refracting telescope

Hold a magnifying glass up to the sky and you have the seed of a refracting telescope. It is the kind most people picture: a long tube with a big lens at the front and a small eyepiece at the back. This is the design Galileo turned on Jupiter in 1609, and the one in toy spyglasses today.

A refractor works by refraction — the bending of light as it passes into and out of glass. The large front lens, called the objective, collects light from a distant object and bends all the rays so they converge to a focus, forming a small bright image. A second lens, the eyepiece, then magnifies that image for your eye or sends it to a camera. The key number is the diameter of the objective: a 100 mm refractor gathers far more light than the pupil of your eye, which is only about 7 mm wide in the dark.

Refractors give crisp, high-contrast images and need little maintenance because the optics are sealed in the tube. But a lens bends different colours by slightly different amounts, smearing a star into a tiny rainbow — an error called chromatic aberration. And a large lens can only be held at its edges, so it sags under its own weight and becomes impractical beyond about a metre across. For these reasons every giant research telescope uses mirrors instead, and refractors survive mainly as small, beautiful instruments.