3 Optical systems
Refracting telescopes use a lens, while reflecting telescopes use a mirror as the primary receptor. The first telescopes were refractors. Although various ideas about using mirrors in a telescope circulated as early as the 16th century, a working version was not built until 1668 by Isaac Newton (Zuidervaart 2004: 409; Hall & Simpson 1996: 2-3). The reflecting telescope did not become common until the 18th century (Zuidervaart 2004). The advantages of reflecting telescopes over the earlier refracting telescopes were that they are shorter and there is no chromatic aberration. Chromatic aberration occurs in refracting telescopes because the lenses cannot focus all the different colours of a ray of white light at one point when refracted (Pedrotti & Pedrotti 1993: 102). This problem with refracting telescopes was not solved until the 18th century.