degree field of view, about 7 degrees. The imaging of the Star points in the peripheral field of the wide-angle model is usually distorted and blurred, reducing the horizon, which is difficult to balance. In addition, the wide-angle model generally shortens the line of sight. The actual field of view (actual and apparent fields of view) is associated. Take the valid field of view (for example, 70 degrees and divide by the rate (for example, 10x), then you will get the value of "actual field of view". In this example, it is 7 degrees. Therefore, the effective field of view is improved, and the actual field of view is improved. However, the actual field of view will decrease when the magnification is increased.

exit pupil
you will also find an exit pupil in many dual-tube telescope manuals). It is the width of the light that is emitted from the mesh. You can divide the aperture (unit: mm) by the magnification to calculate the pupil. For example, 50/7 (7mm) Exclusive pupil occurs in Type 7x50. All 7x50 models have a pupil of 7mm. The pupil of all 7x42 types is 6mm, and so on. You will find that many of the telescope that are recommended for astronomy have a 7mm shot pupil, which makes sense. When your eyes are used to the darkness of the night, the pupil will make more light. Human pupil can expand to 7mm. So as long as the dual-tube telescope emits a 7mm light cone into your eyes, you will probably see the brightest imaging when the eyes gather light to maximize the effect. All the dual-tube telescopes designed to cope with low-light environments have a pupil shot at 7mm. Including: 7x50, 8x56, 9x63, 10x70, and 11x80. Generally, it is most suitable for astronomical observation. However, when you are old, your eyes will not open. A person in his early thirds has a maximum pupil of about 6mm. When I was over 40 years old, I fell between 4.5 and 5mm. If your pupil size is limited to 5mm or 6mm, using a 7mm-wide dual-tube telescope with a light cone will waste some light. Some light cannot enter your eyes, and the dual-tube telescope's light collecting power cannot be fully utilized. For older people, the exclusive pupil 5mm or 6mm of the dual-tube telescope is a good choice. These models include the best-selling 7x42 and 10x50.

Eye relief)
Manufacturers are now paying more and more attention to the "good line of sight" Specification. This specification means that your eyes need to be close to the MIIT to clearly see the distance of the entire field of vision. The good line of sight data is more important for the glasses of myopia. People with myopia can take off their glasses and adjust the focal length of the dual-tube telescope to compensate them, but it is quite inconvenient-wearing glasses and looking at the night sky with clear sight, it is indeed a pleasure to quickly bring the dual-tube telescope into sight without moving towards the target, and then maintain a complete field of view. In addition, if you have several levels of astigmatism at a Glance or in both eyes, your glasses won't be able to be taken off. Wear glasses to see the complete visual eye at least 14mm to 15mm good line of sight, good line of sight from the back of the eye cup start. Models with a good line of sight smaller than 8mm or 9mm may be difficult to observe, even if they do not wear glasses. You have to keep your eyes close to the MIIT to see all the fields of view. a long period of time, it will cause pressure and the grease and dust on your eyelashes will be contaminated with the MIIT.

Prism (prisms)
The body of the dual-tube telescope contains a combination of prism, which can refresh the optical path and enable the imaging to stand upright. The dual-barrel telescope can be divided into two camps: The porro PRISM system and the roof prism system. The porro PRISM system has a Z or N-shaped appearance. The roof prism system is a straight line. Porro prism models are generally cheaper than roof prism models. Roof prism must be well performed, and processing and fixation must require a precise allowable error, resulting in a high production cost. However, the roof prism model tends to be more dense. Generally, the porro prism model is used for astronomical purposes. The porro PRISM system often performs imaging slightly. However, if you use a cheap model (roof prism) to target a bright star or a planet, the stars will experience the annoying dingtalk light. The dingtalk diffraction guide is because the roof prism splits the image into two halves and then combines them. However, you will never find this effect on the advanced roof prism model. If the porro prism is a dual-tube telescope prism that uses the Barium Crown Glass (bak4) as the material, the field of view will be relatively complete. If the prism uses borosilicate glass as the material, the level is low, but the Brightness Difference in practical imaging is very small.

Coatings)
Most dual telescope optical lenses have at least one layer of magnesium fluoride. It will make the lens light blue. Coating can improve the light penetration rate and reduce the internal reflection (internal reflections ctions) and mirror flash (Lens flares), and push the brightness and contrast up. Cheap models usually only rely on external mirrors for coating. Although this model has a certain level, you will find that expensive models have the best quality. In any case, internal reflection is generated and some comparison is lost. These minimum coating models will cause a 40 percentage loss before the incident light reaches your eyes. More advanced dual-tube telescopes will be coated with common Coatings on most lens surfaces, and only one or more sides adopt multi-layer coating (usually plating on the outside of the lens surface in contact with air ). In the best dual telescope, all lenses containing prism will be coated with multilayer film. The light loss of these models will be reduced to a very small percentage of five. The multi-layer coating lens looks dark, and the color is green or dark purple red.

In recent years, there have been some dual-tube telescopes coated with red reflective films. Dishonest shops also say this is an infrared model, especially suitable for evening use. In fact, the layer of red coating reflects a large amount of light out of the telescope, greatly reducing the amount of light reaching the eyes. Therefore, do not purchase this type of telescope.

