Tmt night


Site Selection


The Keck and Subaru telescopes on top of Maunakea, Hawaii

In 2009, the Thirty Meter Telescope International Observatory (TIO) selected Maunakea, in Hawaii, as the preferred site to build and operate TMT. However, in December 2015, the Hawaii Supreme Court ruled that the state’s permitting process was flawed, and the State Board of Land and Natural Resources was ordered to re-do the permit process. The re-do of the permit process took approximately a year, and in September 2017, the BLNR approved a new permit for TMT. Opponents challenged the new permit, and it is before the Hawaii Supreme Court. But, while Maunakea continues to be TIO’s preferred site, TMT has also extensively investigated several alternative sites to ensure that construction can begin in a timely fashion. The alternate sites considered were distributed over the two hemispheres: Two sites in Chile, and two others in Mexico and Spain (respectively San Pedro Martir and Roque de los Muchachos Observatories).


The Roque de los Muchachos Observatory on the Canary Islands, Spain

All sites presented a different set of advantages and challenges. The investigation looked into the various risks associated with each site and many other parameters in addition to site quality, such as the costs for construction and operations, the timeline to first-light, and the legal processes and timescale to obtain the necessary permits. On October 31st, 2016, the TIO Board of Directors selected the Observatorio del Roque de Los Muchachos (ORM), in La Palma, on the Canary Islands (Spain) as the alternate site for TMT. This decision was based on:

 (1) The quality of the ORM site, which will support TMT core science programs

 (2) The range of advantages related to the ORM site, including lower costs of construction and operations, faster timeline to initiate construction and reach ‘firstlight’, and lower project risks based on the existence of support infrastructure

Related Documents
  • See full report describing the site selection process in detail 
  • See one-page document describing the Maunakea and Observatorio del Roque de Los Muchachos (ORM) site characteristics
  • See one-page document describing the characteristics of the five sites studied for TMT

TMT Science and First Light Instrumentation

The Thirty Meter telescope will combine adaptive-optics corrected wavefronts with powerful imaging and spectroscopy capabilities to significantly transform how groundbased exploration of our universe is carried out. With its thirty meter aperture, and diffraction limited imaging capabilities, TMT will provide dramatic improvement in sensitivity and spatial resolution across the visible, near- and thermal-infrared regimes. First-light instrumentation will include:

  • An LGS assisted adaptive optics facility (NFIRAOS) delivering diffraction-limited images in the J,H and K bands to three instrument ports.
  • An AO-fed near-infrared (0.8-2.5µm) imager and IFU spectrograph (IRIS)
  • A UV/visible [0.31-1.0µm] wide-field imaging multi-objectspectrograph (WFOS). Its innovative design, the excellent astronomical quality of its future location, and the scientific capability that will be enabled by its suite of instruments, all contribute to position TIO as a major ground-based facility of the next decade.

Site Comparison

TMT Site Characteristics Comparison Chart
Site Charastetistics
(media values, unless stated otherwise)
Altitude of Site (m) 4050 2250
Fraction of Yearly Usable Time1 (%) 72 72
Seeing at 60 m above ground2 (arcsec) 0.50 0.50
Isoplanatic Angle (arcsec) 2.55 2.33
Atmospheric Coherence Time (ms) 7.3 6.0
Precipitable Water Vapor (% time <2mm) 54 >20
Mean Nighttime Temperature (oC) 2.3 7.6
Extinction (V mag/airmass) 0.111 0.137
Ground Dust Concentration (mg/m3) 0.815 1.006


1 Loss of observation time include time loss due to clouds, high winds, precipitations, dust and any other meteorological conditions.
2 Value calculated at 500nm and zero integration time with turbulence below the TMT enclosure (60m) removed.

Facts about ORM

Although most of TMT science cases can be carried out as nearly as efficiently from ORM in comparison to Maunakea, a lower and warmer site for TMT would cause a lower sensitivity and efficiency, especially at the very shortest and longest wavelengths regimes. For instance, mid-IR observations in Q-band (> 18 microns), which are extremely challenging from the ground, will be significantly impacted from ORM.

Adaptive Optics

The turbulence profile over ORM is very similar to Maunakea’s and our study shows that ORM is the second best site, after Maunakea, to support the AO-assisted observations that will be carried out with NFIRAOS, our AO facility.


There are various stories in the community regarding the impact of dust on observations at ORM. Our analysis shows that the occurrences of increased atmospheric extinction (regardless of the causes) are comparable for all Northern sites we studied. Overall, the presence of atmospheric dust was not a discriminating factor between alternate sites.

Protection of night-sky above La Palma

The entire island of La Palma, where the Roque de Los Muchachos Observatory (ORM) is located, is a UNESCO biosphere reserve, which is protected against light contamination. An impressive series of measures are already in effect to keep the sky above ORM among the darkest skies worldwide for all future generations