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The first reflecting telescope was born in 1668, and Newton decided to use a spherical mirror as the primary mirror. He used a metal with a diameter of 2.5 cm to grind into a concave reflector, so that the condensed light reflected by the main mirror was reflected by the reflector at a 90° angle out of the lens barrel and then reached the eyepiece. This system is called a Newtonian reflecting telescope. Although its spherical mirror will produce a certain amount of aberration, it is a huge success to replace the refractor with a reflector.
James Gregory proposed a scheme in 1663: using one primary mirror and one secondary mirror, both of which are concave mirrors. The secondary mirror is placed outside the focal point of the primary mirror and a small hole is left in the center of the primary mirror. The light is reflected by the primary mirror and secondary mirror twice and then shoots out from the small hole and reaches the eyepiece. The purpose of this design is to eliminate spherical aberration and chromatic aberration at the same time, which requires a parabolic primary mirror and an ellipsoidal secondary mirror. This is theoretically correct, but the manufacturing level at that time could not meet this requirement. So Gregory couldn't get a useful mirror for him.
In 1672, the Frenchman Cassegrain proposed a third design scheme for reflecting telescopes. The structure is similar to that of Gregory telescopes. The difference is that the secondary mirror is advanced before the focus of the main mirror and is a convex mirror. This is the most commonly used now. Cassegrain reflecting telescope. This makes the light reflected by the secondary mirror slightly divergent, reducing the magnification, but it eliminates the spherical aberration, so that the telescope can be made to make the focal length very short.
The primary mirror and secondary mirror of the Cassegrain telescope can have many different forms, and the optical performance is also different. Due to the long focal length of the Cassegrain telescope and the short lens body, the magnification is also large, the resulting image is clear; the Cassegrain focus can be used to study celestial bodies in a small field of view, and it can also be configured with a Newton focus to shoot large areas. Celestial bodies. Therefore, the Cassegrain telescope has been widely used.
At the end of 1918, the Hooke telescope with a diameter of 254 cm was put into use, which was built by Haier. Astronomers used this telescope to reveal for the first time the true size of the Milky Way and where we are in it. More importantly, Hubble's theory of universe expansion is the result of observations with the Hooke telescope.
In the 1920s and 1930s, the success of the Hooker telescope inspired astronomers to build larger reflecting telescopes. In 1948, the United States built a telescope with a diameter of 508 cm. To commemorate the outstanding telescope manufacturer Haier, it was named Haier Telescope. It has been more than 20 years from design to completion of the Haier telescope. Although it can see farther and has stronger resolving power than the Hooker telescope, it has not given mankind a newer understanding of the universe. As Asimov said: "The Haier Telescope (1948) is like the Yekes Telescope (1897) half a century ago, and it seems to herald that a specific type of telescope has almost reached its end. "Before 1976, the Soviet Union built a 600-centimeter telescope, but it was not as effective as the Haier telescope. This also confirms what Asimov said.
Reflective telescopes have many advantages, such as: no chromatic aberration, can record information from celestial bodies in a wide range of visible light, and is easier to manufacture than refracting telescopes. But because of its inherent shortcomings: For example, the larger the aperture, the smaller the field of view, and the objective lens needs to be coated regularly.
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