From their appearance in the early 1600’s to modern orbiting observatories, telescopes have been used to explore the universe and further mankind’s knowledge and understanding.
Today’s amateur astronomers have access to large, high quality telescopes that often provide computer-assisted observing sessions.
The earliest Refractor Telescopes
Hans Lippershey (1570 – September 1619) is generally credited with the earliest recorded design for an optical telescope in 1608, although it is unclear if he invented it.
His work with optical devices grew out of his work as a spectacle maker, an industry that had started in Venice and Florence in the thirteenth century, and later expanded to the Netherlands and Germany.
Lippershey may have made this discovery on his own although there are many stories as to how he came by his invention.
First use of a telescope for Astronomy 1609
Galileo Galilei (15 February 1564 – 8 January 1642) was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution.
His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism. Galileo has been called the “father of modern observational astronomy”.
His contributions to observational astronomy include the telescopic confirmation of the phases of Venus, the discovery of the four largest satellites of Jupiter (named the Galilean moons in his honour), and the observation and analysis of sunspots.
Development of the Refracting Telescope
The first telescopes used to peer into space were refracting telescopes. This means that you would look straight through the telescope from one lens to the other through to the image you were looking at. During the first few decades after refracting telescopes were invented, the race was on for the best telescopes.
However, the lenses used in these telescopes were not the best. The art of grinding and polishing glass lenses was slow going in the beginning. The images seen through refracting telescopes were not perfect. Because of the way the lenses bent the light, the images were always a little blurry. This blurriness is called chromatic aberration.
It was soon discovered that if the telescope lenses were further apart, they would show a clearer image. The distance between the lenses is called the focal length. Soon Telescopes began to get really, really long as astronomers pushed the boundaries of their new technology.
A well-known telescope was made by Johannes Hevelius in 1673. His telescope had a focal length of 158 feet (48m). As you can imagine, these really long telescopes were very hard to work with. They were very difficult to move and set up.
There are two basic ways to magnify an image: Lenses, as we have seen in refracting telescopes; and Mirrors.
James Gregory (November 1638 – October 1675) Described an early practical design for the reflecting telescope – the Gregorian Telescope in 1663
Gregory pointed out that a reflecting telescope with a parabolic mirror would correct spherical aberration as well as the chromatic aberration seen in refracting telescopes. In his design he also placed a concave secondary mirror with an elliptical surface past the focal point of the parabolic primary mirror, reflecting the image back through a hole in the primary mirror where it could be conveniently viewed.
According to his own confession, Gregory had no practical skill and he could find no optician capable of actually constructing one.
The telescope design attracted the attention of Robert Hooke, an Oxford physicist who eventually built the telescope 10 years later.
The first functional Reflecting Telescope 1668
From his work on coloured light, Sir Isaac Newton (25 December 1642 – 20 March 1727) concluded that the lens of any refracting telescope would suffer from the dispersion of light into colours (chromatic aberration). As a proof of the concept, he constructed a telescope using a mirror as the objective to bypass that problem. Building the first known functional reflecting telescope (today known as a Newtonian telescope) This involved solving the problem of a suitable mirror material and shaping technique.
Newton ground his own mirrors out of a custom composition of highly reflective speculum metal, using Newton’s rings to judge the quality of the optics for his telescopes.
In late 1668 he was able to produce this first reflecting telescope. In 1671, the Royal Society asked for a demonstration of his reflecting telescope.