Thirty Meter Telescope Site Testing I: Overview

Thirty Meter Telescope Site Testing I: Overview M. Schöck,1 S. Els,2 R. Riddle,3 W. Skidmore,3 T. Travouillon,3 R. Blum,4 E. Bustos,2 G. Chanan,5 S.G. Djorgovski,6 P. Gillett,3 B. Gregory,2 J. Nelson,7 A. Otárola,3 J. Seguel,2 J. Vasquez,2 A. Walker,2 D. Walker2 and L. Wang3 ABSTRACT 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 Mártir 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. Subject headings: Astronomical phenomena and seeing, site testing, extremely large telescopes 1TMT Observatory Corporation, NRC Herzberg Institute of Astrophysics, 5071 West Saanich Road Victoria, BC V9E 2E7, Canada arXiv:0904.1183v1 [astro-ph.IM] 7 Apr 2009 2Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, Casilla 603, La Serena, Chile 3TMT Observatory Corporation, 2632 E Washington Blvd, Pasadena, CA 91107, USA 4National Optical Astronomy Observatory, 950 N Cherry Ave, Tucson, AZ 85719, USA 5Department of Physics and Astronomy, 4129 Frederick Reines Hall, University of California, Irvine, CA 92697-4575, USA 6Astronomy Department, California Institute of Technology, MC 105-24, 1200 East California Blvd, Pasadena, CA 91125, USA 7Center for Adaptive Optics, University of California, 1156 High St, Santa Cruz, CA 95064, USA –2– 1. TMT site testing and selection basics In April 2008, the Thirty Meter Telescope (TMT) Project reduced its short list of candidate sites to two, Cerro Armazones in northern Chile and the “13 North” (13N) site on Mauna Kea, Hawaii. This decision officially ended the TMT site testing work after five years of in-situ measurements, during which at least 2.5 annual cycles of data were acquired on each of the five candidate sites. The practical work at the sites was preceded by several years of preparatory work, most notably a series of satellite data studies of cloud cover and precipitable water vapor (PWV) of sites in Chile, southwestern North America and Hawaii, on the basis of which the candidate sites were selected. This paper is the first in a series of twelve articles, hereafter referred to as ‘TMT Site Testing’ 1 to 12 (TST-1 to TST-12), describing the TMT site testing process. It con- tains descriptions of the general principles underlying the TMT site testing work and the selection of the candidate sites, as well as summaries of the instrumentation, methodology and the top-level results. TST-2 (Walker et al. 2009) provides are detailed account of the process by which the candidate sites were selected. TST-3 and TST-4 (Riddle et al. 2009b,c) are descriptions of the equipment used, the efforts undertaken to ensure data quality and the methods by which the individual pieces were put together to create systems that operate reliably and autonomously at remote sites. TST-5 to TST-11 (Skidmore et al. 2009b; Els et al. 2009; Travouillon et al. 2009; Skidmore et al. 2009a,c; Otárola et al. 2009; Riddle et al. 2009a) contain detailed descriptions of the results obtained with the instru- ment suite, organized by parameter category: integrated turbulence parameters (TST-5), turbulence profiles (TST-6), turbulence coherence time (TST-7), meteorological parameters (TST-8), cloud cover and light pollution (TST-9), precipitable water vapor (TST-10), and combinations and correlations of parameters (TST-11). The final paper in the series, TST-12 (Schöck et al. 2009), describes how this wealth of information was interpreted and used to determine which sites are qualified to host TMT as far as their atmospheric parameters are concerned. The selection of a site is a critical issue for TMT on many levels. Obviously, the TMT site needs to be suited for producing astronomical data of superb quality and for maximizing the scientific productivity of the observatory over its lifetime. In addition, the site has tangible consequences beyond its direct impact on science. It strongly affects the cost and ease of observatory construction and operation. It affects the activities of management, technical support, and personnel recruiting. On a more subtle level, a detailed characterization of the site can affect the telescope and dome design (for example, due to the wind speed distribution, mechanical properties of the soil, seismicity, etc.), and the adaptive optics (AO) design (through the various atmospheric turbulence properties). Site selection and –3– testing were therefore given high priority from the very beginning of TMT and its precursor projects, the Giant Segmented Mirror Telescope (GSMT), the California Extremely Large Telescope (CELT) and the Very Large Optical Telescope (VLOT). [For simplicity, we refer to all these efforts as ‘TMT site testing’, even when