Still can't see clearly? It seems that the telescope should be upgraded

In 1609, astronomer Galileo built a 4.4cm telescope and used it to observe the moon, sun, stars and the Milky Way. This was the world's first astronomical telescope with scientific research output. Galileo used it to discover Jupiter's satellites and measure the sunspot cycle.

Galileo is teaching others how to use a telescope (Source: Wikipedia)

If you want to do your work well, you must first sharpen your tools. Since telescopes play a vital role in astronomical research, how to increase the clarity has naturally become a difficult problem that scientists need to overcome.

Want to see more clearly? Please increase the diameter

As scientists have come to understand the optical principles of telescopes, they have concluded that the main factor affecting telescope resolution is the aperture. The larger the aperture of a telescope, the greater its ability to collect light and the more information it can capture.

The relationship between the aperture and the resolving power of an optical system is clearly expressed in the formula named after the physicist Lord Rayleigh.

It doesn’t matter if you don’t understand the formula. What it actually means is that the larger the aperture of the telescope, the higher the resolution of the telescope, and the more details that can be observed.

Image source: self-made by the author

With the advancement of technology, the aperture of telescopes manufactured by the astronomical community has become larger and larger. In 1789, William Herschel, a British-German, built a reflecting telescope with an aperture of 1.22 meters. In 1975, the six-meter reflecting telescope BTA-6 built by the Soviet Union was officially unveiled. It was the largest telescope in the world at that time.

But in actual operation, the observation results of BTA-6 are almost the same as those of a one-meter telescope. Despite its huge aperture, atmospheric turbulence still firmly limits its observation capabilities. BTA-6 can only observe less than half of the nights a year, and its resolution is far from the result calculated by Rayleigh's formula.

Atmospheric turbulence that interferes with observations

The twinkling stars at night, the distorted cars in the distance on a hot summer road, and the view behind an airplane engine are all caused by atmospheric turbulence. Under the influence of atmospheric turbulence, objects appear to be distorted. Atmospheric turbulence causes light to distort as it passes through the atmosphere, greatly reducing the quality of images observed by telescopes.

Atmospheric turbulence causes distortion in images of the moon observed through telescopes (Source: Wikipedia)

Initially, in order to minimize the impact of atmospheric turbulence, scientists chose to build the telescope in a place with relatively good atmospheric conditions. Before the construction of BAT-6, 16 expedition teams were sent to various regions of the Soviet Union, and the final site was selected in the North Caucasus Mountains at an altitude of 2,070 meters. At present, most of the world's more important observatories are located in places with relatively good atmospheric conditions, such as Hawaii and the Canary Islands.

Despite this, the troubles caused by atmospheric turbulence are still unavoidable. The scientists who suffered from it thought: Doesn't atmospheric turbulence distort the light? So can we "twist" the light back? Inspired by this idea, adaptive optics technology came into being.

Adaptive optics with twisted mirrors

As early as 1953, scientists proposed the concept of adaptive optics, but it took decades for a real breakthrough to be achieved. The core of adaptive optics is a deformable mirror and a Shack-Hartmann wavefront sensor that detects the distortion of light waves. The use of a deformable mirror to correct the distortion caused by atmospheric turbulence can greatly improve the performance of the optical system.

(Image source: self-made by the author)

But how can the mirror be deformed? One idea is to make a very thin mirror and apply pressure behind it to cause it to deform.

For example, the deformable mirror system in the European Space Agency's VLT survey telescope is like this:

The densely packed small holes on top will be equipped with small drivers, which will then be covered with a very thin lens. The drivers will cause the lens to deform.

Ultra-thin lenses covering the actuator (Source: ESA official website)

When a telescope equipped with an adaptive optics system is working, it will shoot a laser toward the sky. The role of this laser beam is to measure how much distortion is caused by atmospheric turbulence, and the measured data is a reference for the deformation of the deformable mirror. The deformable mirror can adjust its deformation hundreds of times in one second to cope with the ever-changing atmospheric turbulence.

Images obtained by the VLT astronomical telescope. The left image is obtained after the adaptive optics system is turned on, and the right image is obtained without the adaptive optics system.

