We welcome the European Southern Observatory's decision to site the European Extremely Large Telescope (E-ELT) at Cerro Armazones in Chile.

In his Correspondence on the subject, Francisco Sánchez argues that the high seismicity of Chile's Armazones region could damage the telescope's optical systems (Nature 464, 977; 2010). However, he is misinformed about the vulnerability of telescopes under seismic conditions and about the costs of preventing possible damage to them.

There have been several large earthquakes in northern Chile during the past 40 years, the most recent being the Maule earthquake on 27 February 2010. None of the observatories — including the Very Large Telescope at Paranal — suffered any significant damage.

Seismic isolation reduces structural accelerations and forces by 6–8 times compared with those affecting a conventional structure. To guarantee full operational continuity for the E-ELT, approximately 52 seismic isolators will be required, each one costing up to US$10,000 — a total of half a million dollars. This is a long way from doubling the cost of the structure (€1.1 billion; US$1.5 billion), as Sánchez claims.

The costs of controlling the effects of seismicity are negligible in comparison with those of constructing the E-ELT and its instrumentation.