Thirty Meter Telescope: Alternative telescope site is a good back-up

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Your report on discussions about a possible location for the Thirty Meter Telescope (TMT) calls for some clarification (Nature (2017); see also Nature; 2016). Although the TMT board's preferred site is Mauna Kea in Hawaii, the alternative location at the Observatorio del Roque de los Muchachos (ORM) on La Palma, Canary Islands, is not as disastrous as you imply.

Last year's evaluation by the TMT committee revealed that turbulence and average dust at ORM are similar to those at Mauna Kea (see Indeed, dust densities above 100 micrograms per cubic metre, when the telescope's enclosure must be shut, are statistically more frequent at Mauna Kea. The fraction of clear nights is therefore about the same for both sites.

The committee concluded that ORM is an excellent alternative to Mauna Kea for operating a telescope that has to use adaptive optics. The only observations affected are those of the thermal infrared, which can be improved by flexible scheduling.

In addition, the atmospheric stability is better in La Palma than at the European Southern Observatory's Extremely Large Telescope on Cerro Armazones, Chile — the TMT's leading competitor. Because the heights of the locations are similar, the disadvantages in the thermal infrared are nearly the same relative to Hawaii.

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  1. Wooptix, San Cristobal de La Laguna, Tenerife, Spain.

    • Sergio Bonaque-González

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