As part of the conceptual and preliminary design processes of the Thirty Meter Telescope (TMT), the TMT site testing team has spent the last five years measuring the atmospheric properties of five candidate mountains in North and South America with an unprecedented array of instrumentation. The site testing period was preceded by several years of analyses selecting the five candidates, Cerros Tolar, Armazones and Tolonchar in northern Chile; San Pedro M´artir in Baja California, Mexico and the 13 North (13N) site on Mauna Kea, Hawaii. Site testing was concluded by the selection of two remaining sites for further consideration, Armazones and Mauna Kea 13N. It showed that all five candidates are excellent sites for an extremely large astronomical observatory and that none of the sites stands out as the obvious and only logical choice based on its combined properties. This is the first article in a series discussing the TMT site testing project.
Over the last eight years, the Thirty Meter Telescope and its precursor projects have invested considerable resources into identifying the best possible candidate sites for TMT. The work began by considering as complete a list as feasible of all potentially interesting sites on Earth. This list was first narrowed down to three regions in the western hemisphere,
northern Chile, southwestern North America, and Hawaii based on existing information from previous site testing work and general knowledge about the atmosphere. Through analyses of cloud cover and precipitable water vapor using satellite data, five candidate sites were selected for in-situ testing, namely Cerros Tolar, Armazones and Tolonchar in northern Chile; San Pedro M´artir in northern Baja California, Mexico and Mauna Kea 13N, Hawaii.
TMT then began an extensive on-site testing effort for which we equipped each of the five candidate sites with a large array of instrumentation for analyzing atmospheric properties, with a special emphasis on turbulence and turbulence profiles. All equipment was calibrated
and characterized carefully before deployment to the candidate sites, where it operated for 2.5 to 5 years. The results show that all candidate sites are excellent and are clearly among the best ground-based telescope sites on Earth. They also show that not a single site is perfect or is the best (or worst) in all parameters. This is, of course, a welcome outcome, as
it means that the selection of candidate sites was successful and that the final site selection can safely also take into account other considerations, without compromising the expected scientific output of TMT.
Based on the site testing data and on other technical and programmatic considerations, the TMT Project selected two sites for further consideration, one in the northern hemisphere,one in the southern hemisphere. They are Cerro Armazones and Mauna Kea 13N.