He later devised means for shaping and grinding the mirror and may have been the first to use a pitch lap to polish the optical surface.
He chose an alloy ( speculum metal) of tin and copper as the most suitable material for his objective mirror. In late 1668 Isaac Newton built his first reflecting telescope. If this were true, then chromatic aberration could be eliminated by building a telescope which did not use a lens – a reflecting telescope. During the mid-1660s with his work on the theory of colour, Newton concluded this defect was caused by the lens of the refracting telescope behaving the same as prisms he was experimenting with, breaking white light into a rainbow of colours around bright astronomical objects. Colour distortion ( chromatic aberration) was the primary fault of refracting telescopes of Newton's day, and there were many theories as to what caused it. Newton built his reflecting telescope because he suspected it could prove his theory that white light is composed of a spectrum of colours.
Newton may even have read James Gregory's 1663 book Optica Promota which described reflecting telescope designs using parabolic mirrors (a telescope Gregory had been trying unsuccessfully to build). Galileo Galilei and Giovanni Francesco Sagredo had discussed using a mirror as the image forming objective soon after the invention of the refracting telescope, and others, such as Niccolò Zucchi, claimed to have experimented with the idea as far back as 1616. Newton's idea for a reflecting telescope was not new.