Euro50 Telescope. Composite artwork of the 50m European Telescope (Euro50) which, if built, will comprise a primary mirror 50 metres in diameter. This will make it the biggest optical telescope in the world. The Euro50 also includes a secondary mirror located above the primary mirror, a design known as a Gregoriansystem. It will utilise adaptive optics to compensate for light distortion by the Earth's atmosphere. The telescope will be 85 metres tall and weigh 3500 tons. It is one of two proposals for the European Extremely Large Telescope (ELT) project. The other design is the Overwhelmingly Large Telescope (OWL).