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The US-Extremely Large Telescope Program

The US ELT Program


New research frontiers in astronomy and astrophysics will open with the advent of ground-based Extremely Large optical-infrared Telescopes (ELTs,  with primary mirrors in the 25-m to 40-m range). Two projects with US leadership plan to build and operate ELTs in the next decade, one in the Northern hemisphere (the Thirty Meter Telescope, 30-meter in diameter), and the other in the Southern hemisphere (the Giant Magellan Telescope, 25-meter in diameter). A third project, the E-ELT, led by the European Southern Observatory, aims to build a 40-meter diameter ELT also in the Southern hemisphere.


USELT strategy

The US-ELT Program will provide access to all night time sky and a unique suite of instruments to the US science community.

U.S. scientific leadership will be significantly enhanced if the broad U.S. science community can take advantage of the power of the two US ELT projects. In that context, the National Science Foundation’s (NSF) National Optical Infrared Astronomy Research Laboratory (NOIRLab), the Giant Magellan Telescope Organization (GMTO), and the Thirty Meter Telescope International Observatory (TIO) have embarked on developing the U.S. Extremely Large Telescope Program (US-ELTP). These organizations plan to submit for peer-review proposals that seek a federal contribution to complete the telescopes and make at least 25% of the observing time available to the US astronomical community on each of these two facilities.  The US-ELTP will enable scientists anywhere in the US to create and lead projects taking advantage of the combined full-sky coverage, as well as diverse and complementary instrument capabilities of GMT and TMT, to address the most compelling and challenging research questions in astrophysics.

USELT Sky Coverage

Sky coverage provided by the bi-hemispheric US-ELT Program

The combined apertures of TMT and GMT are comparable to that of the single European ELT, but for many scientific programs, access to two telescopes and a diverse instrument suite offers clear advantages.  All-sky coverage enables observation of relatively rare phenomena (e.g., the number of observable rocky planets in the habitable zone is predicted to be small), unique targets in each hemisphere (e.g., Magellanic Clouds in the Southern Hemisphere, Andromeda group galaxies in the North), and key survey fields with unique multiwavelength data sets (e.g., GOODS-North and -South; LSST Deep Drilling fields; the Kepler main survey field; ecliptic pole deep survey regions for TESS, Euclid, WFIRST and JWST). Together, GMT and TMT will access the full celestial sphere, with roughly 50% sky overlap allowing their complementary instrument suites to be used for joint or even simultaneous investigation of many objects.  In addition to separation in latitude, their different longitudes create valuable opportunities for time-domain research, especially for rapidly changing phenomena that benefit from high-cadence observations. Finally, two platforms can offer a greater variety of instrumentation than would be available on a single telescope, and more observing hours to support long term variability and large scale programs that would otherwise require many years to complete. A description of the science programs expected to be carried out with the TMT and GMT can be found on their respective science books: TMT Detailed Science Case, and GMT Science Book. An overview of the instruments planned for these two facilities can be found on their webpages: TMT science instruments, and GMT science instruments.

The Astro2020 Decadal Survey


The on-going Decadal Survey on Astronomy and Astrophysics (also named "Astro2020") is a partnership between the National Academy of Sciences and the astronomical community aiming at identifing science priorities in astronomy and astrophysics for the next decade. While the survey is sponsored by NASA, NSF, and DOE, it is driven by input received by the community in the form of white papers, either about specific science programs (science white papers), or about more general considerations for state of the profession (APC white papers).

Concretely, the Astro2020 process will produce a report (e.g. last decadal report from 2010) presenting a broad vision for developing transformative science, and will provide guidelines and recommendations to scientists, policy makers, and federal agencies to develop a comprehensive and strategic scientific investments for the next decade.


The US-ELTP and Astro2020

All 573 science white papers submitted as part of Astro2020 can be accessed on a dedicated AAS webpage. Many of these papers make reference to the need for the US community to have access to ELTs for carrying out the transformative astronomical program they have designed for the upcoming decades. The 294 submitted APC white papers can similarly be accessed on the same AAS website, and US-ELTP team members have themselves submitted two such white papers: A white paper presenting the details of the US Extremely Large Telescope Program, and another white paper on Observatory operation costs.

US-ELT key-science programs

In preparation for the Astro2020 Decadal Survey, the US science community was invited to design key-science programs to address questions of fundamental scientific importance through access to TMT and/or GMT, and by exposing a well-posed observing program describing the measurements to be acquired by the suite of instruments available at both facilities. The Astro2020 science white papers that were based on these USELT key-science programs can be found on this website hosted by the NOIRLAB.