In the latter half of the 17th century, astronomical observation was constrained by a significant optical limitation inherent in the era’s refracting telescopes. The passage of light through a primary lens caused it to disperse into its constituent colors, an effect known as chromatic aberration. This flaw produced a distracting halo of false color around observed objects, fundamentally limiting the clarity and effective power of lens-based instruments. Isaac Newton, after extensive experimentation with prisms and light, concluded that this aberration was an inescapable consequence of refraction.
To overcome this challenge, Newton abandoned the principle of refraction entirely and turned instead to reflection. His revolutionary design, first constructed in 1668, became known as the reflecting telescope. In place of a large objective lens, Newton employed a concave primary mirror to gather and focus starlight. This mirror was cast from a polished alloy of copper and tin called speculum metal. The light was then directed by a small, flat secondary mirror, set at a 45-degree angle, to an eyepiece mounted on the side of the main tube.
This Newtonian configuration elegantly solved the problem of chromatic aberration, as a mirror reflects all wavelengths of light to a single focal point. Although his initial model was modest in size, its performance was a profound proof of concept. Upon its presentation to the Royal Society in 1672, the instrument was recognized as a monumental advancement. It established a new and superior paradigm in optical design, enabling the construction of the larger, more powerful telescopes that would define the future of astronomy.