Mechanical features)
Most of the dual-tube telescopes have a central focusing device that can simultaneously move two pairs of Enis. These models are the easiest to operate. There are some dual-cylinder telescopes, which have two pairs of yundis with a focal ring. These independent focus models are tightly sealed to defend against moisture, but focus is less convenient. Then again, when you look at the stars, every target is infinitely far away, and the focal length does not need to be adjusted frequently. It is advantageous to lock the telescope on a tripod when it is used at night. Many dual telescope bodies are attached with threaded holes to connect a tripod adapter that is normally compatible. If a threaded hole is missing from the telescope, you may need to use a adapter that can be clipped to the central connecting rod between two frames or a coating package to enclose the entire body. The dual telescope designed for maritime purposes is often waterproof. These models are in a closed structure, and the internal air is extracted and then filled with nitrogen. Waterproofing is not necessary, but it has a place in astronomy-we expose the dual-tube telescope to the wet and cold night climate, and then bring it back to the warm house, more moisture is generated. After misuse, moisture and mold will gather in the dual-tube telescope, the optical lens paste into a piece, the repair cost is high.

First Choice for Star watching
The dual-tube telescope between the range of 40mm and 63mm (Standard Specification) is of the dense type, facilitating the "burst" scanning of the stars. A 70-mm or larger large dual-tube telescope requires considerable effort (you can frame it on a tripod or on a chair dedicated to the dual-tube telescope ).

Standard Specifications (40 to 63)
The 7x50 model has always been regarded as the best all-around dual-tube telescope for astronomy, with a pupil shot of 7mm. Generally, the field of view is 7 to 10 degrees, which is suitable for handheld applications. The application is also good during the day, although it is a little larger than the day. Because the models are the most popular for maritime purposes, many models have waterproof functionality and advanced optical design. 10x50. This model looks a little darker than the 7x50 star background (only 5mm of pupil shot), but it will be better to look at the monthly table and cluster. The extra magnification is also helpful for the observation of dark stars. Despite the high magnification, many 10x50 models show the same star ranges (about 5 to 7 degrees) as 7x50 models ).

10x50 models
The disadvantage of this model is that it is not easy to hold hands and may weaken the clarity of the target imaging you are searching. X dual-tube telescope should use a tripod. The high-rate model also has a short line of sight, which may cause inconvenience when wearing glasses for observation.

7x42, 8x40, or 8x42 Models
If you are over 40 years old, the 5mm to 6mm injection pupil is a good choice for 7x42, 8x40, or 8x42 models. This type of Dual-tube telescope is lighter than the 7x50 model, and is also suitable for daytime views and natural observation. Some models can focus close-up on a few feet away, which is ideal for watching birds. If you want a full-featured dual-barrel telescope, this model is competent.

8x56 and 9x63 Models
This model is a compromise between a convenient 40-50mm model and a large 70-80mm model. This is especially true for people who do not like to use a pair of telescopes for daily purposes as night telescopes. Very few manufacturers supply this type of model, the price of good quality is very expensive, to choose a larger lens and limited production products.

10x40 and 10x42 Models
This model emits a pupil of 4mm, which can certainly be used for astronomical observation. However, it is mainly designed for many bird watching users to use high-rate models in daytime environments. If this is your main goal, watching the second place, 10x40 is a good choice, and the roof prism advanced model is particularly worth recommending.

What can I see?
Through the dual-tube telescope, you can see many distant stars, clouds, and even several bright but galaxy. You can also look at Jupiter's satellites and observe their movements every night. You can observe the profit and loss of Venus and the moon. Others include the new moon, Uranus, Neptune, mercury, minor, lunar eclipse, eclipse, and comet. What can't be seen in a dual-tube telescope? You cannot see the details of the monthly table or the planetary surface. If you want to see Jupiter's clouds and earth-star rings, you need a single telescope.

Roofprism and Abbe prism
Porroprism is the most commonly used prism. The dual-cylinder lens with proprism is wider, and the spacing between the two objective lenses is greater than that of the precision lens. In this way, the stereoscopy is strong when observing near objects. Some compact dual-cylinder mirrors use an inverted prism. The spacing of objective lenses is smaller than that of the precision lens, and the stereoscopy decreases. Proprism is easy to manufacture and cheaper than Ridge Prism of the same optical quality.

Roofprism is a compact, dual-cylinder lens that is usually used for compact dual-cylinder mirrors because it is small in size and can place the objective lens and the eyepiece in a straight line. Compared with the proprism, The Ridge Prism has two main disadvantages: one is the loss of light and the image is darker; the other is the high requirement on assembly accuracy, which is difficult to manufacture and expensive, well-manufactured roof prism can catch up with but won't surpass proprism in terms of performance.

The key to the Ridge Prism is that the ridge Surface exists. The so-called ridge surface is a reflection surface in the optical path, which is shaped by a ridge shaped by two reflective surfaces. The two edges are in the middle of the optical path, so some Ridge Prism can see a dividing line in the middle, but it can also be understood as dividing the beam into two halves and then combining them. Make up two mirrors at a right angle to form a ridge surface. Try it on your own and see if it is exchanged with the left and right sides of a common mirror? The principle of the most common loose prism should be reflected six times. Correspondingly, there is also a kind of Abbe prism commonly used by modern Zeiss telescope, which is also a kind of roof prism with a slightly larger length, but it only needs to be reflected four times without a reflective layer, therefore, the efficiency is similar to that of normal Paul prism.