The telescope is an instrument which increases our ability to observe far away objects through the collection of electromagnetic radiation, the most prevalent type of telescope is the optical telescope which collects light, however there are other kinds of telescopes which collect UV and X -Rays. Optical telescopes use lenses to redirect light to a specified point. Different lenses are used for different magnifications of celestial objects. This essay will be mainly discussing optical telescopes.

As we go through the history of optical telescope we can see that there are a few concepts that are fundamental to all the different types of optical telescopes, from the reflecting to the refracting to the hybrid of the two these are as follows.

The lens and its properties were known prior to the invention of the optical telescope with simple lenses made from rock crystal being found before recorded history (George & Yannis, 1987). The properties of light such as reflection, refraction and colour were discussed by Ptolemy in his work ‘Optics’ written in the 2nd century (Salih, Al Amri, & El Gomati, 2005) and by the 12th Century ‘reading lenses’, the predecessors to the magnifying glasses, were in use by people suffering from sight deficits (White, 1961).

All telescopes have a hole or opening through which light travels called the aperture. This relates to the telescope’s ability to collect light through a lens or mirror, with the bigger the aperture the more light it is able to collect and bring to focus, and the brighter the final image. The focal length of a telescope is the distance required by the objective lens or primary mirror to bring all the light it has collected to a single point (focal point). As a general rule of thumb, the longer the focal length of a telescope the more capable of the telescope to deliver higher magnifications. Finally, the resolution of a telescope is the ability to see fine details in an image, it also moderates the effectiveness of its ability to separate, or resolve, two close object. Resolution depends on the aperture of the telescope, the observing conditions and the quality of the optics.

From these developments spectacle makers named, Hans Lippershey, Sacharia Jansen and Jacob Metius, are often credited with the invention of the telescope. Hans was the first known person to apply for a patent on the invention in October 1608 as well as distributing designs for the first practical telescope. However there is much speculation that these people was not the first to make the telescope, nevertheless they were the leading people in making the instrument widely known(King, 2003)

Refracting telescopes

As Lippershey brought news about this new gadget to light, the story about this instrument spread throughout Europe, including to Galileo Galilei who heard about the telescope whilst at Padua. After working out the basic principles behind how the telescope operates, Galileo manufactured his own eight power telescope.

As Galileo began to refine his method of making telescopes (e.g. the way in which he grinded the lenses) he gradually improved the power of the telescopes and used them for observing the sky. Through the use of the telescope Galileo was able to provide key evidence which supported the Sun-Centred / Copernican model of the solar system which contradicted the Ptolemaic, Earth Centred model and thus settled one of the largest debates at the time. Furthermore Galileo found the 4 moons of Jupiter as well as the different phases of Venus, mountains on the moon and discovering that the Milky Way containing many stars. This showed how critical the practical use of the telescope is in the growing field of astronomy.

The telescopes which Galileo created are known as refracting telescopes. These telescopes work like a magnifying glass in that it uses a primary concave lens where the light enters telescope.