As discussed in Chapter 3, there are three projects underway in the world to construct and operate a new generation of extremely large telescopes with diameters in the range of 23 to 42 meters (Figure 7.9). The Giant Magellan Telescope (GMT) is composed of seven 8.4-meter mirrors and has an aperture equivalent to a single 23-meter mirror; it will be sited at the Las Campanas Observatory (Chile). The GMT design builds on the success of the two 6.5-meter Magellan Telescopes. The Thirty-Meter Telescope (TMT) is composed of almost 500 1.44-meter segments, has an aperture equivalent to a single mirror 30 meters in diameter, and will be sited at Mauna Kea (Hawaii). It builds on the success of the two 10-meter Keck Telescopes. The European Extremely Large Telescope (E-ELT) has a segmented mirror design with an aperture equivalent to a single mirror 42 meters in diameter. Its recommended site is at Cerro Armazones in Chile. The project is led by the European Southern Observatory (ESO) and has a mirror segment design similar to that of TMT.

The committee concluded that more than one GSMT will be required in the world to fully exploit the identified science opportunities. The reasons are that there are advantages to having capability in two hemispheres, or two in the same hemisphere with different instrument capabilities requiring different optimizations of telescope design, and that so many new scientific problems can be addressed that any credible number of GSMTs is likely to be oversubscribed. It is imperative that at least one of the U.S.-led telescope projects have U.S. federal investment. Such a federal role will leverage the very significant U.S. private investment, will maximize the potential for the project’s success, will help to optimize the U.S. scientific return on other federal investments (ALMA, JWST, and LSST), and will position the NSF for leadership in future large-telescope projects beyond GSMT. Since both GMT and TMT are already international public-private partnerships, federal involvement with either one is consistent with the international collaboration strategy that is a recurring theme in this survey and would ensure U.S. leadership in one international large telescope. Such leadership would further another important strategy advocated in this report: cooperation with other countries so as to develop complementary capabilities that will maximize the science output. In the case of GSMT this means coordination with ESO on technology development and instrument selection to create a global system of GSMTs with optimal complementary and scientific reach. The committee notes that public time on a GSMT would, in principle, be subject to the open skies policy in effect for all federally supported U.S. telescopes. It is the committee’s hope that a result would be corresponding reciprocal access to major optical-infrared telescopes abroad.

The committee reviewed a technical risk assessment and sensitivity analysis of the anticipated cost and schedule for GMT and TMT that indicated the risk is medium to medium high. A cost sensitivity study based only on the telescope optics and instruments concluded that the construction costs of GMT and TMT would be $1.1 billion and $1.4 billion, respectively (at a 70 percent confidence



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