Space telescopes: a different way of imaging

Not long ago, NASA officially released the first batch of photos taken by the Webb Space Telescope, one of which is a galaxy cluster SMACS0723 containing thousands of galaxies. The galaxies captured are fainter than those in the Hubble Space Telescope's Extremely Deep Field Photo. It is the most distant and clearest deep space image ever taken by humans. So, how does the space telescope take these distant galaxy photos? How is it different from taking photos with our common cameras?

Schematic diagram of the Webb telescope

Ground photography is seriously affected by the atmosphere

German philosopher Kant once said: "There are two things that the more deeply and persistently I think about them, the more the wonder and awe they arouse in my soul will change and grow day by day. These are the starry sky above my head and the moral law in my heart."

Humans are naturally curious about and yearn for the starry sky, and are eager to learn more mysteries and knowledge. However, the naked eye is too weak to observe. Starting from the simple telescope in the time of Galileo, humans have built countless ground-based telescopes. Currently, European and American countries are building 30-meter and 40-meter-class ultra-large-aperture ground-based telescopes, which will greatly expand the human vision of the universe.

However, the complex and ever-changing atmosphere is a stumbling block for ground-based telescope observations. Even if we have binoculars or cheap primary astronomical telescopes, we will often encounter interference from weather conditions such as clouds, rain, fog, and wind. The disturbances caused by atmospheric turbulence not only make the stars visible to the naked eye flicker continuously, but also make them flicker and jump even more violently in the telescope.

Seeing refers to the clarity of the image in the telescope, which is used to measure the degree of disturbance of atmospheric turbulence. Affected by seeing, the resolution limit of ground-based telescopes is about 1 arc second, which is much smaller than the diffraction limit of the telescope.

With the emergence of adaptive optics technology, seeing is no longer an insurmountable obstacle, but the atmosphere's absorption of light waves in many bands has left ground-based telescopes helpless. The thick atmosphere has absorbed light waves in infrared and other bands, and the rapid expansion after the Big Bang caused the starlight in the early universe to be severely redshifted to the infrared band. No matter how large the ground-based telescope is, it is powerless.

Schematic diagram of the Webb telescope

In order to see more clearly and deeply and understand more mysteries of the starry sky and the universe, scientists have proposed the idea of ​​launching a space telescope. This way, they will no longer be troubled by the atmosphere and the optical capabilities of large telescopes can be maximized.

People use cameras on the ground to take pictures of the starry sky. Whether it is the cameras of mobile phones, SLRs and telescopes, or the super telescopes and high-performance CCD sensors used by scientists, they cannot see the infrared light absorbed by the atmosphere, and cannot see the distant and faint starlight from the early days of the Big Bang like the Hubble Space Telescope.

Different materials have different imaging

Optical telescopes can be divided into two categories: refraction and reflection, as well as mixed catadioptric systems. The most common and commonly used optical telescopes are mobile phone cameras and SLR camera lenses that use the principle of refraction to form images. Although the refraction optical system is simple and reliable, it also has unavoidable problems: chromatic aberration and weight.

In the refractive optical system, light passes through the convex lens body, which requires a high light transmittance of the lens. However, the light-transmitting material has different refracting abilities for light in different wavelengths, so chromatic aberration is very likely to occur.

Telescope manufacturers discovered this phenomenon very early, so they used lenses made of different materials to develop refracting telescopes, which basically solved the chromatic aberration problem. However, in order to obtain better light-gathering capabilities and observe fainter stars, telescopes became larger and larger, and the lenses of refracting mirrors became heavier and heavier. At the same time, the huge lenses of refracting mirrors are difficult to grind and are easily deformed, which basically makes them withdraw from the field of astronomical observation.

Space telescopes have extremely high requirements on size and weight. They all use reflective optical systems, which are very different from the optical systems of mobile phones and camera lenses.

When a reflecting telescope forms an image, light does not need to pass through the primary mirror, but is focused by reflection, thus getting rid of the problem of chromatic aberration. The lens of a reflecting telescope only needs to process a reflective surface, which is simple to manufacture and the lens is light, making it easier to build a larger-diameter telescope.

The advantages of reflecting telescopes have made them quickly become the mainstream of astronomical telescopes. In order to obtain better imaging quality, the Hubble Space Telescope and the Webb Space Telescope in the United States chose the Ritchie-Chrétien hyperbolic reflector design, while the survey telescope being developed in my country uses an unobstructed off-axis three-reflector design with better imaging quality. These high-performance reflective imaging systems are extremely rare on ordinary cameras and even amateur telescopes.

For space telescopes, their lenses also need to take into account the huge temperature difference environment in space, so lens materials with extremely low thermal expansion coefficients will be selected. The acceleration and vibration during the launch of the carrier rocket must also be considered, so the material must also have a certain degree of elasticity.

The Hubble telescope uses quartz glass with an ultra-low expansion coefficient, while the Webb telescope chooses metal beryllium as its lens material. Space telescopes in Europe and my country prefer silicon carbide materials with low expansion coefficients and densities.

Infrared telescopes such as the Webb telescope also need to weaken the infrared rays they emit, and their mid-infrared cameras need to be actively cooled to an ultra-low temperature of 6K, which is a world of difference from ordinary cameras on the ground.

New concept telescope has more unique imaging

The Hubble telescope ushered in a new era of space astronomy, and the Webb telescope showed people distant and dim galaxies in the early days of the Big Bang. Today, in order to explore more mysteries of the universe, many new concept telescopes have become or will soon become a reality. They use strange imaging methods and will bring more scientific exploration surprises to mankind.

Hubble Space Telescope

At the press conference for the release of the first photo by the Webb telescope, NASA Administrator Bill Nelson made a vivid metaphor: "The range of the starry sky covered by this photo taken by Webb is equivalent to a grain of sand on your fingertips seen at a distance of an arm's length."

The Hubble Extreme Deep Field and the Webb Deep Field photographs are both about spying on distant and dim celestial bodies in an extremely small range, which is commonly known as a detailed investigation. However, astronomy not only focuses on a single "grain of sand" in the starry sky, but also cares about the details of the large universe, which requires a survey telescope responsible for the general survey.

The Euclid telescope, which is responsible for the sky survey mission of the European Space Agency, is about to be launched into space. my country's sky survey optical telescope will also be launched in 2023. The Roman telescope in the United States is also used for sky survey observations. They are all characterized by a wide field of view.

The most radical concept of a space telescope is the Terrestrial Planet Finder (TPF) telescope concept of the United States. Currently, China has invested in the Square Kilometer Array (SKA), which is an antenna array composed of more than 3,000 15-meter diameter antennas, and obtains high-resolution radio images through very long baseline interferometry technology.

The SKA operates in a very long wavelength band, while the wavelength of visible light and even infrared light is very short, making interferometric imaging extremely difficult. TPF is a space telescope concept of a visible light interferometric array. Through revolutionary imaging technology, it can directly photograph distant exoplanets, measure their size, temperature and other information, and answer the question of whether there is extrasolar life.