Photos of the reddish lunar soil are black and white? Space cameras are very different from everyday cameras!

On June 25, my country's Chang'e-6 returner landed accurately in the designated area of ​​Siziwang Banner, Inner Mongolia. The Chang'e-6 lunar exploration project was a complete success, achieving the world's first return of samples from the far side of the moon. Interestingly, after sampling on the far side of the moon, Chang'e-6 took a photo of a "Zhong"-shaped mark left on the lunar soil. In this photo, the color of the lunar soil is reddish, but the image taken by the lander's landing camera before was black and white.

Why are the colors in the photos different when they are all taken of the moon? It turns out that this is because the sampled photos have not been color corrected. So, what is the difference between the spacecraft camera and the digital cameras and mobile phone cameras that people use every day, and why do they need special color correction?

Traces left on the lunar soil after Chang'e 6 sampled

Images taken by the Chang'e-6 lander's landing camera

Space cameras are very different

Mobile phone cameras are special digital cameras. With the popularity of smart phones, people are accustomed to taking photos. Many readers think that since they are both photo machines, aren’t the cameras used in space and the cameras people use in daily life the same?

The principles of optical cameras are indeed similar, but the specific designs are quite different. Due to the different environment in which they are located, the design focus of space cameras is quite different from that of the common digital cameras.

The cameras people use in daily life usually perform automatic white balance adjustment, assuming that the scene being photographed is neutral gray as a whole, and then correct the color of the photo. This method rarely causes obvious problems on the ground, but it is different in space. Due to the different light intensity, target reflection characteristics and material composition in space and on the ground, automatic white balance is not easy to use.

The cameras installed on the satellite have a unique way of imaging

The Chang'e-6 sample photos were taken with a surveillance camera. The photos were first used to determine whether the table-taking robotic arm was working properly, so no color correction was performed, resulting in a severe red cast. On the contrary, it is difficult for commonly used digital cameras to encounter such a serious color cast problem.

In order to solve the color correction problem in the space environment, Chang'e 6 performed color correction on the camera on the ground, so that the photos after color adjustment are closer to the real color of the shooting target. However, a more effective way is to carry a color standard plate to obtain the color correction coefficient, and correct the color of the color standard plate when shooting, so as to obtain the real color of the target. Therefore, color correction of space cameras has always been a difficulty and focus of camera design, which is an inevitable result of the difference between space and ground environments.

In addition, in the harsh environment of space, such as heat, vacuum, and radiation, other aspects besides camera color also have to pay a lot of price, such as temperature control system. When people use mobile phones to take pictures on the ground, sometimes the camera or even the entire phone cannot be used normally in extremely cold conditions. In the case of drastic temperature changes in space, temperature control becomes a top priority that must be considered.

Visible light space cameras generally require a temperature control accuracy of less than 1 degree, and infrared band cameras must work in ultra-low temperature environments. They must either use a sunshade or install an active cooling device, or both.

Unique imaging method

Color correction is just a common example of the differences between space cameras and ground cameras. Due to different environments and uses, there are many other differences between space cameras, and even the imaging methods are completely different.

Commonly used digital cameras generally use area array CCD or CMOS as photosensitive elements. When taking pictures, the picture comes out with a "click". However, the cameras installed on spacecraft are not so easy. The cameras they carry usually use linear array CCD to achieve on-orbit imaging. When linear array remote sensing CCD is used for imaging, the target is perpendicular to the CCD linear array direction and must be in relative motion, which is exactly the opposite of the requirement that people need to keep it relatively still when taking pictures with their mobile phones.

So, why is there relative motion? The area array CCD in a digital camera can directly obtain two-dimensional photos, while the linear array CCD camera obtains a line of information and generates a photo by scanning it line by line. If it does not move, it can only scan out a line, not a photo.

Although linear array CCD cameras cannot directly form images, they also have unique advantages: on the one hand, they are low-priced and high-resolution, and on the other hand, they are highly accurate and have a wide field of view. Linear array CCD scanning is also very common in people's daily lives, and scanners usually use linear array CCDs. Remote sensing satellites move at high speeds relative to the ground, and push-broom cameras fly with the satellites, allowing them to form images on orbit without scanning back and forth.

Linear array CCD cameras are the main sensors for high-resolution earth remote sensing. For example, France's SPOT5 satellite and Pleiades satellite both use linear array push-broom cameras.

Of course, linear array CCD is not the only space camera. With the advancement of microelectronics technology, the originally expensive area array CCD has become cheaper. In addition, cameras using CMOS as a photosensitive element have also been increasingly used. Meteorological satellites such as the US GOES have begun to use area array CCD cameras for ground imaging.

Lighter new concept space camera

At present, there are significant differences between the space cameras used by various satellites and probes and the digital cameras people use on the ground. In the future, the technical principles of the new concept space cameras and the digital cameras and mobile phone cameras or "cannons" used by people will be even more different.

According to the resolution limit formula, the resolution index of traditional optical cameras is directly related to the lens aperture. The rocket carrying capacity and fairing volume are limited. If a large reconnaissance satellite is to be launched into space, it will be greatly restricted. Therefore, researchers are developing new concept technologies for space cameras, with the aim of completely changing the appearance of existing space cameras.

Currently, the US Defense Advanced Research Projects Agency is funding military enterprises to develop film-type optical instant imager (MOIRE) technology, which uses diffraction rather than refraction imaging. This technology uses a foldable and thin diffraction film, which is as thick as household plastic wrap. If future spacecraft use advanced MOIRE technology, the weight of a reconnaissance satellite carrying a 20-meter aperture camera can be reduced to about 5 tons.

In addition, the U.S. Defense Advanced Research Projects Agency is also funding Lockheed Martin to develop the Segmented Planar Imaging Detection Electro-Optical Reconnaissance System (SPIDER). This camera has evolved from the stacking technology of optical lenses to a revolutionary micro-optical lens array. Unlike traditional refractive or reflective telescopes, it uses interferometry to form images, which can also be understood as phased array technology in the optical field. SPIDER technology can greatly reduce the size and weight of space cameras. Compared with traditional optical cameras of the same imaging quality, the camera volume after applying SPIDER technology is only a few tens of times smaller than the original.