Thirty Meter Telescope - Background Information & FAQ

The US TMT Science Working Group (SWG) and the Thirty Meter Telescope project office have compiled this page of information about the observatory, its planned instrumentation, the partnership, and potential US national participation in TMT.   The FAQ was originally compiled for a community survey carried out by the SWG in September-October 2014.  It is far from being comprehensive, but it should provide a useful introduction to TMT for interested scientists, and there are links to other web sites and presentations with more detailed information.

Since 2015, plans for TMT construction on Maunakea have been on hold for review of legal permitting issues.      While Hawaii remains the project's first choice for building TMT, an alternate site on La Palma, Spain, has been selected in case construction on Maunakea is not feasible.   This FAQ has only been minimally updated to take these issues into account.  In particular, new information on timelines and construction costs are not yet available.

For any other questions, or if you have comments about this FAQ or TMT in general, please write to the US TMT SWG.


Telescope, sensitivity, site, timeline

Primary mirror: 30m diameter, 492 hexagonal segments, f/1 focal ratio, 20 arcmin field of view.

Nasmyth focus: f/15, plate scale 0.46 arcsec/millimeter

Diffraction limited angular resolution: 0.017 arcsec at 2 microns

Sensitivity: A basic set of TMT exposure time calculators are available to calculate imaging and spectroscopic sensitivities for the TMT first-light instruments.

Site: In 2009, the TMT project selected the Maunakea Observatories (MKO) in Hawaii as the TMT site after an extensive evaluation process. On 25 July 2014, the Hawaii Board of Land and Natural Resources (BLNR) approved the Maunakea sublease for TMT.  In 2015, protests blocked construction access to the Maunakea site, and in December 2015 the Supreme Court of the State of Hawaii ruled that the state's permitting process for TMT was flawed, and BLNR was ordered to redo the process.  This concluded in September 2017 with the issuance of a new permit.  In 2018, the relevant issues are being considered by the State Supreme Court.

Meanwhile, the TMT project has identified an alternative site in case timely construction on Maunakea is not possible.  This is the Observatorio del Roque de los Muchachos (ORM) on La Palma, Canary Islands, Spain. A permitting process there is underway, and the observatory design modifications that would be necessitated for this site are now well understood.

The TMT project web site provides detailed information about MKO, ORM, and the alternate site selection process.

Project timeline:  The continuing uncertainty about the observatory site leads to a corresponding uncertainty in the project timeline.  If the construction phase restarts in 2019, then early science operations could be expected circa 2028.

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First-light adaptive optics (AO) system:

TMT is designed for extensive use of adaptive optics (AO) from the beginning of science operations. The Narrow Field Infrared Adaptive Optics System (NFIRAOS) is a multi-conjugate adaptive optics (MCAO) system that will operate with a laser guide star facility, and which will feed up to three instruments.

NFIRAOS is designed to deliver a high degree of AO correction over a 30 arcsec diameter science field, and useful (but not diffraction limited) AO correction over an unvignetted field 2 arcmin in diameter. At zenith and in median seeing conditions, the NFIRAOS Design Requirements Document (DRD) specifies maximum wavefront errors of:

  • 191 nanometers RMS over a 17 arcsec diameter field
  • 208 nanometers RMS over a 30 arcsec diameter field

This corresponds to the following minimum Strehl ratios:

RMS wavefront error Wavelength
1.0 μm 1.25 μm 1.65 μm 2.2 μm
191 nm: 0.24 0.40 0.59 0.74
208 nm: 0.18 0.34 0.53 0.70

Potential future upgrades ("NFIRAOS+") would improve the wavefront errors to approximately 133 nm RMS over a 30 arcsec diameter field of view, corresponding to Strehl ratios of 0.50 at 1.0 μm and 0.87 at 2.2 μm.

