Refracting, or refractor, telescopes use lenses to bring light to a focus, as opposed to reflecting telescopes, which focus light using a mirror.

Invention

The invention of the refractor telescope predates Sir Isaac Newton’s construction of the first successful reflector by about 60 years. Although it is not known for sure who constructed the first telescope, Hans Lippershey, Zacharias Janssen and Jacob Metius are generally credited with developing the first refracting telescopes in Holland in around 1608.

It is not known whether they each came up with the idea independently; however, Lippershey, an eyeglass manufacturer, was the first to apply for a patent for his ‘spyglass’ or ‘perspective glass’, as the invention was originally known. Although it was alleged at the time that he stole the idea from Janssen – the acknowledged inventor of the compound microscope, also an eyeglasses maker working in the same town of Middelburg – Lippershey claimed that he developed the idea after watching children in his shop use two lenses to magnify a distant weather vane. Complicating the matter further, Jacob Metius, also from Holland, applied for a patent for a similar device just weeks after Lippershey.

Galileo

Galileo Galilei (1564 to 1642) was one of the first people to use a telescope to study the night sky, and is often known as the ‘Father of Astronomy‘.

Galileo’s contributions include the discovery, in 1610, of the four largest satellites of Jupiter – Io, Europa, Ganymede, and Callisto – now known as the Galilean moons, in his honour. He was also the first to observe the phases of Venus, and the rings of Saturn, although due to the limitations of his telescope, he mistakenly believed them to be two large moons, or even ‘handles’ either side of the planet.

Galileo also observed the planet Neptune in 1612. Although he didn’t realise that it was a planet, he noted that it was moving in relation the stars, before he lost track of it.

As well as this, Galileo also studied sunspots and the topography of the Moon, and was the first to realise that the Milky Way isn’t nebulous, as was believed at the time, but made up of miriad individual stars.

Disadvantages of Large Refractors

The largest practical lenses that can be built are around 1 meter in diameter – much smaller than the largest reflector telescopes, which can have mirrors reaching tens of metres across. Large lenses made from glass can be very heavy and usually suffer from defects such as air-bubbles. Glass also flows like a liquid over long periods of time, so that the lens is eventually deformed by gravity.

Refracting telescopes also suffer from spherical and chromatic aberration, as light of different wavelengths is refracted by different amounts, leading to rainbow-like fringes around objects. To some extent this can be mitigated by using a long focal length, or an objective lens made from two pieces of glass with different refractive indices. The largest such ‘achromatic refractor’ telescope was built for the The Great Paris Exhibition of 1900, which had an objective lens of 1.25 metres, but was dismantled after the event. The Yerkes Observatory, in Wisconsin USA, is the largest refracting telescope still in existence, with a lens of 40 inches or 102 cm.