Who invented the reflecting telescope?

In 1668, the English scientist Isaac Newton built the first practical reflecting telescope, now called the Newtonian telescope. He presented an improved version to the Royal Society of London in 1671-72.

Before Newton, telescopes were refracting telescopes, which use lenses to collect and focus light. The first telescopes were made in the Netherlands around 1608, and the Dutch spectacle-maker Hans Lippershey is often credited with the first patent application. In 1609-10, Galileo Galilei built his own refracting telescope and used it to make important astronomical discoveries, such as the four largest moons of Jupiter, the phases of Venus, sunspots and craters on the Moon. Kepler improved the design with a convex eyepiece.

Refracting telescopes suffered from a problem called chromatic aberration. Because a lens refracts different colours (wavelengths) of light by different amounts, the images had coloured fringes and were blurred. Newton, through his experiments with prisms, understood that white light is made of different colours. He realised that a mirror reflects all colours of light at the same angle and does not cause chromatic aberration.

Newton's design used a concave primary mirror at the bottom of the tube to collect and focus the light, and a small flat secondary mirror placed at 45° near the top of the tube to reflect the light out to an eyepiece on the side of the tube.

Reflecting telescopes have other advantages: large mirrors are easier and cheaper to make than large lenses, and they can be supported from behind. That is why almost all large modern telescopes are reflectors, such as the James Webb Space Telescope (launched in 2021), the Hubble Space Telescope and India's Devasthal Optical Telescope (3.6 m) at Nainital.

Why not the other options?

• ✗Galileo Galilei used a refracting telescope for astronomy (1609-10).
• ✗Hans Lippershey is credited with the first refracting telescope (1608).
• ✗Johannes Kepler designed an improved refracting telescope (1611).