Other NFIRAOS requirements include:

  • >50% sky coverage at Galactic poles
  • Throughput >80%
  • Thermal background <15% of the ambient sky + telescope
  • High astrometric and photometric accuracy (see the Science-based Requirements Document for specifics)

Among the first-light instruments, IRIS will use NFIRAOS to achieve diffraction-limited performance over a 15 arcsec field of view, while IRMS will benefit from AO-assisted (but not diffraction-limited) images over a 2 arcmin field of view.

References on TMT AO:

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First-light instrumentation:

The TMT Science Advisory Committee selected three first-light instruments for TMT that are funded as part of the construction project and will be operational when early science operations begin. They are:

  • Wide Field Optical Spectrograph (WFOS): Seeing-limited optical imager and multi-slit spectrograph. Spectral range of 0.3-1.0 microns with estimated throughput above 40% at all wavelengths.  The original design featured a total slit length of 500 arc sec (8.3 arc min), with spectral resolution modes at R ≈ 1000 and 5000 (for an 0".75 slit width).  The multiplex at R ≈ 1000 would be up to 100 for 5" slit lengths.  In 2017-2018, WFOS has undergone significant re-design studies that have led to several alternative designs.  The prime contenders are a monolithic, articulated imaging spectrograph, similar to that described above, and a modular design with multiple spectrographs, each operating at R ≈ 5000, fed by fibers over a field approximately 10 arcmin in diameter.  The TMT Science Advisory Committee is reviewing these options for future down-select.
  • Infrared Imaging Spectrometer (IRIS): 0.8 to 2.5 micron integral field spectrometer (field of view up to 4.4"x2.3") and imager (field of view 32"x32"). Spectral resolution R = 4000 to 8000. IRIS is designed for diffraction-limited performance behind the NFIRAOS AO system.
  • Infrared Multi-object Spectrometer (IRMS) (see TMT Forum presentations from Simard 2013 and Konidaris 2014): 0.8 to 2.5 micron imager and multislit spectrograph with a 2 arcminute diameter field of view. Spectral resolution R~5000. IRMS is closely based on the successful Keck MOSFIRE instrument, which provides up to 46 movable slitlets which can also be configured as a long slit. IRMS will have AO-corrected but not diffraction-limited performance over the full 2 arcmin NFIRAOS field of view.  Note: At present, IRMS is no longer being prioritized as a first-light instrument.

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Later-generation instruments:

Studies have been carried out for a variety of 2nd- and later-generation instruments, although none have yet been selected for development. These include:

  • Multi-IFU imaging spectrometer (IRMOS) - multiple near-infrared IFUs deployable over a field >5 arcmin diameter.
  • Mid-IR AO-fed Echelle Spectrometer (MIRES)
  • Near-IR AO-fed Echelle Spectrometer (NIRES)
  • High-Resolution Optical Spectrometer (HROS)
  • Planet Formation Instrument (PFI) - extreme high-contrast AO imaging
  • Wide-Field AO imager (WIRC) - diffraction limited imaging over a 30" field

Priorities and designs for second-generation instruments will be revisited by the TMT project, partners, and Science Advisory Committee in the relatively near future. The TMT Instrument Development Office will open in 2016, and will coordinate the development of new instruments in response to the needs of the TMT user community, with the first 2nd generation instrument expected to be delivered 2 to 3 years after early science operations begin.

References on future-generation instruments:

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Other operations issues:

Slew time and target acquisition: With no change of instrument, the time between ending an exposure on one target and starting an exposure on another target anywhere on the sky, including telescope and enclosure motions, target acquisition, and guiding, is required to be less than 5 minutes. With a change of instrument, the requirement is 10 minutes.

Instrument switching and availability: all instruments are mounted on the Nasmyth platforms, and are fed by a fully articulated M3 mirror. The platforms will accommodate up to eight instruments, permanently mounted and aligned. All instruments will be kept operational except during periods of maintenance. However, it is planned that only as many as 3 to 4 instruments will be scheduled for a given night, and hence fully calibrated and supported by expert observatory support staff. Switching from one instrument to another is required to take no more than 10 minutes (see "Slew time and target acquisition" above).

