TMT Construction Proposal - Thirty Meter Telescope

4 Site
4.1 Overview
TMT needs to be built on the best available site to obtain the maximum return from its science potential.
Careful site selection has therefore been extremely important to TMT from the very beginning of the project.
The site selection process started in 2001/2002, in a collaboration between the Giant Segmented Mirror
Telescope (GSMT) and the California Extremely Large Telescope (CELT), with the pre-selection of five
candidate sites (see Section 4.3) to be studied in detail. On-site testing via the operation of small but complex
remote site monitoring observatories has been in progress since 2003 and will continue until final selection of
the TMT site. The site evaluation process employs a site ranking metric which allows for an objective
comparison of the technical properties of the candidate sites and their science producing implications.
Selection of the TMT site is expected in mid 2008.
Detailed descriptions of the site testing program, the requirements on this program, the candidate sites and
their pre-selection, the instruments and methods used, and the results are given in the "Site Selection
Requirements and Strategy Document"[1], the “Site Selection Intermediate Report”[2] and the “Site
Qualification Report”[3].
4.2 Site requirements
The TMT site needs to be suited for producing astronomical data of superb quality and for building and
operating an observatory of the size and complexity of TMT. Strict technical requirements as they apply to
other parts of the project do not exist for the TMT site, because there are no hard cut-offs for the parameters
entering the TMT site decision beyond which a site becomes unsuitable for TMT. Instead, the site selection
process involves measuring and predicting the properties of the sites, both technical and programmatic, and
balancing them so as to determine the site that best meets the TMT needs.
4.3 Candidate sites
Five TMT candidate sites were chosen from a list of potentially interesting sites based on existing information
(previous site characterization campaigns and data from existing observatories) and satellite data studies of
cloud cover and precipitable water vapor[4,5]. The names and properties of these sites are summarized in
Table 4-1, with photographs of each shown in Figure 4-1.
4.4 Characterization of candidate sites
The TMT site decision will be based on both technical and programmatic aspects. Technical site properties are
assessed predominantly through data acquired in a multi-year study of the site conditions using identical
equipment. To acquire these data, the TMT site testing team has been operating remote site monitoring
observatories at each of the candidate sites. Considerable effort has gone into calibrating all equipment
through side-by-side comparisons of identical instruments and, when possible, by comparison with other
instruments. The instruments and methods as well as the data and results verifications are summarized in this
section. This is followed by a description of the programmatic aspects being investigated. Details of all aspects
of the site selection program can be found in References [1-3].
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