Why are telescopes coated?Apexeloptic
Telescope coating is to improve the transmittance of this wavelength of light, the more common blue film, which is one of the simplest antireflection films. Why are telescopes coated? Next, we will learn about the reasons and some relevant knowledge of telescope coating by studying this article.
1. Why are telescopes coated?
To minimize light loss, when light passes through a telescope’s lens, some light is reflected off its surface and lost. Manufacturers deposit thin chemical coatings, or coatings, on the surface of the lenses to reduce reflection loss and improve light transmission. The quality of the coating is as important as the quality of the lenses.
Without coating, each lens could lose up to 5% of its light. Lenses with multi-layer coatings can reduce light loss by a tenth. As a result, poor optics may lose as much as 35% of the light entering the objective, while high-end designs may lose less than 5%.
The coating also improves image quality because light reflected inside the optical elements washes out details and blurs colors. Premium optical device manufacturers add up to 80 coating layers to optimize the transmittance of each basic color.
2. Telescope coating type
(1) Red film
Generally used in snow and another highly reflective environment, the use of red film telescope in the daily environment, there will be a serious problem of dim imaging, this coating is called an infrared telescope, in fact, with infrared ray without a dime, is completely deceive consumers.
(2) Green film
Someone called the broadband green film, or through the green film, is now one of the best domestic coating processes, this coating process makes the lens in different angles of different ribbon, this is the performance of multi-layer coating characteristics, this coating image is clear, high color reduction, if you choose, try to choose this green film.
(3) Blue membrane
Domestic use of one of the most widely used coating methods, compared to broadband green film, look some dim, blue film some plating three layers, good plating five layers, small factory production and only a layer of plating, so a penny goods truth, we all want to understand.
(4) Amber coating
In addition to these, there are also amber coatings, which use the blue light band as the central frequency to reflect green and red light and make the lens look amber. Amber coatings are also common in telescopes, providing a more balanced color reduction.
3. Influence of telescope coating on transmittance
In fact, the surface reflection of the coated lens is influenced by two factors: optical interference reflection caused by film thickness and natural reflection caused by refractive index. According to the interference theory, when two light waves with the same frequency and equal amplitude propagate along the same path, if the phase difference of half-wavelength is even times, the combined amplitude is 100%, then the light shining on the surface of the medium is completely reflected.
When the phase difference is an odd multiple of half wavelength, the combined amplitude is 0, then the reflection caused by the interference phenomenon is the smallest, and the reflection of the surface of the medium is closest to the natural reflection level determined by its refractive index. Anti-reflection film applies the principle of interference so that the reflected light and incident light on the surface of the film are interfered to cancel to reduce the intensity of reflection caused by light wave interference.
4. Improvement of telescope coating on color reduction
In addition to improved light transmittance, antireflective films should also have neutral color reduction. In order to design a single-layer antireflective film with neutral color reduction, the central wavelength is generally selected in the blue-cyan light segment of 480nm-540nm, and it is mainly 480nm-520nm.
Because of the absorption of light by the optical glass, the loss of light at short waves is relatively large. In order to balance the color, the central wavelength of single-layer anti-reflection film is as short as possible, and the loss of short wave light is as little as possible. However, the central wavelength is shorter than 460nm, so it is difficult to distinguish the difference of the central wavelength of the film color visually, so the central wavelength of the monolayer film shorter than 460nm is generally not used.
If the color factor is not considered, the central wavelength can choose 550nm yellow-green light, the reason is that the human eye is the most sensitive to this wavelength of light, and this wavelength is longer to dust and mist has a strong penetration. But at the same time, in this case, the diffuse reflection of short wave light is serious in the lens, which will seriously reduce the contrast of the scene after entering the light path, and form a serious gray fog phenomenon. In order to filter short-wave stray light, yellow glass is often used to filter blue stray light.