Primary mirror

A primary mirror is the most important part of a reflecting telescope. It serves as the principal light-gathering surface, often called the objective. This component is vital because it collects light from distant objects in space. The mirror must be shaped into a specific disk to work correctly. These shapes can be spherical, parabolic, or hyperbolic.

By using these precise shapes, the telescope can focus light to create a clear image.

To work well, the mirror must be highly reflective. In the past, makers used a material called speculum metal. They would polish this metal into a disk to reflect light. Later, scientists changed how they made mirrors to get better results. They began using glass as a base instead of solid metal. They would then add a reflective layer on top of the glass. One method was using silver on glass, which was seen in the 19th-century Crossley reflector.

Modern telescopes often use vacuum deposited aluminum on glass. This process involves placing a thin layer of aluminum onto the glass inside a vacuum.

Building these mirrors has changed significantly over the centuries. One of the first known reflecting telescopes was Newton's reflector. Isaac Newton created this device in 1668. His primary mirror was made of polished metal and was quite small. It measured only 3.3 cm in diameter. As technology improved, mirrors became much larger and more precise. The 200-inch Hale telescope represented a major leap by using aluminum on glass. Each new material and method allowed astronomers to see much further into the universe.

There is a major physical limit to how large a single mirror can be. A solid primary mirror must be strong enough to sustain its own weight. It cannot deform or bend under the pull of gravity. If the mirror bends even slightly, it will not focus light correctly. To solve this problem, engineers use segmented mirror configurations. Instead of one huge piece, they use several smaller mirrors together.

This allows telescopes to reach much larger sizes than a single piece of glass would allow.

Different telescopes use different mirror setups depending on their goals. The Giant Magellan Telescope is a great example of segmented design. It will use seven 8.4 meter primary mirrors. When used together, they provide a resolving power equivalent to a 24.5 m (80.4 ft) optical aperture. The Hubble Space Telescope uses a single primary mirror that is 2.4 meters (7 feet 10 inches) wide.

Other large telescopes also use multiple mirrors to achieve high resolution.

Some of the world's largest telescopes use unique mirror arrangements. The Subaru telescope has an 8.2 m (27 ft) single mirror. It is located at the Mauna Kea Observatory in Hawaii and has been there since 1997. The Large Binocular Telescope uses two 8.4 m (28 ft) mirrors. These two mirrors can be used together in an interferometric mode. Additionally, the dual Keck telescope uses segmented primary mirrors that are 10 m in size. These different designs help scientists study different parts of the sky.

Radio and submillimeter telescopes work differently than optical telescopes. They use much larger dishes or antennae to collect signals. These dishes do not need to be as precisely shaped as optical mirrors. For example, the Arecibo Telescope used a massive 305 m dish. At one time, it was the world's largest single-dish radio telescope fixed to the ground. The Green Bank Telescope has a 100 m diameter dish. It is the world's largest steerable single radio dish. While radio arrays of multiple dishes have better resolution, they sometimes have less sensitivity than single dishes.