Principle and Light Path Diagram of the Galileo Telescope
The Galileo telescope uses a converging lens as the objective lens and a diverging lens as the eyepiece. The objective lens forms a real image at its focal point, which is located behind the eyepiece. This real image acts as a virtual image for the eyepiece, resulting in an enlarged, upright virtual image after refraction through the eyepiece. The magnification of a Galileo telescope is determined by the ratio of the focal length of the objective lens to that of the eyepiece. One advantage of this design is the short length of the lens barrel, making the telescope more portable, though it offers a relatively small field of view. A device that aligns two Galileo telescopes with low magnification, using a bolt button in the middle for simultaneous clarity adjustment, is also available.
To build a Galilean-style telescope affordably, you can purchase an ophthalmic lens with a larger diameter and focal length for the objective and a lens with a smaller focal length and diameter for the eyepiece from a stationery store. By using glue and small grooves to mount the lenses in a cardboard tube and constructing a simple pedestal, you can create a telescope capable of viewing mountains on the Moon, stars in the Milky Way, and Jupiter’s moons. Galileo himself used such a design for his discoveries. However, be cautious not to observe the Sun directly through the telescope to avoid damaging your eyes from intense light. Galileo’s refractors had a notable drawback of producing ‘false colors’ around bright objects. This issue arises because what is commonly known as ‘white light’ is actually a mixture of all colors from red to violet. When light enters the objective lens, different colors refract at different angles, causing the image to blur.
In 1611, the astronomer Johannes Kepler improved magnification by using two biconvex lenses as the objective lens and eyepiece, respectively. This optical system is known as the Kepler telescope. While modern refracting telescopes have been developed, the problem of ‘false color’ persists.
Lippershey, though not an astronomer, did not initially aim his new invention at the sky. However, news of his discovery spread quickly. Fortunately, Galileo Galilei, a professor at the University of Padua in Italy, learned about it and soon constructed his own refracting telescope. He used plano-convex lenses as objective lenses and concave lenses as eyepieces. Light from the observed object passes through the objective lens, where it is refracted and focused to form an image at the focal point. This image is then magnified by the eyepiece lens and viewed through the human eye.