Telescope Coatings AR Coatings and Light Transmission

When light passes through the interface between glass and air, the light will not only be refracted, but also reflected. Reflection will not only lose energy and cause the image to become darker, but also form multiple reflections inside the telescope. If the internal extinction is not good, part of the light reflected multiple times will pass through the eyepiece and enter the eye, resulting in a decrease in the contrast image of the image and a clear sense of fog
The surface of the early telescope lens is not coated with anti-reflection coating, and the white glass surface will lose about 4% of the energy per reflection
Zeiss company proposed the concept of anti-reflection coating in the 1930s, coating a layer of magnesium fluoride (mgf2) on the surface of the lens, the thickness is a quarter of 550nm, so that the middle band of visible light can be completely transmitted, visible light The light in the edge band cannot be completely transmitted and there is residual reflection. The reflection in the blue band is relatively strong, so the coating we see shows blue reflection, commonly known as blue film. The average reflectance of the blue film is about 1.5%. This coating is the first generation of anti-reflection coating, because there is only one layer of coating, also called single-layer coating.
The characteristics of full transmission, by increasing the number of coating layers, high transmittance can be obtained in the entire visible light band, and the transmittance curve is flatter and close to 100%. Generally, 2-layer or more than 2-layer coatings can be called multi-layer coatings, because the reflection of this coating is green, commonly known as green film. The average transmittance of the green film is above 99.5%, and the corresponding reflectance is only 0.5%.
In top-level telescopes, the anti-reflection coatings coated on the contact surfaces of each glass and air are not consistent. Different lens surfaces correspond to the highest transmittance of different wavelengths. In this way, the entire telescope’s light transmission in the entire visible light band The power curve is not only very flat but also close to 100%, which can obtain bright imaging without color cast. When we observe the reflection of the telescope, we can see that different lenses reflect different colors. This method of processing is called membrane collocation
From the above simple analysis, it can be found that if all the mirrors of the telescope are not coated, the reflection will lose 40% of the light. If all the mirror surfaces of the telescope are coated with blue film, the reflection will lose 15% of the light. They are all coated with green film, and the reflection only loses 5% of the light. However, due to cost considerations, many telescopes are only coated with blue or green coatings on some surfaces, and some surfaces are not coated, so it is difficult to calculate the actual reflection loss. The above analysis is only for the Paul prism telescope. For the roof prism telescope, the prism has 2 more glass-air reflective surfaces than the Paul prism, and there are 2 more glass-air reflective surfaces of the focusing lens, at least 4 more reflections In addition, if the reflective surface of the roof prism is silver or aluminum, there will be 5-10% more reflection loss, so generally speaking, the brightness of the low-end roof prism telescope is darker than that of the low-end Paul prism telescope. In fact, when light passes through each lens of the telescope, in addition to the light reflected by the glass-air contact surface, the reflective film of the roof prism will absorb the light, and the light will also be absorbed by the glass when it passes through the glass. Therefore, the light transmittance of the whole telescope will be further reduced