While the first telescopes gathered and focused starlight with clear glass lenses, modern research telescopes use polished glass mirrors. Starlight “bounces” off a series of mirrors and into a spectrograph, camera, or other electronic detector. The largest reflectors are far bigger than the largest refractors, allowing them to see objects that are much fainter and farther away.
The heart of a reflector is its primary mirror at the base of the telescope. (Older or smaller mirrors are mounted inside tubes, while larger, more-modern designs use an open framework.) This mirror gathers starlight and brings it to a focus at a point above the mirror.
The earliest reflectors used the same type of glass as wine bottles, but this type of glass expands and contracts a great deal as the temperature warms during the day and cools at night. Beginning in the 1930s, telescope makers switched to Pyrex, the same type of glass that’s used in oven-safe cookware. Today’s mirrors use a similar type of glass with more high-tech properties. The mirror is coated with aluminum to provide a bright reflection.
The greatest challenge in building a reflector is grinding the mirror to the proper curved shape. An error of a millionth of an inch can create blurred vision, known as spherical aberration, as happened with the primary mirror aboard Hubble Space Telescope. It typically takes months or years to carefully grind the surface, measure the curvature, and fine-tune the precise shape.
The primary mirror’s diameter determines the size of the telescope. The Harlan J. Smith Telescope at McDonald Observatory, for example, has a primary mirror that is 107 inches (2.7 meters) in diameter, so the telescope is typically referred to as the “107-inch” or “2.7-meter.”
The size of the mirror also determines how much light the telescope gathers. The 200-inch telescope at Palomar Observatory in California, for example, collects almost four times as much light as the 107-inch Harlan J. Smith Telescope at McDonald Observatory.
The mirror’s power is that it gathers a great deal of light from a small patch of sky and “pours” it all into the telescope’s scientific instruments. It’s like gathering rainwater with a barrel and having it funnel into a drinking glass: The large collecting area allows you to catch a lot of water.
While the mirrors in most reflectors are made from a single piece of glass, giant mirrors face the same problem as giant glass lenses in refractors: gravity. As the telescope turns, gravity distorts the shape of the mirror, and can cause the mirror to sag under its own weight. These problems are overcome by mounting the mirror on a rigid structure that maintains its shape, and by making the mirrors as thin as possible to reduce their weight.
Another solution for this problem is to make the mirror out of several pieces of glass that fit together like tiles on a floor. This design reduces the weight of the final mirror and makes it easier to manufacture. It requires precise computer control systems to keep the mirror segments aligned, however. Every telescope with a primary mirror greater than about 8.5 meters in diameter uses this design, including the Hobby-Eberly Telescope at McDonald Observatory and the twin Keck telescopes in Hawaii. Engineers are designing even larger segmented-mirror telescopes, which could have diameters of 30 meters (100 feet) or more.