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In 1611, the German astronomer Kepler used two biconvex lenses as the objective lens and the eyepiece respectively, which greatly increased the magnification. People will call this optical system a Kepler telescope in the future. People use refracting telescopes in these two forms, and astronomical telescopes generally adopt Kepler type.
It should be pointed out that since the telescopes at that time used a single lens as the objective lens, there was serious chromatic aberration. In order to obtain a good observation effect, a lens with a very small curvature was required, which would inevitably cause the length of the lens body. So for a long time, astronomers have been dreaming of making longer telescopes, and many attempts have ended in failure.
In 1757, Dulong established the theoretical basis of achromatic lenses by studying the refraction and dispersion of glass and water, and made achromatic lenses using crown glass and flint glass. Since then, achromatic refracting telescopes have completely replaced long-lens telescopes. However, due to technical limitations, it is difficult to cast larger flint glass. In the early days of achromatic telescopes, only 10 cm of lenses could be polished.
Lens lenses absorb radiation in the ultraviolet and infrared bands very strongly. It is also very difficult to cast huge optical glass. By 1897, when Ye Kaishi's 1-meter telescope was completed, the development of refracting telescopes reached the climax. In the following hundred years, no larger refracting telescopes appeared. This is mainly because it is technically impossible to cast a large piece of perfect glass to make a lens, and the deformation of a large-size lens due to gravity will be very obvious, thus losing the sharp focus.
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