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, which is a measure of the light-gathering power and resolution of a reflecting telescope. These giant telescopes use huge mirrors to collect light from distant stars, planets, and galaxies. Some mirrors can be even larger than the space they use, and special techniques like aperture synthesis through interferometry 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 and can take very clear pictures over long periods. Earth-based telescopes, however, can be updated more easily and often use active optics and adaptive optics to remove the 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, which is the diameter of a circle with the same light-collecting area as the telescope. Aperture is important because it tells us how much light a telescope can gather and how detailed the pictures it takes can be.

The list includes telescopes with an effective aperture of at least 3.00 metres (118 inches) that work at visible or near-infrared wavelengths. Some telescopes with multiple mirrors on the same mount are ranked by their combined aperture, while others that use optical interferometry 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 either being built now or are plans for future construction.

The Extremely Large Telescope in Chile will be 39.5 metres wide when 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.