How does a telescope work?

From Ulugh Beg's observatory to James Webb

A telescope's main job is not to magnify but to collect light. The eye's pupil is 7 mm; a 20 cm telescope gathers 800 times more light, so it shows faint stars. There are lens telescopes (refractors — Galileo, 1609) and mirror telescopes (reflectors — Newton, 1668); all big telescopes use mirrors, because a giant lens sags under its own weight.

The sky was measured before telescopes too: in Samarkand, Ulugh Beg (1420s) built a giant sextant with a 40 m radius — he measured star altitudes so precisely that his tables give the length of the year to within 25 seconds. That was the peak of 'naked-eye' astronomy.

Air makes stars 'twinkle' — so big telescopes are placed on high mountains (Chile, Hawaii) or launched into space. Hubble (1990) orbits 600 km up; James Webb (2021) sits 1.5 million km away, and its 6.5 m golden mirror sees in infrared — through dust, to newborn stars and the universe's very first galaxies.

Radio telescopes (the Event Horizon Telescope) 'see' radio waves; others see X-rays and gamma rays. The Maidanak Observatory near Tashkent (2,600 m) is one of the best observing sites in the world — Ulugh Beg's legacy lives on. You can start too: ordinary binoculars show the Moon's craters, Jupiter's moons and the Andromeda galaxy.