A giant telescope for giant discoveries

Published June 11, 2025

Standing tall at 10.4 meters, the Gran Telescopio Canarias (GTC) is the largest and one of the most advanced telescopes in the world. Gathering precious information about our universe, this major reflecting telescope in La Palma, Spain, is built using a combination of curved, thin, and lightweight mirrors. Through their smooth and perfect surface, these mirrors can collect and focus light toward the image plane. However, the pursuit of clarity and precision in its observations demands meticulous attention to detail, as even the slightest imperfection can compromise the brilliance of its findings.

Gran Telescopio Canarias telescope with the dome open

Image 1. Gran Telescopio Canarias telescope with the dome open.

Credit: Instituto de Astrofìsica de Canarias (IAC).

Protecting each segment from optical aberrations
GTC’s many applications
Researcher profile
For further information
References

Protecting each segment from optical aberrations

The exceptional performance of the GTC is enabled by both its inherent design and its strategic location at the Roque de los Muchachos Observatory in La Palma, Canary Island, renowned as one of the top astronomical sites in the Northern Hemisphere. Since 2009, when the first scientific observations began, the GTC has been instrumental in advancing our understanding of fundamental astrophysical phenomena, including black holes, exoplanets, dark matter, and the origins of the universe.

Traditionally, the sensors used in technical instruments for telescopes are CCDs, however, when the GTC team designed new focal stations, they decided to adopt a different paradigm: three additional focal stations, one Cassegrain and two Folded-Cassegrain, each of them using a single technical instrument much simpler than the ones used in Nasmyth foci.

Previous instruments at Nasmyth focal stations using CCDs

View of the three previous instruments at Nasmyth focal stations using CCDs.

The technical instrument is a wide field of view low-order Shack-Hartmann wavefront sensor, sampling a Ø20 arcsec field of view across 18 pupil sub-apertures on 1024 × 1024 pixels of an sCMOS camera. This configuration permits simultaneous slow and fast guiding at speeds of up to 200 fps alongside wavefront sensing to feed the closed-loop active optics. Additionally, the field of view is big enough to perform field acquisition since the blind pointing error of GTC is very low.

To equip these new technical instruments, the GTC team reached out to Hamamatsu Photonics, renowned for its high-speed, low-noise scientific cameras. Instead of using multiple instruments as before, the team decided to consolidate them into a single unit, using a board-level sCMOS camera from Hamamatsu. Integrated into a wider system, these sCMOS cameras are ideal due to their low maintenance and reliability.

The high sensitivity and high speed delivered by the cameras have been the crucial characteristics that have permitted the building of this robust unit within each of the new focal stations of the telescope. The result is a long-term, low-maintenance operation, enabling all technical functions essential for scientific observations, including acquisition, guiding, and precise control of telescope alignment.

The instrument shown above is not operational and serves for size comparison only.

GTC’s many applications

GTC observes an incredible number of projects brought by scientists from all over the world. Since its creation, GTC has also been instrumental in numerous discoveries, each one captivating and mesmerizing. Among its notable achievements is the uncovering of a new ring system around a dwarf planet on the edge of the Solar System (image 2), a giant black hole at the center of the Milky Way (image 3), or the formation of the first galaxies in the universe (image 4).

Thanks to Hipercam, an ultra-fast camera installed at GTC, these rings could be detected when the material of these structures hid the brightness of the planet.

Further information here [2].

Image 2. Artist impression of the Quaoar ring system

Image 2. Artist impression of the Quaoar ring system. Credit: ESA, CC BY-SA 3.0 IGO

The ring system around the dwarf planet, named Quaoar, which is approximately half the size of Pluto and orbits the Sun beyond Neptune. Rings are not usually common around small planets, and even less so far from their surface, as is the case with Quaorar. The image above shows a recreation of this ring.

Image 3. Color scale in the image shows the amount of infrared (heat) radiation coming from warm dust particles in the filaments and luminous stars within a light year of the Galactic centre.

Image 3. Color scale in the image shows the amount of infrared (heat) radiation coming from warm dust particles in the filaments and luminous stars within a light year of the Galactic centre. Credit: Roche et al. 2018, MNRAS, 476, 235

Additionally, the GTC has contributed to our understanding of the cosmos by uncovering the existence of a massive black hole at the center of our Milky Way galaxy (referenced as Image 3).

