Purchasing dual-tube telescope

Some people think that the stars must be clearly viewed through astronomical telescopes and astronomical observation at a high rate. In fact, these are incorrect ideas, however, there is no dual-tube telescope, which is not only difficult to carry, but also does not have the advantages of the dual-tube telescope's wide field of view, just like, etc. The astronomical telescope's high rate search is inconvenient, although there is a star-searching mirror, however, almost all the star-seeking mirrors are inverted images. Therefore, for beginners or gamers, the dual-tube telescope is definitely important in usage. Do not think that the dual-tube telescope is only a low-level, entry-level telescope.
The dual-tube telescope is the most basic equipment for an astronomy enthusiast or a bird watching enthusiast.
Basic knowledge of Dual-tube Telescope (purchase and use of dual-tube telescope instructions ):
(1) caliber: the diameter of the objective lens. The aperture size determines the light collecting force and resolution force of the telescope (and determines the telescope price at the same time). The larger the aperture, the higher the resolution force, the more expensive it is. Take the 7x50 dual-cylinder mirror as an example. 50 represents a diameter of 50mm. 2 3cm dual-tube mirrors are suitable for viewing birds during the day, but lack of light collecting power is not suitable for astronomy, handheld dual-tube mirrors are suitable for less than 5cm, more than 5cm need to be supported by a tripod.
(2) magnification: The 7x50 dual-cylinder mirror is used as an example. The 7X represents a magnification of 7 times, and the recommended magnification of the handheld dual-cylinder mirror is less than 10 times. The higher the magnification, the better. The higher the magnification, and the higher the shaking will affect the image quality. It requires support from a tripod and is inconvenient to use. The narrow field of view will make the novice feel afraid of a needle in a haystack.
(3) Exit Pupil: Take the dual cylinder a little farther away from you, and the small bright circle on the eyepiece is the pupil diameter. The larger the diameter, the brighter it will be. Formula: Shot pupil diameter Mm = caliber mm/rate. Because the pupil of the human eye can be enlarged to about 7mm at night, the closer the pupil diameter is to 7mm, the higher the brightness. The more suitable it is for astronomical use, but the pupil of the human eye is only 2 to 3mm during the day, therefore, during the day, you do not need to use a pair of large pupil tubes to view birds. As you grow older, the more difficult the pupil to enlarge to 7mm. The viewing pupil can be used to check the quality of the prism. The bak4 prism is the best, and there is no prism masking. The seeing pupil is a complete circle, and the viewing image is clear and bright. The prism of BK7 has a prism shadow phenomenon, and the circumference of the circular pupil will produce angular (edges ).
(4) field of view: the range seen through the telescope is divided into two types: the actual field of view rfv and the eye field of view (Angle of View) AFV, which are usually expressed as degrees. Formula: actual field of view X rate = eye field of view (angle of view ). If you think a mirror tube looks narrow and has the feeling of looking at the sky, the reason is that the angle of the eye is not large enough. The narrow angle or increased magnification of the eyepiece will make the actual field of view smaller.
(5) coating: A special metal compound is plated on the lens surface to reduce the reflection and increase the light transmittance. Generally, at least one layer of magnesium fluoride (MgF2) is coated, it looks pale blue. Multi-layer film shows a touch of green light or blue violet light. The optical lenses and Prism inside the higher telescope use full multi-layer film (fully Multilayer Coating ), this greatly improves brightness, contrast, and resolution. Pick up the dual cylinder and let the objective lens face itself and observe the reflected light. The darker the reflection light, the better. It means the higher the light transmittance. some top-level astronomical products, from the objective lens, it seems like seeing a black hole. Reflected light colors from dark green, dark blue to dark purple. However, the so-called Ruby coating is the coating of a reflection red wavelength on the surface of the objective lens. This coating will only reduce the image light and color bias, so that the image is dimmed and green, there are no infrared and night vision capabilities that poor manufacturers boast. Do not be deceived.
(6) eye relief: it is also known as the visual line of sight. It refers to the distance from the binocular vision to the Focal Point. The longer the Pupil Distance is, the whole field of view can be seen on the side a little farther away from the binocular vision, it is suitable for use with glasses. The Exit Pupil Distance is short (less than 12mm. Most of the dual tubes on the market have 8 ~ 14mm, some ultra-long mydrion are more than 20mm away from HP (High Eye point) telescopes.
(7) imaging quality: resolution force, contrast, color reduction, chromatic aberration and aberration, etc. These are more experienced users who will pay attention when purchasing dual-cylinder.
(8) structural features: the dual-cylinder mirror optical system can be divided into two types:
1. porro Prism: two groups of PRISM are used internally to convert the image into a portrait. The image is large and easy to create. There are forward Paul and reverse Paul designs.
2. Dash Prism: Also known as roof Ridge Prism, Dash is smaller and easier to carry. The cost of the same optical quality is higher than that of Paul.
(9) other signs or nouns:
CF (center focusing) And if (individual focusing): the focus mode can be divided into two types: Cf central focusing and if dual-eye independent focusing. The dual-eye independent focusing design is inconvenient for focusing during viewing, the astronomical objects are all at an infinite focal length, so they have no impact on astronomical observation. If the CF focus Linkage Mechanism considers waterproofing, it requires a considerable technical level, therefore, the IF type is used in the air-tight waterproof telescope.
WP (water proof) Airtight waterproof design: Vacuum inner mirror body filled with high-pressure dry nitrogen, in order to avoid wet air intrusion and long mold, it is suitable for high mountains without light damage, but the temperature difference is large and easy to expose, and the view is good but wet seaside use, so professional astronomy and bird watching are very fond of, but the waterproof mirror price is relatively expensive.
Ed (Extra Low dispertion): some advanced telescope objective lenses use the Ed ultra-low dispersion glass to minimize the color aberration. However, the prism and the eyepiece still produce chromatic aberration.
Aspherical Lens: This optical technique can eliminate the blurred deformation of the image generated by the spherical aberration at the periphery of the field of vision, so that the visual field can reach a consistent sharp image from the center to the periphery, excellent optical quality.
Zoom, for zoom telescopes of the same optical quality, the price may be 5 times the fixed price. As a result, like Leica, Zeiss and other top-level well-known manufacturers, there are few zoom models available.