China's FXT space X-ray telescope captures' supermoon '

Chinese scientists used the FXT Space X-ray Telescope (FXT) on the Einstein Probe (EP) satellite launched earlier this year to send back a complete X-ray image of the full moon from space. This is the first time that Chinese scientists have observed a complete X-ray picture of the moon with a space telescope developed by themselves.

X-ray images of the moon taken by the Popular Sky X-ray telescope during the Mid-Autumn Festival

Taking pictures of the moon in X-ray energy is very difficult.

X-rays from the Sun can excite elements on the moon to produce X-ray fluorescence radiation. Because the X-ray characteristic energies of different elements are different, the distribution of various elements on the lunar surface can be revealed by studying the X-ray images with different energies.

However, X-ray radiation cannot penetrate the Earth's atmosphere and therefore cannot be detected on the ground. On the other hand, X-rays are difficult to focus imaging, generally can only use grazing incident focusing, resulting in the X-ray telescope field of view is very small, usually only about 20 arc minutes, can not cover the entire moon.

To this end, scientists usually use X-ray satellites to observe outside the atmosphere, but so far have not succeeded in capturing a complete X-ray full moon image, and the moon is like a layer of mystery.

At the moment, solar activity is at its peak, with frequent solar flares. When there is a solar flare, the X-ray flow of the sun will increase dramatically, the energy spectrum will harden, and the X-ray radiation on the lunar surface will also increase. This provides an advantage for taking X-ray pictures of the lunar surface.

"Einstein Probe" satellite

What is valuable is that this year's Mid-Autumn Festival coincides with the moon in the Earth-moon orbit near the perigee, the closest to the Earth is only 357,400 kilometers, at this time the moon than the ordinary full moon area increased by about 14%, the apparent diameter of 33.4 Angle minutes, and higher brightness, so it is called "supermoon". This is a good time to take clear X-ray pictures of the moon.

However, to take a full X-ray picture of the "supermoon," several conditions need to be met:

● The satellite needs to have the ability to track the moon, that is, according to the lunar calendar, adjust the telescope's pointing at any time, so that the moon is always in the center of the field of view.

● The telescope must have a large enough field of view (at least 33.4 arc minutes in diameter) so that a complete picture of the Moon can be taken.

● The telescope should have a high angular resolution, which is an important prerequisite for taking clear pictures.

● The visible light radiation of the "supermoon" is very strong, and the detector needs to have a thick blackout film in front of the visible light, and at the same time, it should be able to pass X-rays.

● The detector energy resolution is good, so that X-ray fluorescence images of different elements can be obtained, and then the lunar surface distribution of each element can be studied.

"FXT" space X-ray telescope

"FXT" meets all the requirements for photographing the "supermoon" and is the best choice to uncover the mystery of the moon.

It adopts multi-layer nested Wolter-1 metal focusing mirror, and the focal plane detector adopts parallel readout imaging detector.

The image field of view of the "FXT" is square, and the side length is about twice the diameter of the moon, so that you can see the "supermoon" at a glance. The angular resolution of the "FXT" is very high, about 20 arcseconds, corresponding to a circular region on the Moon with a diameter of about 37 km. The "FXT" is also equipped with a filter wheel, which has a variety of shading films to choose from, of which the "medium film" has a strong ability to block visible light, and can be selected when observing the "supermoon".

Compared with other international X-ray satellites in orbit, the "FXT" carried by Einstein Probe satellite has better X-ray energy resolution and higher effective area, which can reveal the distribution of oxygen, iron, magnesium, aluminum and silicon in the lunar surface.

"Tonight, I do not know who is thinking of autumn." The "FXT" conducted X-ray observations of the moon during the Mid-Autumn Festival, and successfully returned clear X-ray photos of the complete lunar surface, sharing the fun of enjoying the moon from another perspective. The EP satellite science team is conducting data analysis and processing and is looking forward to producing fruitful results in moon-related scientific research.

Source: Voice of the Chinese Academy of Sciences