Though the night sky may appear to the naked eye to be filled with stars, only the brightest ones are actually visible, because the human retina can only take in so much light. But by using large reflecting telescopes, it’s possible to capture and focus large quantities of light. Thus, even galaxies located more than 10 billion light-years away become visible.

The first telescopes were simple spyglasses in which light was focused by lenses. Not until the invention of the reflecting telescope in 1668 were observations accelerated. Isaac Newton is generally thought of as the father of the reflecting telescope, in which the light came in at the top, was reflected by a concave mirror at the bottom, and was cast out to the observer by a tip-tilt mirror.

In their early stages, telescopes were smaller, because accurate, perfectly curved mirrors were difficult to make. But in the late 1850s, the method of applying a thin coat of silver to a glass surface was introduced, resulting in lighter, cheaper mirrors that reflected 50 percent more light than metal ones.

Today, mirrors can have diameters larger than 39 feet; even larger ones are constructed by putting several together. These telescopes have led to increased knowledge of nebulas, supernovas and distant planets.