List of largest optical reflecting telescopes

This list of the largest optical reflecting telescopes with objective diameters of 3.0 metres (120 in) or greater is sorted by aperture. Aperture measures how much light a reflecting telescope can gather.

These big telescopes use huge mirrors to collect light from faraway stars, planets, and galaxies. Some mirrors can be larger than the space they use. Special techniques can combine signals from different mirrors to see even finer details.

Largest does not always mean the best telescope. For example, space-based telescopes, such as the Hubble Space Telescope, fly above Earth’s atmosphere. They can take very clear pictures over long periods. Earth-based telescopes can be updated more easily. They often use active optics and adaptive optics to remove blurring caused by Earth’s air. By placing these telescopes on mountains with steady air and good weather, scientists can see the universe in amazing detail.

Table of reflecting telescopes

This list shows the largest optical reflecting telescopes, ordered by their aperture. The aperture is the size of the telescope's main mirror, and it tells us how much light the telescope can gather.

The list includes telescopes that are at least 3.00 metres (118 inches) wide and that work with visible or near-infrared light. Some telescopes have more than one mirror together, and these are ranked by how wide they are all together. Others use a special method called optical interferometry and are listed separately.

Chronological list of largest telescopes

These telescopes were the biggest in the world when they were built, measured by how wide their main mirrors are. This list shows them in the order they were created. Each one was the largest at the time until a new, even bigger telescope was made.

Future telescopes

These are telescopes that are being built now or are plans for future building.

The Extremely Large Telescope in Chile will be 39.5 metres wide when it is finished, with its first light planned for 2029. The Thirty Meter Telescope in Hawaii began building in 2014 but stopped and has not continued as of 2022. The Giant Magellan Telescope in Chile will use seven large mirrors to act like a single 21.4 metre telescope, also aiming for first light in 2029. Other telescopes under construction or planning include the San Pedro Martir Telescope in Mexico, the Magdalena Ridge Observatory Interferometer in New Mexico, and the Timau National Observatory in Indonesia.

Some telescopes are still only ideas but are being designed, like the Habitable Worlds Observatory which would be a space telescope launched in the 2040s, the MUltiplexed Survey Telescope, and the Chinese Giant Solar Telescope.