Through the Looking Glass

Yerkes telescope

The 40-inch telescope at Yerkes Observatory, shown here in the early 1900s, remains the largest refractor in the world. [Yerkes Observatory]

When most people think of a telescope, they think of a refractor: A long, skinny tube with glass lenses at each end. Galileo Galilei used his hand-crafted refractors to discover Jupiter’s moons, the phases of Venus, and the mottled appearance of the lunar surface, and refractors remained the leading instruments for astronomical research through the 19th century.

A refracting telescope consists of a large lens (or lenses) at the front of the telescope tube, with a smaller lens at the back of the tube to magnify the image.

The large lens, known as the objective, refracts or bends the parallel rays of light that strike it so that they all come to a focus behind the lens. Different telescopes over the years have used convex lenses, in which the surface curves outward; concave lenses, in which the surface curves inward; or a combination of the two.

As the size of the objective increases, so does the distance behind the lens at which the light rays come to a focus. The world’s largest refractor, at Yerkes Observatory, has an objective that is 40 inches (102 cm) in diameter, with a tube that is 60 feet (18.3 meters) long.

This great length is one factor that limits the size of refracting telescopes; large refractors are so long that they require giant domes to house them. Another limiting factor is the size of the objective. A large lens is so heavy that gravity causes it to “flow” like a batch of thick fudge, ruining its view of the sky.

Another problem is that a refractor’s lenses bend different wavelengths of light at different angles, so the colors don’t come to a focus at the same point. This flaw, known as chromatic aberration, can be corrected with additional lenses or longer tubes, but that increases the weight and complexity of the telescope.

Despite the problems, a properly built refractor can provide crisp views of the night sky, it’s easy to operate, and it requires relatively little maintenance. For these reasons, refractors are still popular among beginning skywatchers, and the classic research refractors of the 19th and early 20th centuries remain useful scientific tools.

Galileo's refractor

A refractor uses a series of glass lenses to gather and focus light. In this diagram, light passes through the large primary 'objective' lens, which focuses the light to a point near the back of the telescope tube. A second lens magnifies the image and corrects for some distortions caused by the primary lens. More complicated refractors use even more lenses.