The Overwhelmingly Large Telescope (OWL) was a conceptual design by the European Southern Observatory (ESO) organisation for an extremely large telescope, which was intended to have a single aperture of 100 metres in diameter.

[1] While the original 100 m design would not exceed the angular resolving power of interferometric telescopes, it would have exceptional light-gathering and imaging capacity.

[2] The OWL could be expected to regularly see astronomical objects with an apparent magnitude of 38, or 1,500 times fainter than the faintest object that has been detected by the Hubble Space Telescope.

However, the projected cost (of around €1.5 billion) was considered too high, so the ESO is now building the smaller Extremely Large Telescope around 39 m in diameter.

[7] It has been estimated that a telescope with a diameter of 80 meters would be able to spectroscopically analyse Earth-size planets around the forty nearest sun-like stars.