23/04/2026
Our next feature is ‘T’ for ‘Telescope’. ✨
Telescope – from the Greek: tele, meaning 'far', and skopein: meaning 'to look or see' – hence teleskopos 'far-seeing'.
Galileo was one of the first to look at the sky through a telescope in 1609, and what he and others saw soon changed our understanding of the universe: a rough, cratered surface to the Moon, moons around Jupiter, the phases of Venus. (Galileo even saw and sketched Neptune on at least two occasions in 1612 and 1613, but thought it was a star.)
A telescope does basically 2 things: makes objects seem closer (and therefore larger), and gathers more light than our eyes do, allowing us to see fainter objects. The first telescopes were very simple devices with just 2 lenses, but the view through them was a bit fuzzy and with false colours affecting the image. Later, we learned how to create optical systems that reduced the false colour (achromatic), as well as learning how to make better-quality lenses for a sharper view.
Those early telescopes had a lens to gather the light and focus it, and a smaller lens (the eyepiece) to magnify the image from the large one. Isaac Newton was the first to build a telescope that used a curved mirror instead of a lens to gather the light, and eventually, various designs of these ‘reflecting’ telescopes began to appear. The lens-based or ‘refracting’ telescope reached its maximum size at the end of the 19th century with the 40-inch (1-metre) refractor at the Yerkes Observatory, because larger lenses would sag and change shape under their own weight and produce degraded images. By this time though, larger reflectors had been built (because mirrors can be supported from below, to prevent sag) and most large telescopes built thereafter were reflectors, such as the 100-inch (2.5-metre) at Mt Wilson, the 200-inch (5-metre) at Mt Palomar, and the 3.9-metre Australian Astronomical Telescope.
While telescope lenses and mirrors are now usually made of glass, early mirrors were made of polished speculum metal (a mix of mainly copper and tin) which tarnished and needed frequent repolishing. Glass mirrors coated with reflective silver (and later, aluminium) eventually replaced speculum, though some telescopes, such as the James Webb Space Telescope, use beryllium – a very lightweight metal – covered with gold, which improves the mirror’s ability to reflect infrared.
It was once thought that we couldn’t make usable mirrors bigger than 6 metres. However, single mirrors as big as 8.4 metres are now in use, as well as larger mirrors that are composed of numerous smaller segments, such as the 10-metre Keck reflectors, and the 6.5-metre JWST, while a telescope with a 39.3-metre mirror is under construction in Chile – the Extremely Large Telescope, or ELT.
Fun fact: there was a notion nearly 30 years ago to build a 100-metre telescope, but it didn’t happen. It was to have been called OWL – the Overwhelmingly Large Telescope!
📸: Wikipedia
