Color photos of the moon, you may not have seen it? It looks so beautiful, so real.

Although I didn't enjoy the Perseid meteor shower in the bright moonlight. But do you know that the familiar old face, the moon, can also shoot different colors! Under normal circumstances, we can only get a black-and-white image when shooting the details of the moon surface. In recent years, some technologies have enabled enthusiasts to photograph the moon in color and reveal some mineralogical features of the moon's surface.

1967, the French astronomer Ludao drew a map of Zhang Yue, which was different from the previous map of the moon in color. Lu Dao carefully observed the colors in different positions on the moon, and magnified and colored this color difference. This may be the only true-color hand-painted picture of the moon in the world. In this picture, we can see several areas with relatively rich colors on the moon: for example, the colors of Jinghai and Chenghai are significantly different, the colors near Plato crater and Hongwan are more complicated, and the colors around Aristarch crater are more obvious. Can the moon really have colors? Why do we look like the moon is black and white?

No matter how you look at it, the surface of the moon is always lacking in color. When we shoot the moon with an astronomical telescope, except for a short time when the moon rises or falls, the moon will appear orange-red due to Rayleigh scattering in the atmosphere. The normally photographed moon is almost without exception, and no color can be seen. The photos taken by Apollo mission on the surface of the moon have no bright colors, and the latest photos taken by Chang 'e probe are also monotonous.

This is because the rocks on the surface of the moon are not as rich as those on the earth. Basalt and breccia are common rocks on the moon. Their colors are black or gray, black or white, and some are slightly brown. That is to say, there is little difference in color, so it is impossible to shoot a colorful moon by normal means. The so-called "color moon" photos we can find now are basically false color images of height or other information.

In actual shooting, 9 to 16 photos are used for superposition. The second problem comes from the photo format of SLR cameras. Ordinary JPG format can only record 8-bit gray scale, and RAW mode can only record 12- 14-bit gray scale, so the gray scale and color scale will be broken in post-processing. Relatively speaking, the later shooting space of RAW is relatively large. The third problem comes from white balance, which is influenced by the response curve of SLR camera. When saturation increases, slight inaccuracy of white balance will lead to overall color cast. The general solution is to gradually adjust the color balance while increasing the saturation, so that the entire lunar surface is in a neutral gray state as far as possible. By solving the above three problems, most people can easily shoot full moon color images, but this is not the ultimate pursuit of this shooting method.

On the moon, there are complex geological stories in some places, which make the distribution of minerals around it have certain characteristics. Although the color difference of lunar minerals is very small, this small difference can be revealed by multi-spectral photography. The so-called multispectral photography refers to imaging the same area of the lunar surface with multiple filters with medium bandwidth or narrow bandwidth. Because the spectral characteristics of each mineral on the surface of the moon are different, the distribution of this mineral can be highlighted by finding the characteristic wavelength of this mineral and shooting it with the corresponding filter.

A camera specialized in shooting multi-spectral images of the moon is called a multi-spectral camera, which has the near-infrared shooting function. Common devices are miniCAM6F, equipped with corresponding filters, such as 1008nm near infrared, 9 10nm near infrared, 750nm near infrared and 4 15nm visible light. Sometimes longer wavelength filters are used, such as 1250nm, 1550nm, etc. , but this requires a professional near-infrared camera to complete. For amateur lunar photography, the vicinity of 900 nanometers is already the shooting limit.

Firstly, in the visible light band, the contrast between 4 15nm and 750nm can well reflect the distribution of titanium oxide, while the distribution of iron oxide needs to be described by the contrast between 750nm and 950nm. Therefore, simple color images of lunar mineral distribution can be taken with 4 15nm, 750nm and 950nm. There are absorption bands in the spectra of other minerals, so shooting in the corresponding bands can reflect the distribution of these minerals. For example, the absorption band of basalt is 950- 1000nm, that of pyroxene is 950-970nm, and that of olivine is 1 100nm, while plagioclase, which is common on the moon, has no obvious absorption band in the infrared region.

After obtaining images of different bands, each band needs to be synthesized. The processing methods are divided into two categories: one is color enhancement after channel synthesis, and the other is to obtain the intensity ratios of different bands, and then carry out pseudo-color processing on different ratios. This processing already belongs to the shooting method of reflecting mineral distribution by using pseudo-color, which is different from the coloring method introduced in this paper and will not be introduced here.

Spectral response curves of different minerals

With shooting ideas and shooting skills, how to choose a suitable location on the moon to shoot? Let's go back to the map of the moon drawn by Lu Dao in 1967. This map gives us a lot of inspiration. We first focus on the area around the Ali starks crater. Aristakes crater is one of the brightest structures on the surface of the moon, and its color is also the most obvious. The highlands around the crater are obviously yellow-this is almost a color that can be perceived by the naked eye and is called "wood spot"-1922. Wood found that the spectrum of this area is similar to the sulfur emitted by volcanoes on the earth, and the lunar waters around the crater should be inconspicuous blue.

Photography with enhanced color near Aristakes crater /Alberto Fernando, José Ribeiro, Filipe Alves

Chenghai and Jinghai are opposite in color, and they are also good subjects. The sea of tranquility is blue, Chenghai is brown, and some blue areas of the sea of tranquility extend to Chenghai. The color composition of Yuhai is also complicated. The east corner of Yuhai is brown, and there are blue and brown spots in Yuhai.