We found that no matter the telescope or camera lens, manufacturers will spare no effort to put on the "color coat" for the lens. Why ?

Different coatings can bring different gains, such as filtering out more interference light, improving the experience of use, protecting lenses, prolonging service life, etc. But at the same time, there will be reflection on the surface of the lens. The more light is reflected, the less light is entered. Then the object we observe will be dimmer, so coating is very important.

How much improvement can coating bring to the telescope and which coating is the best?

To sum up, there are three key points:

1. Coating can increase the transmittance of glass, improve the brightness of observation, reduce the glare and ghost of lens, and improve the contrast of observation.
2. The common color of antireflective film is red film, blue film and green film, and its transmittance is red film < blue film < green film. Full surface multilayer coating (FMC) is the best coating.
3. When purchasing a telescope, pay attention to the degree of reflection of the lens. The dimmer the reflection, the better the coating. Otherwise, the worse.

When light passes through the optical glass lens of a telescope, it will not only refract, but also reflect. Reflection can cause light loss and darken the image. If there are multiple lenses, more light will be reflected, which is quite large.
If the internal extinction is not good, part of the light reflected many times will penetrate the eyepiece and enter the eyes, resulting in the reduction of the contrast image, and the feeling of gray and fog is obvious. Therefore, coating is needed to reduce the reflection. By controlling the material of the film, the reduction or enhancement of the reflected light can be controlled to achieve the effect of antireflection or antireflection, so as to increase the transmittance of the lens or prism and improve the observation brightness.

But for the prism reflecting light, the higher the reflectivity is, the better it is. It also needs coating to improve its performance. Different coating schemes need to be selected for different lens functions.

Full surface multilayer coating (FMC)

There are many layers of antireflection and antireflection coatings on the surface of glass in contact with air, which is the best coating.

The telescope with full surface multi-layer broadband coating has the best light transmittance and bright field of vision; the color restoration is balanced and realistic, with little reflection, which can control glare and ghosting well, and can easily see the objects in the shadow even in the backlight or bright background.

The coating needs to cover several or dozens of nanoscale films on the glass surface, which is a great challenge to the manufacturing process, so the price is relatively expensive. FMC coating is usually used for advanced telescopes.

Little knowledge: under the same level of conditions, Paul's telescope has a brighter field of vision than roof's telescope, so its quality performance is higher. Because the roof type telescope has more internal glass structure than the Paul type telescope, it has more reflection and greater light loss, so the transmittance of the whole telescope will be reduced, but the top telescope is generally the roof type telescope.

The lenses of the above five specifications are all double cemented. Among them, 23F62 is glued blue film, 26F35 is glued green film, 30F120 is glued blue film, 34F106 is glued blue film, 35F135 is glued green film, and 28F100 is glued green film. Suitable for DIY binoculars, viewing mirrors, starfinders, etc.

In a word, the main significance of coating is not only to improve the brightness, but also to reduce the glare caused by the reflection of our different lenses, reduce the dazzle and turbidity caused by the reflection of the telescope, improve the color saturation, make the observation object more prominent, and let us more immersed in the viewing experience of the telescope.