Level 1 completely dark sky: the ecliptic, the zodiac, and the sunshine can see. The ecliptic reaches a striking degree, and the zodiac extends to the entire sky. Even with just the naked eye, M33 is an extremely clear object. The galactic region in Scorpio and Sagittarius can cast a faint shadow on the ground. Limitless eyes of the naked eye can reach 7.6 to 8.0, etc.; Jupiter or Venus in the sky will even affect the naked eye's adaptation to darkness. Aura (a natural glow that normally appears on the horizon at 15°) is also stably visible. Using a 32-cm telescope, you can see stars as dark as 17.5 through hard work, and use a 50-cm telescope to reach 19 at moderate magnification. If you are observing on the grass surrounded by trees, you can hardly see your telescope, companions and your car. This is an observer's paradise.
Level 2 Typical real dark observations: faintly visible along the horizon. M33 can be easily seen. The summer galaxy is rich in detail and its brightest part looks like a marble with lines in the ordinary binoculars. The light of the ecliptic before dawn or after dusk is still very bright, and it is possible to cast shadows of darkness, which is quite yellow as compared with the blue-and-white galaxy. Any cloud that appears in the sky is like a hollow in the sky. Unless under the starlight, you can only see your telescope and surrounding things in a blur. Many globular clusters in Messier objects are targets that can be seen directly with the naked eye. The naked eyes can reach 7.1 to 7.5, and the 32cm telescope can reach 16 to 17, etc.
Level 3 starry sky: There are some signs of light pollution in the direction of the horizon. Clouds are slightly illuminated at the horizon, but dark at the top of the head. The galaxy is still rich in structure, and globular clusters such as M4, M5, M15, and M22 are still visible to the naked eye. M33 is also easy to see. The ecliptic light is obvious in spring and autumn, but its color is hard to discern. Telescopes 6 to 9 meters from you have become blurred. Limits of the naked eye can reach 6.6 to 7.0, etc., 32cm reflector telescope can reach 16, and so on.
Level 4 Rural/Suburban Transition: Light pollution is visible in the direction of the populated area. The ecliptic light is clearer, but the extent of the extension is small. The Galaxy still impresses people but lacks most of the details. The M33 is hard to see and is barely visible when the ground level is greater than 50°. Clouds are lightly illuminated in the direction of light pollution and are still dark in the direction of the head. You can recognize your telescope within a distance. The limit of the naked eye can reach 6.5, and the 32-cm telescope can reach 15.5 at moderate magnification.
Level 5 suburban sky. Only in the best night of spring and autumn can you see the ecliptic light. The galaxy is very weak and not visible in the horizontal direction. The light source is obvious in most directions. In most of the sky, clouds are brighter than the sky background. The limit of the naked eye is 5.5 to 6.0, etc., and that of the 32 cm reflective telescope is 14.5 to 15.
6 bright suburban sky: Even on the best nights, the ecliptic cannot be seen. The galaxy can only be seen in the zenith direction. Levels below 35° in the sky emit grayish light. The clouds in the sky are brighter everywhere. You can easily see the eyepiece on the table and the telescope on the side. Without binocular M33 is impossible to see, for the naked eye M31 is only a relatively clear goal. With the naked eyes and other magnitude 5.5, 32cm telescopes can see stars as dark as 14.0-14.5.
Level 7 Suburban/City Transition: The entire sky presents a blur of gray and white. Strong light sources are clear in all directions. The Galaxy is completely invisible. M44 or M31 barely visible and not very visible. The cloud is bright. Even with medium-sized telescopes, the brightest Messier objects still look pale. After a real attempt, the naked eye is limited to a magnitude of 4.5 to 5.0, and the 32-cm telescope can barely reach 14.0.
Grade 8 city sky: The sky emits white, gray or orange light, and you can read newspapers without difficulty. M31 and M44 can only be seen with the naked eye by experienced observers only on the best nights. With medium-sized telescopes only the brightest Messier objects can be found. Some familiar constellations have become illegible or disappeared entirely. In the best case, the naked eye has a limit of 4.0 to 4.5, and the 32 cm reflector has a 13 and so on.
Level 9 Downtown Sky: The entire sky is illuminated, even in the direction of the zenith. Many familiar constellations are no longer visible, and weak constellations such as Cancer and Gemini simply cannot be seen. Maybe in addition to the Pleiades, no Messier objects can be seen with naked eyes. Only the moon, the planets, and some bright star clusters can bring some fun (if observed) to stargazers. The limit of the naked eye is 3.0 to 4.0 or less.
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A mirror is an optical element that uses the law of reflection. Mirrors can be divided into plane mirrors, spherical mirrors and aspheric mirrors according to their shapes; according to the degree of reflection, they can be divided into total reflection mirrors and half mirrors (also known as beam splitters). According to the coating, it can be divided into aluminum-plated mirrors, silver-plated mirrors, and copper-plated mirrors.
Aluminum-plated mirrors have relatively high reflectivity from ultraviolet to infrared. The optical film is more stable than other metal materials, because the surface of the aluminum film after coating can produce an aluminum oxide film in the atmosphere. Aluminized The reflector uses magnesium fluoride and lithium fluoride as a protective layer in the ultraviolet band, and uses silicon monoxide or aluminum oxide in the visible and infrared bands.
Silver-plated mirrors have high reflectivity in the visible and infrared bands, with small polarization effects, silver is easily affected by sulfides, and has a short service life.
The gold-plated reflector has a higher reflectivity in the infrared band. The lightness and stability of the gold-plated reflector is better than that of the silver-plated reflector, but it cannot be wiped when it is stained.
The use of a high reflectance mirror can double the output power of the laser; and it is reflected by the first reflecting surface, the reflected image is not distorted, and there is no ghosting, and it is the reflection of the front surface. If an ordinary reflector is used as the second reflecting surface, not only the reflectivity is low, the wavelength is not selective, but also the ghost is easy to produce. However, the use of a coated film surface reflector not only has high brightness, but also is accurate and unbiased, the picture quality is clearer, and the color is more lifelike. The front surface mirror is widely used for optical high-fidelity scanning reflection imaging.
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