the two projects be made as soon as possible for a federal partnership at a level of about a 25 percent investment in one of them. A schedule and budget plan should then be developed. The survey appraises a total GSMT construction cost in the range of $1.1 billion (GMT appraisal) to $1.4 billion (TMT appraisal) and assumes that the federal share of the capital cost will be borne by MREFC, while recognizing that the total share may be secured through whatever combination of capital cost, operating funds, and instrumentation support is most favorable. The operations federal cost share is expected to carried by NSF-Astronomy. Both telescope projects estimated their annual operations costs (including facility and instrument upgrades) at around $50 million ($36 million, GMT; $55 million, TMT). Although the committee did not analyze these estimates in detail, they are far below the usual rule of thumb for large projects (10 percent of construction costs per year).

The committee believes that a GSMT will, as large telescopes have in the past, transform U.S. astronomy because of the telescope’s broad and powerful scientific reach, and that federal investment in a GSMT is vital to U.S. competitiveness in ground-based optical astronomy over the next two decades. These are the main reasons for the committee’s strong recommendation of GSMT.

The third-place ranking also results from the requirement in the committee’s charge that the survey’s prioritization be informed not only by scientific potential but also by the technical readiness of the components and the system, the sources of risk, and the appraisal of costs. LSST and several of the concatenation of candidates for the Mid-Scale Innovations Program were deemed to be ahead of GSMT in these areas. The committee also took into account programmatic concerns such as the time it will take to implement the committee’s recommendation for a choice to be made on which one of the two U.S.-led GSMT concepts NSF will partner, and the time it would take for any MREFC decision to be made and federal funds awarded. The committee’s setting of the relative positions of its top three ranked activities resulted from its consideration of all these various factors.

Atmospheric Čerenkov Telescope Array (ACTA)

The past decade has seen the coming of age of very high energy tera-electron-volt (TeV) gamma-ray astronomy. Plans are underway to capitalize on recent scientific advances by building a large facility that uses light created as gamma rays interact with the atmosphere and that will achieve an order-of-magnitude greater sensitivity compared to current telescopes. This new gamma-ray observatory will detect a wide variety of high-energy astrophysical sources and seek indirect evidence for dark matter annihilation. Two facilities, the European Čerenkov Telescope Array (CTA) and the U.S. Advanced Gamma-ray Imaging System (AGIS), have been proposed. The survey appraised the full AGIS project cost to be in the $400 million range. The technical risk was judged to be medium low. The committee recommends



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