The year 2009 was designated by the United Nations as the International Year of Astronomy. This year was chosen because 2009 is the 400th anniversary of Galileo's invention of the astronomical telescope. The following is a brief historical development of astronomical telescopes.
In 1608, a Dutch spectacle craftsman aligned a convex lens and a concave lens and found that distant objects became closer and larger. This was the first telescope with records.

But it is now recognized that the first astronomical telescope was invented by the Italian astronomer Galileo in 1609. He made a telescope with a magnification of 30 in one day, and pointed the telescope to the sky, so the astronomical telescope was born. He became the first person in human history to see the moons of Jupiter (the four bright moons of Jupiter are therefore also called Galilean moons), the first person to see the rings of Saturn, The first person to observe that Venus has waxing and waning like the moon, the first person to see the craters and valleys on the surface of the moon, the first person to prove that the sun rotates by observing sunspots, the first person to discover that the Milky Way is composed of countless stars, his instrument was also called a 'telescope' for the first time, and later specifically called his refracting telescope a 'Galilean telescope'.

Because light of different wavelengths has different refractive indices in the medium, refracting telescopes inevitably have the problem of chromatic aberration. In the 17th century, the solution that people thought of to solve the chromatic aberration was to lengthen the focal length. When the focal length is long enough, the chromatic aberration becomes negligible. For refracting telescopes, the length of the lens barrel must be as long as the focal length. The focal length of the refracting telescope with the longest lens barrel made at that time reached 45 meters.