On Saturday 15 October, the European Southern Observatory (ESO) opened the doors of its headquarters in Garching bei Munchen, Bavaria, Germany, to the public.

Throughout the day, thousands of visitors had the chance to help build a full-size mock-up mirror of the largest planned telescope in the world - the European Extremely Large Telescope (E-ELT) - and to experience many other aspects of ESO's work.

Around 3100 people came to ESO's headquarters on Saturday, curious to find out more about ESO's world-class facilities and front-line scientific results. Visitors to the ESO Open House Day 2011 had a wide variety of activities to choose from.

The highlight was to take part in the construction of a mock-up mirror for the European Extremely Large Telescope (E-ELT), giving participants a unique chance to experience the true scale of the world's biggest eye on the sky.

Enthusiastic visitors queued up to put the 798 cardboard hexagons in place, each measuring around 1.4 meters across. Using pegs and an outline of the mirror on the ground, visitors placed the hexagons on the ground to slowly form the giant mirror.

Meanwhile, a time-lapse video of the assembly was filmed from the roof of the ESO building. When finished, the full-size mock-up measured 39.3 meters across and occupied an area of nearly 1000 square meters next to ESO Headquarters.

There were talks on current hot topics in astronomy in ESO's auditorium, and video links were established to the Paranal Observatory in Chile, giving people the opportunity to chat live with ESO astronomers working on site.

Children and their parents enjoyed having their photo taken in the infrared, and looking at the Sun through telescopes equipped with suitable filters.

Among other fun activities, visitors were able to write postcards from the edge of the Universe to their friends and family - to be sent by ESO - and travelled between the stars in a Sky-Skan planetarium and with the Stellarium program.

Three exhibitions were dedicated to ESO's observatories: the Very Large Telescope (VLT), the Atacama Large Millimeter/submillimeter Array (ALMA) and the European Extremely Large Telescope (E-ELT).

Visitors saw prototype components for the E-ELT, including a giant deformable mirror, and were able to control a real segment of the main E-ELT mirror. They also took a virtual tour through the E-ELT and steered a virtual giant transporter for the ALMA antennas.