Non-sidereal guiding: Enabled for rates that are up to 10% different than the sidereal rates (maximum of 16.5 arcsec per second), without significant degradation to image quality. This is a TMT system-level requirement on all necessary hardware and software components.

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Observing modes:

TMT will initially support two main observing modes:

  • PI-directed mode: Classically scheduled blocks of observing time (generally in units of full or half-nights, but in some circumstances shorter) will be assigned to PIs or to TMT Partners. The PIs are responsible for carrying out the observations. Remote observing will be supported.
  • Service observing: TMT staff members execute shorter, pre-defined observations on behalf of PIs in an ordered sequence. PI eavesdropping will be supported.

Queue scheduling: Initially, TMT will not use observatory-wide adaptive queue scheduling. However, tools will be provided to allow adaptive scheduling within a PI- or partner-directed block of observing time. Partners may then run their own queues. TMT could implement observatory-wide adaptive queue scheduling at a future time, depending on the wishes of the partner communities, but additional staffing would be required, beyond what is in the baseline science operations plan.

Cadence observing: Programs that require monitoring observations at particular times will be accommodated in the pre-planned service observing mode (see above).

Target of opportunity (ToO): There is a TMT policy for cross-partner ToO interrupts, with detailed accounting for how ToO observing time is charged to the partners. Typical response time from trigger notification to ToO observing, including telescope slew and acquisition time, is expected to be 10 to 15 minutes.

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Data management and archives:

All TMT instruments will be delivered with standard data reduction software pipelines. TMT will maintain these pipelines after a hand-off from the instrument teams. TMT will support community-led open source development and enhancement of the pipelines.

The TMT Data Archive will be the main system used by observers to retrieve their science data. It will be searchable with associated metadata and calibration frames. Science data will be accessible to all astronomers within the TMT partnership after a proprietary period, currently planned to be 18 months.

In the current TMT operations model, the observatory will not process or calibrate data for users - observers are responsible for reducing and analyzing their own data. There will be no routine pipeline processing by the observatory, and the TMT Archive will not hold reduced TMT data. Partners may establish (and fund) their own data centers, where they could carry out standardized data reduction and archive reduced data products.

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Time allocation:

In the current TMT operations plan, each TMT partner will establish and operate its own observing proposal and time allocation process. At present, there is no plan for a Master Time Allocation Committee (TAC) to oversee or coordinate telescope time allocation, or to enable coordinated proposals between partners. TMT will not check for duplication of observations between partners. Partner shares of observing time will be divided into equal amounts of bright, gray and dark time, evenly spread across each semester's available time. TMT will receive the information on time allocation from the partners and will produce a merged observing schedule for each semester. Observing time used by each partner in each lunation category will be tracked and accounted at the level of fractional hours.

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The TMT International Observatory - status and partnership

The TMT International Observatory (TIO) was incorporated as a non-profit limited liability company (LLC) on 6 May 2014. The initial members of TIO are the California Institute of Technology, the National Astronomical Observatories of the Chinese Academy of Sciences, the National Institutes of Natural Sciences in Japan, and the University of California.  On 24 September 2014, the government of India formally approved that country's full participation in TIO (see press release). On 6 April 2015, the office of the Prime Minister of Canada announced his government's commitment to funding TMT (see press release).  The Association of Canadian Universities for Research in Astronomy (ACURA) represents Canada in TIO. As described below, the Associated Universities for Research in Astronomy (AURA) is also an Associate of TIO on behalf of the US community, but there is not yet any commitment for the NSF to provide funding for full membership in TIO.