(Image source: European Space Agency official website)

After solving the problem of atmospheric turbulence, the astronomical community has shown a vision of "becoming bigger and stronger, and creating greater glory". The large telescopes that have been built include:

The 10-meter-diameter Keck Telescope on the Pacific island of Hawaii;

Keck Telescope (Source: Wikipedia)

The 10.4-meter-diameter Canary Telescope on the Spanish island of La Palma;

The 11-meter-diameter Salter Telescope at the South African Astronomical Observatory. These telescopes are like a battle of gods, with equal strength.

Currently, plans are underway to build the TMT telescope with a diameter of 30 meters, the Giant Magellan Telescope with an equivalent diameter of 21.4 meters, and the ELT telescope with a diameter of 42 meters.

ELT telescope rendering (Source: European Space Agency official website)

The emergence of adaptive optics has not only made great contributions to the astronomical community, but has also been widely used in other fields.

In medical imaging equipment, the application of adaptive optics enables us to obtain clearer images of the human eye's tissue structure, promoting medical progress; in the field of nuclear fusion, the most ideal energy source for mankind in the future, adaptive laser optics can produce better quality laser beams, proposing new possibilities for mankind's energy future.

Atmospheric influence? Then go to space

In addition to adaptive optics, there is a more direct way to eliminate atmospheric turbulence: go to space.

The Hubble telescope is the first telescope that humans have that works outside the atmosphere. Its aperture is 2.4 meters. Because it is located above the atmosphere, it is not affected by atmospheric turbulence. The emergence of the Hubble telescope has successfully made up for the shortcomings of ground-based observations, helped scientists solve many basic problems in astronomy, and also allowed humans to have a better understanding of astrophysics.

The recently launched James Webb Telescope (JWST) is the new king of space telescopes. Compared with the 2.4-meter diameter of the Hubble Telescope, it not only has a larger diameter (6.5 meters), but is also equipped with an adaptive optics system.

Atlas of the world's major telescope sizes (Source: Wikipedia)

Due to the size of the rocket, the JWST mirror is not a whole piece, but is composed of 18 hexagonal lenses. The main mirror of the telescope enters space in a folded manner, unfolds in space, and uses an adaptive optical system to correct the position deviation of different lenses. In order to avoid the influence of the sun on observation, JWST also deliberately went to the second Lagrange point 1.5 million kilometers away for observation.

Among the space telescopes under construction is the Sky Survey Telescope designed and manufactured by the Changchun Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences. It is expected that in 2024, the Sky Survey Telescope will be launched into space and will co-orbit with the Tiangong space station.

Chinese scientists are also studying the plan of manufacturing and assembling telescopes in orbit to make up for the limited loading capacity of rockets. Perhaps in the near future, we will have a 30-meter diameter space telescope working in space.

Large astronomical telescopes are the epitome of human wisdom and modern technology. Humanity's relentless pursuit of space has promoted the continuous advancement of astronomical telescope technology. And human exploration of the universe will continue.

References:

[1]https://www.tmt.org/blog/tmt20180419

[2]https://www.eso.org/public/

[3] Jiang Wenhan, Jiang, Wenhan, et al. A review of the development of adaptive optics[J]. Optoelectronic Engineering, 2018, 45(3):15.

[4] Angeli GZ, Dierickx P, Neill D, et al. Overview of the LSST active optics system[C]// Modeling, Systems Engineering, & Project Management for Astronomy VI. Modeling, Systems Engineering, and Project Management for Astronomy VI, 2014:91500G.

[5] Ellerbroek BL, Gilles L, Vogel CR. A Computationally Efficient Wavefront Reconstructor for Simulation of Multi-Conjugate Adaptive Optics on Giant Telescopes[J]. Proceedings of SPIE - The International Society for Optical Engineering, 2003.

[6] A brief history of astronomical telescopes - jjjastronomy's article - Zhihu https://zhuanlan.zhihu.com/p/33304114

Produced by: Science Popularization China

Author: Salted Fish in the Sea

Producer: China Science Expo