Fifty years have passed since I first arrived in the Canary Islands to determine whether the conditions on the heights of these Islands really were good enough for modern astronomy with large telescopes. After several years of astronomical prospecting, I became convinced that the Canary Islands’ skies were outstanding and that it was necessary to open them to the international scientific community.

Astronomers from a variety of countries continued testing throughout the 1970s, confirming that conditions for astronomy at the Teide Observatory in Tenerife, and the Roque de los Muchachos Observatory on La Palma, were indeed exceptional. Many long years were then spent negotiating the terms for our observatories to be opened up, which ultimately became a multinational treaty: the Agreement and Protocol on Cooperation in Astrophysics and the various Third Level Agreements that were signed with the user institutions.