List of largest optical telescopes in the 20th century

Diverging methods of construction and use for reflecting telescopes in that area make comparing synthesized aperture irregular.

However, many famous optical telescopes have had large apertures on their primary mirror with corresponding good angular resolution.

The list covers from about 1901 to 2001, with some flexibility to accommodate ambiguity in classification, for example some 2002 telescopes, if it was nearly operation in 2001.

Multiple mirror telescopes are ranked by their equivalent optical area, not peak interferometric aperture unless it is not relevant for the design.

This table does not include all the 20th largest mirrors manufactured; the Steward Observatory Mirror Lab produced the 6.5-metre f/1.25 collimator used in the Large Optical Test and Integration Site of Lockheed Martin, used for vacuum optical testing of other telescopes.