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Partnership shares & available, unsubscribed share

Members of TIO receive TMT observing time in proportion to their contributions to construction and operations costs, with some consideration for the time at which the funds are committed to the project. The University of Hawaii (UH) will receive a 7.5% share of TMT observing time. The current TIO Members each nominally subscribe a share of approximately 10% to 20% of the available time (after subtracting time for engineering and UH).  A similar share (of order 15%) is currently unsubscribed and available for additional partners.

After subtracting time for observatory activities (engineering, calibration, instrument commissioning, etc.) and the UH allocation, roughly 300 nights per year of observing time are expected to be available to the TMT partners. I.e., a 10% share (for reference) would correspond to about 30 nights of observing time per year. Partner shares of observing time would normally be divided into equal allocations in bright, gray and dark lunar conditions.

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Observatory construction and operations costs

Note: The following information was current in 2014, before the delay in construction imposed by legal challenges in Hawaii.  It has not been updated to reflect current estimates for a later start date.

Construction: $1.2 billion in 2012 dollars, and $1.55 billion in "as spent" dollars tracking inflation and spending profiles.

Operations: $27 million per year running costs + $12 million per year for instrumentation (2012 dollars). The running costs include observatory operations and staffing, as well as data management and archiving. Instrument teams and partners are expected to identify instrumentation funding in addition to the $12M/year baseline. The TMT Operations Model was developed with a bottoms-up cost analysis including comparison to currently-operating observatories, and was externally reviewed. It is sometimes described as an "enhanced and corrected" Keck operations model.

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Cost of observatory membership

Note: The following information was current in 2014, before the delay in construction imposed by legal challenges in Hawaii.  It has not been updated to reflect current estimates for a later start date.

Based on the projected construction and operations costs, a 10% share of TMT (for reference) would entail a commitment of:

  • $155 million for construction
  • $3.9 million per year for operations, broken down as
    • $2.7 million/year operations & maintenance
    • $1.2 million/year instrumentation development

In detail, real costs depend on proper accounting for the dates when the partner contributions are made and rates of inflation. If the NSF became involved on behalf of the at-large US community, it would be expected that the capital contribution would be via the NSF MREFC program and the operations costs would be borne by NSF AST.

For reference and comparison, see this summary of NSF construction and operations investments in major astronomical facilities.

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TMT membership and benefits

Full membership in TMT International Observatory (TIO) comes with a commitment to fund the construction and operations of the observatory. The minimum commitment for full membership in TMT is a share of 5% of the construction and operations costs, or 10% for partners making in-kind contributions.

Membership in TIO provides:

  • Observing time.
  • Representation on the TIO Board of Governors and the TMT Science Advisory Committee (SAC), and thus a say in observatory governance and in planning for instrumentation, operations, and science.
  • Access to archived TMT data (from all partners), subject to standard proprietary times (nominally 18 months).
  • Opportunity to participate in international multi-partner TMT collaborations and large/survey programs.

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US participation in TMT

The 2000 and 2010 Astronomy and Astrophysics Decadal Surveys identified the need for a Giant Segmented Mirror Telescope (GSMT) to address a wide range of forefront problems in astrophysics. US federal participation in a GSMT was listed as one of the top priority ground-based programs in both Decadal Surveys. Three GSMT projects (also known as Extremely Large Telescopes - ELTs) are now underway: The Thirty Meter Telescope (TMT), the Giant Magellan Telescope (GMT), and the European Extremely Large Telescope (E-ELT). All three expect to begin science operations in the 2020s. At a national level, the US is not currently a full partner in these projects, although several US universities and institutions are partners in TMT (Caltech; the University of California) or GMT (Carnegie Observatories; Harvard-Smithsonian CfA; Texas A&M; University of Texas at Austin; University of Arizona; Arizona State University; University of Chicago). If TMT is built in Hawaii, the University of Hawaii would be granted observing time, as is the case for all telescopes on Maunakea.

