Galileo
Galileo built his own telescope and improved its magnification. He saw craters on the Moon, the phases of Venus and four moons of Jupiter. These moons proved that not everything orbits the Earth.
Lenses and mirrors
Lens telescopes produced coloured fringes. So Newton built a telescope with a mirror. All large telescopes today use mirrors, some of them tens of metres across.
In space
The atmosphere blurs the image, which is why telescopes are sent into space. Since 1990 Hubble has taken iconic images, and since Christmas 2021 James Webb has been observing in infrared light. Along the way, the Slovak optics pioneer Petzval laid the foundations for calculating optical systems.