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On January 11, 1672, Newton showed off his reflex telescope to members of the Royal Society. Unlike the lens imaging refraction telescopes from Galileo's time, Newton's telescopes used mirrors to image. Later, historians of science said that Newton invented the telescope. However, this brief historical narrative masks a more interesting and complex story. Newton's idea of inventing the telescope was immediately questioned by two other competitors, James James Gregory and Laurent Laurent Cassegrain. What is even more puzzling is that the idea of focusing light with a curved mirror was put forward more than 1500 years before Newton, while the final realization of the practical reflection telescope was more than half a century later.

The optical principle of the Royal Society reflective telescope was proposed a long time ago and can be traced back to the reflective optics of Hiro of Alexandria. Hiro was an Egyptian mathematician, engineer, and inventor in the first century AD. He proved that a paraboloid mirror can focus parallel incident light and image it. There is no evidence that Hiro actually made reflective optics. But now the "pot lid", a common TV antenna on the roof, takes advantage of Hiro's theory, which focuses broadened radio signals to one point.

Medieval Islamic opticians also discovered the focusing characteristics of curved mirrors, the most famous of which is the 10th century Persian scholar Abu Saeed Allah Ibne Sal's "Burning Mirror and Lens". Unfortunately, Ibne Sal's work seemed to have little influence at the time and was not taken seriously until the 20th century. Hiro was a favorite writer for Renaissance Italian artists and engineers, so it's not surprising that the principle of the reflection telescope reappears in Leonardo da Vinci's unpublished manuscripts. But Leonardo da Vinci's interest in telescopes was only about writing and painting, and he did not try to actually build such a telescope.

The honor of actually building the first telescope fell to the Italian astronomer Nicolo Zucchi, who in his book philosophy of Optics mentioned that he had built a telescope with curved copper mirrors in 1616. The image formed by the mirror at the focus needs to be magnified to be seen. Zuji added the eyepiece of a traditional refracting telescope to the optical path to enlarge the image focused by the mirror. But he found that the resulting image was distorted beyond recognition. He discovered a major problem with reflective telescopes: it was difficult to grind and polish a suitable curved metal mirror. Although grinding lenses can also cause defects on the surface, the light deflection caused by defects on the surface of the mirror is six times stronger than that of defects of the same size on the surface of the lens. The bending of light in the lens weakens the effect of the defect, while the mirror amplifies the effect of the defect.)

Even if Zucci's telescope works, it has only one mirror, so it is not a total reflection telescope. All real reflection telescopes have at least two mirrors: the primary mirror for imaging and the secondary mirror that projects the image from the main body of the telescope. The design of the first double-mirror telescope appeared in the harmonious Universe by French mathematician and physicist Marin Mersenne in 1636. He never tried to implement his design, probably because he realized the great difficulty of making parabolic mirrors.

A paraboloid containing any line (L), focus (F), and vertex (V). The parallel light into the parabolic mirror is focused on the focus F, the vertex is V, and the axis of symmetry passes through V and F. For the off-axis reflector (only the partial paraboloid between point P "and point P"), the receiver is still placed on the focus of the paraboloid, but it does not cast a shadow on the reflector. Picture Source | Wikipedia at this time, Newton appeared. In about 1668, according to his own design, Newton made a mirror out of copper-tin-arsenic alloy and successfully built a working telescope. He played a little clever and built a spherical mirror. Although spherical mirrors do not produce completely accurate images, they are easier to grind and polish than parabolic mirrors. However, we should not underestimate Newton's ability as a craftsman. His telescope is large enough to observe the four large moons of Jupiter. It was as powerful and smaller as many refracting telescopes of the time.

A few years before Newton publicly demonstrated his telescope, Scottish mathematician and physicist James Gregory (James Gregory) published a slightly different design of reflective telescopes in his Optical Promotion. His design uses a parabolic primary mirror and an ellipsoidal secondary mirror. Unlike Mason, Gregory tried to actually build his telescope; unlike Zucci, his telescope had a complete double-mirror reflector. Although Gregory hired Richard Reeve, the best lens maker in London, to grind the lenses, the quality of his mirrors did not meet the imaging requirements.

It is reported that Newton's old adversary, Robert Hook (Robert Hooke), successfully built a working reflection telescope based on Gregory's design in 1674. But it was quickly damaged and could not be preserved. Two years before that, Hook was also the main force in support of Gregory's accusation of Newton. Also in 1672, doctor Jean-Baptiste Jean-Baptiste Denys published a letter in a French magazine claiming that his assistant, Laurent Cassegrain, was the one who designed the first reflector telescope. Little is known about Cassegrain, and it is not known whether anyone tried to build telescopes according to his design in the 17th century. He uses a paraboloid primary mirror and a hyperboloid secondary mirror, which is very complicated to polish.

Gregory and Cassegrain's designs were not immediately put into practice, but neither did Newton's designs. Although the reflective telescope has many advantages over the refracting telescope, Newton's spherical mirror cannot be magnified effectively and can only be turned into an interesting scientific toy.

In the early refraction telescope, there are two main optical problems that are difficult to solve, namely spherical aberration and chromatic aberration. First, the surface of a simple lens is a spherical crown-the only configuration that 17th-century technology can do. It cannot focus light on a single point, so it naturally produces a distorted image. This spherical aberration can be mitigated by using extremely long focal length lenses, but this solution causes telescopes to become longer and heavier. The second problem, chromatic aberration, was what prompted Newton to study reflective telescopes. Light of different colors bends at slightly different angles as it passes through the lens. In the rainbow, the effect is beautiful. However, in the refracting telescope, the red light image and the blue light image will be separated by a short distance, resulting in color stripes, so the image becomes blurred.

A schematic diagram of the light path of the Newton telescope. Image source | Wikipedia the image of a reflection telescope without a lens is clearer than that of a simple refraction telescope. However, it was not until 1721, more than 50 years after Newton built the first telescope, that the British inventor John Hadley succeeded in building a large Newtonian reflector telescope (not exactly Newton's spherical mirror design). Members of the Royal Society compared it to Christian Huygens' 120-foot refracting telescope and announced that Hadley's telescope was better. Hadley then built a working Gregory reflector telescope. More importantly, he taught the major instrument manufacturers at the time a mature method of grinding and polishing metal lenses, making it possible to mass produce high-quality telescopes.

Today, all major observatories, including the Hubble Space Telescope, use reflective telescopes. To whom should we give credit for inventing it? Hiro who came up with the concept? Try the actual construction of Zucci? Newton who successfully built the forerunner of the modern telescope? Or the Hadley that makes reflector telescopes feasible? They are all indispensable, but none of them has enough work.

The revelation of this story goes far beyond the example of a telescope. For almost all industrial equipment, it is inaccurate to claim that the inventor is a particular person. Concepts, prototyping and large-scale applications can be very different, and this process is usually not a smooth road, but a long, tortuous and challenging journey.

Author: Thony Christie

Translator: Tibetan idiot

Revision: * 0

Original link: How many great minds does it take to invent a telescope

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: Thony Christie

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