they happened before the existence of the actual TMT Project.] Another early decision was not to create requirements for the TMT site in form of limits for certain parameters, as there are generally no hard cut-offs beyond which a site becomes unsuitable. Instead, TMT opted to measure and predict both the technical and programmatic properties of the sites with the highest accuracy and longest temporal baseline possible within the framework of the program. The methodology by which these parameters are balanced against each other was developed during the course of the site testing process. This included input from the different telescope design teams by means of quarterly internal reviews, as well as approximately annual external reviews which were accompanied by the issuance of comprehensive reports of the site testing process and results. The balancing of the atmospheric parameters is the subject of the last paper of this series, TST-12. Other technical considerations such as geological and geotechnical conditions of the site (seismic activity, mechanical integrity of the soil, vibration transmission properties, etc.) and programmatic considerations such as construction and operating cost and methods, cultural, environmental and land use considerations, labor force availability, proximity to astronomers and astronomy infrastructure, the economic impact of siting TMT, permitting, land ownership, availability of infrastructure and transportation, and customs and immigration issues are not discussed in this series as they were not assessed as part of the site testing work. 2. Selection of TMT candidate sites TMT started its site testing and site selection efforts by considering all potentially interesting sites on Earth as potential candidates. This work started in the late 1990’s with a series of meetings and exchanges of ideas and was originally led by the Cerro Tololo Inter- American Observatory (CTIO). This phase is described in a dedicated paper in this series, TST-2. In summary, existing information from previous site testing campaigns and from existing observatories was combined with general knowledge of atmospheric behavior. This process produced the list of ‘usual suspects’ of regions of interest. For the most part, these can be divided into three types of superb sites in terms of atmospheric properties: 1. Coastal mountain ranges next to a cold ocean current with stable subtropical anticy- clone conditions. The proximity to the coast allows for unperturbed laminar air flow. –4– The cold ocean, whose influence may extend to some distance inland, keeps the inversion layer low. These conditions exist on the coasts of California, Baja California, northern Chile and Namibia. 2. Isolated high mountains on islands in temperate oceans, where the weather is good and a laminar air flow and large thermal inertia of surrounding ocean keep the inversion layer low. There are two such locations known on this planet: the Hawaiian and the Canary Islands, although other locations might exist, such as Réunion off the eastern coast of Africa. 3. A number of high points of the Antarctic plateau where katabatic winds and the absence of the jet stream cause low turbulence (above a thin ground layer) and the low temperatures and high altitude create low thermal background and water vapor content. The first two types of sites have been know for a long time (see, for example, Walker (1971) and references therein). The third type has only recently been tested and analyzed quantitatively (e.g., Lawrence et al. (2004); Swain & Gallée (2006)). While it is generally believed that most inland sites would be inferior to sites in these three categories, mainly due to turbulence-generating topography, the situation is not always clear. An example demonstrating that superb sites (at least in terms of seeing) do not all fit this pattern is Maidanak in Uzbekistan (Ehgamberdiev et al. 2000). It is possible that some excellent and so far undocumented sites exist in, for example, northern Mexico, or the high mountains of northern Africa. Site testing in the high Arctic is also beginning to investigate whether sites with comparable conditions can be found there (Steinbring et al. 2008). A detailed study of all regions fitting the descriptions above is, of course, impractical even for a project of the magnitude of an extremely large telescope (ELT). Thus, a first cut, based on both the expected atmospheric properties of the sites and practical concerns, reduced the regions of interest for TMT to northern Chile, the southwestern continental United States, northern Mexico and the Hawaiian Islands.

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