Most galaxies that have been studied so far have a supermassive black hole at their centers. The one that houses our galaxy has a mass of about four million times the mass of our Sun. Sagittarius A, as this giant is called, is about 26,500 light-years from Earth, however, its observation is hindered by the amount of gas and dust along our line of sight.

However, thanks to the GTC’s infrared detection capabilities with spectroscopic, coronagraphic and polarimetric functionalities with very high spatial resolution, researchers were able to observe, for the first time, the effects of this black hole on its surrounding environment. This includes disturbances to the magnetic fields and the heating of surrounding gas and stars that surround it (see Image 3).

The color scale in the Image 3 shows the amount of infrared radiation coming from warm dust particles in the filaments and luminous stars within a light year of the Galactic Center. The polarization of the infrared light traces the magnetic field in the filaments of hot gas containing dust grains.

Further information here [3].

Moreover, the GTC has provided invaluable insights into the structure and composition of cosmic phenomena such as the Abell 370 cluster (referenced as Image 4).

Image 4. Abell 370 cluster view from GTC+OSIRIS and with the Hubble Space Telescope (HST)

Image 4. Abell 370 cluster view from GTC+OSIRIS (left) and with the Hubble Space Telescope (HST) (right). Credit: GRANTECAN/HST.

The image above compares the centre of the cluster Abell 370 as seen with GTC+OSIRIS (left) and with the Hubble Space Telescope (HST) (right). The data from the HST have better spatial resolution because they are not affected by the turbulence in the atmosphere. However, data from GTC goes in more depth by showing the existence of some galaxies previously unknown and not detected by the HST. Abell 370 is a cluster located at a distance of about 6 million light years. Its core is made up of several hundred galaxies that act as a gravitational lens, deforming and amplifying the universe behind it, as if they were natural telescopes. In this way, astronomers can study the first galaxies, stars, and other structures that formed in the early universe, which would otherwise be too faint to detect.

Further information here [4].

Acknowledgment: The design and implementation of these new technical instruments of GTC has been financed by the European Regional Development Fund (ERDF) within the Operational Program of the Canary Islands (2014-2020).

Researcher profile

Head of Development Group at Gran Telescopio Canarias S.A.F. Javier Castro López Tarruella has spent all his professional career working on telescopes.
He joined the Instituto de Astrofísica de Canarias in 1991 to work on the proposal for an 8-10 m telescope for the Canarian observatories. Once the project was funded, he joined GRANTECAN S.A. in 1996 as head of the Optics Group for the construction of the GTC telescope. Once the construction of the GTC was completed and the operation of the telescope started, he became responsible for the new developments for the GTC. He has participated in design reviews for the next generation of telescopes as TMT and the E-ELT and has been a consultant for several companies developing telescope subsystems.

For further information

[1] B. Morgado et al. "A dense ring of the trans-Neptunian object Quaoar outside its Roche limit." | Nature, volume 614, pages 239–243 (2023): www.nature.com/articles/s41586-022-05629-6

[2] "El Gran Telescopio Canarias juega un papel fundamental en el descubrimiento del anillo del planeta enano Quaoar" | Instituto de Astrofìsica de Canarias (IAC), (16/02/2023): https://www.iac.es/es/divulgacion/noticias/el-gran-telescopio-canarias-juega-un-papel-fundamental-en-el-descubrimiento-del-anillo-del-planeta

References

[3] "CanariCam studies the polarization produced by the gas and dust around the black hole at the centre of the Milky Way" | Instituto de Astro- fìsica de Canarias (IAC), (02/20/2018): https://www.iac.es/en/outreach/news/canaricam-studies-polarization-produced-gas-and-dust-around-black-hole-centre-milky-way

[4] SCIENCE COMMUNICATION AND OUTREACH UNIT, "Images from the Hubble Space Telescope and GRANTECAN help to show how the first galaxies were formed" | nstituto de Astrofìsica de Canarias(IAC), (10/21/2021): https://www.iac.es/en/outreach/news/images-hubble-space-telescope-and-grantecan-help-show-how-first-galaxies-were-formed

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