In the top imported 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 transmittance of the entire telescope in the entire visible light band The luminous 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 processing method is called film matching. The easiest way to judge whether a coating is good or not is to see if the reflection is weak. If it is faint and dark, it is a good coating. From a photography point of view, it is difficult to photograph a good coating because of weak reflection.
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 Amoy brand telescope we bought for disassembly and analysis is only coated with blue film on the surface of the objective lens, and the prism and eyepiece are all uncoated, so the reflection loss is very serious. The above analysis is only for the porro 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 Porro prism telescope. In fact, when light passes through the lenses 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. For example, the glass air surface of the Forester 10X50 only loses 5% of the energy reflected by the air surface, but the glass absorbs more than 15% of the light, and the light loss of the whole telescope is more than 20%. The light transmittance of the machine is less than 80%, while the light transmittance of a well-known model roof 8X42 is about 70%!
Therefore, different coatings can bring different benefits, such as filtering out more interfering light, improving the user experience, protecting the lens, prolonging the service life, and so on.