In 2013, the National Science Foundation and TMT established a cooperative agreement after TMT's response to an NSF-AST solicitation, "Planning a partnership model for a Giant Segmented Mirror Telescope". The cooperative agreement states that "the primary deliverable of this award is to be a partnership model that might allow NSF to join the TMT Project on behalf of the US astronomical community." However, this cooperative agreement "does not imply that a GSMT will be approved for further funding."

As a consequence of this cooperative agreement, AURA is now an Associate Member of the TMT International Observatory (TIO). AURA has representatives on the TIO Board of Governors and on the TMT Science Advisory Committee (SAC). NOAO is executing the responsibilities, privileges and participation activities of AURA, representing the US-at-large astronomical community. NOAO has formed a US TMT Science Working Group (SWG), consisting of astronomers from universities and other astronomical institutions outside the current US TMT partners. The SWG is tasked to engage with the US astronomical community about TMT and potential NSF participation therein, to represent that community's interests and aspirations, and to transmit them to the TMT project, the SAC, and the Board of Governors.

The SWG and the AURA representatives to the TIO Board, working with the TMT Project, are developing a US TMT Participation Plan for the NSF. This document will discuss the science justification for US participation in TMT, as well as discussions of the telescope, instrumentation, and AO capabilities needed to address the US community's scientific aspirations for TMT. It will also discuss how the telescope would be used in order to maximize benefits for the US community, including issues of operations, scheduling, time allocation, data management and archiving, etc. Finally, there will be an integrated science and education plan, addressing broader impacts of workforce development, education, and public outreach.

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NSF construction and operations investments in major astronomical facilities

As a point of reference, and for comparison to the potential cost for NSF participation in TMT, here we summarize NSF contributions to construction (through MREFC) for several major new and upcoming astronomical facilities, as well as the expected or estimated NSF share of their operations costs. This information is taken from references indicated by numbers in brackets, with links provided below.

Construction and operations costs for major upcoming facilities:

Atacama Large Millimeter Array (ALMA):

  • International partnership (North America, ESO, and East Asia, in cooperation with the Republic of Chile). The North American contribution share is 37.5%
  • Total construction cost approximately $1.4 billion (as-spent dollars)
  • NSF MREFC contribution to construction: $499 million (as-spent dollars, mainly 2005-2013) [1]
  • Projected NSF contribution to operations, FY2017: $43.25 million/year (2017 dollars) [4]

Daniel K. Inouye Solar Telescope (DKIST, formerly ATST):

  • Primarily funded by the NSF
  • NSF MREFC contribution to construction: $344 million (as-spent dollars, 2009-2019) [2]
  • Projected NSF contribution to operations and maintenance, FY2020: $19.5 million/year (2020 dollars) [2]

Large Synoptic Survey Telescope (LSST):

  • Partnership between the NSF, DOE, and international institutions
  • Total construction cost estimated at $670 million (as-spent dollars)
  • Expected NSF MREFC contribution to construction: $473 million (as-spent dollars, 2014-2022) [2]
  • Preliminary estimate of total operations costs: approx. $36.6 million/year (2013 dollars) [2]
  • Preliminary estimate of NSF share of operations costs: approx. $18 million/year (2013 dollars) [2]

Other NSF-AST observatory operations costs

  • NOAO, FY2016: $17.5 million/year + supplemental $3.5 million/year for Mayall 4m if DOE+NSF proceed with DESI [3]
  • Gemini - international partnership, US share 65.5% assumed for CY2017 [5]:
    • Total international budget: $32.2 million/year = Operations & maintenance @ $28.2 million/year + Instrumentation development @ approx. $4.0 million/year
    • US 65.5% share: $21.1 million/year = Operations & maintenance @ $18.5 million/year + Instrumentation development @ approx. $2.6 million/year
  • NRAO Jansky VLA, FY2017: $29.5 million/year [4]


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Useful TMT references

General references and resources:

AO and instrumentation:

TMT and the US astronomical community:

Last updated or reviewed April 18, 2018.