The first telescope was most probably constructed by a Dutch optician H. Jansen in 1604. He might have used the existing Italian construction. Galileo Galilei improved the construction in 1609 by increasing the magnification to 30 times.
A telescope consists of a simple lens ( a convergent lens) and an eyepiece which is a divergent lens, both placed in a tube at a meticulously matched distance. When Galileo looked into the sky through this apparatus, he saw the sky magnified by hundreds and thousands of times more than what has been observed by the scientists of all past epochs.
Thanks to this discovery we can observe distant stars at the edges of the Universe.
In a telescope an objective (with a long focal length) and an eyepiece (with short focal length) are positioned in such a manner that their focal points meet.
Galileo's telescope gives a virtual image, which is upright. Armed with this powerful research work tool, Galileo began observing the sky. He found out that the surface of the Moon is not smooth, as was widely believed, that the Milky Way consists of millions of stars. He discovered that Jupiter has its own satellites, today called Galilean. The latter one was a very important discovery. It proved that the Earth is not the only celestial body around which other bodies do orbit. Galileo watched also other phenomena and objects such as the Sunspots, phases of Venus, Saturn's rings. He published the results of his work in the book 'Suderius nuntius' in 1610.
The angular multiplication P for simple Galilean telescope is described by the following formula:
P = fb/fk ,
in which fb - is the focal length of the objective and fk - the focal length of the eyepiece.
The magnification power of the telescope cannot be enhanced freely by the application of an eyepiece with shorter and shorter focal length. It is determined by so called angular resolution of an objective limited by diffraction phenomena which depend on the width of the objective, termed its "aperture" d.
The objective can distinguish two separate objects, provided the angular resolution is described by the following formula:
ΔΨ = 1.22λ/d ,
in which: - λ the wavelength (the Rayleigh criterion).