Very Large Telescope: The First Fifteen Years of Discovery

July 2013
VLT Survey Telescope (VST) at Cerro Paranal

Europe’s flagship visible-light observatory, the Very Large Telescope array (VLT) is located on Cerro Paranal (elevation 2, 635m) in the Atacama Desert of northern Chile. Astronomers at the European Southern Observatory (ESO) recently celebrated the 15th anniversary of the telescope achieving “first light.” The facility consists of a quartet of four 8.2m instruments, known as the Unit Telescopes, complemented by four 1.8m Auxiliary Telescopes. The eight telescopes can be combined to form a giant interferometer, the ESO Very Large Telescope Interferometer (VLTI), which is capable of an angular resolution of 0.001 arc-second.

The VLT has been fully operational since 2000, when its fourth and last 8.2m entered service. Its main operational mode is as a set of independent giant telescopes, with the four Auxiliary Telescopes working as the VLTI; there are only a few nights each year when all eight are united interferometrically. In December 2012 a ninth spyglass came on stream, the VLT Survey Telescope, with a 2.6m mirror. It is the world’s largest telescope dedicated to optical light sky surveys.

ESO is justifiably proud of the big discoveries made by the VLT. These include: observing the stars orbiting the supermassive black hole at the center of the Milky Way; the first direct image of an exoplanet; and providing the definitive proof that gamma-ray bursts are spawned by supernova explosions.

To shed more light on the extent to which the ESO telescopes are driving research in observational astronomy, ScienceWatch searched the Thomson Reuters Web of Science to assess the impact of papers that cite VLT data. The publication dates searched were the years 1999 to 2012, encompassing non-review papers indexed in the Web of Science category of "Astronomy & Astrophysics." The search was conducted on the topic “Very Large Telescope” and its acronyms, by examining the fields “Title,” “Abstract,” “Author Keywords,” and “Keywords Plus.” Filtering in this way eliminates papers in which the VLT played a minor secondary role: for example, papers that consolidate observations made by many telescopes across a range of wavebands from X-rays to microwaves are excluded. The sampling process produced 3,257 papers that have been cited 79,635 times in 34,595 articles.


In Table 1, 25 top papers illustrate the wide variety of investigations in which the VLT made a major impact. The fields of research range from the very distant universe to our local Milky Way. There is cosmology, and there are detailed follow-up programs in the optical and near-infrared to support sky surveys at all wavelengths. Also featured are stars lurking at the center of our Galaxy where they orbit its supermassive black hole.

Table 1
Highly Cited Papers Deriving from the Very Large Telescope, 1999-2012

Listed by citations

Rank Paper Citations
1 A.G. Riess, et al., Type Ia supernova discoveries at z > 1 from the Hubble Space Telescope: Evidence for past deceleration and constraints on dark energy evolution,” Astrophys. J., 607(2): 665-87, 2004. 2,107
2 P. Astier, et al., “The Supernova Legacy Survey: measurement of Ω(M), Ω(Λ) and w from the first year data set,” Astronomy & Astrophys., 447(1): 31, 2006. 1,371
3 M. Pettini, et al., “The rest-frame optical spectra of Lyman break galaxies: Star formation, extinction, abundances, and kinematics,” Astrophys. J., 554(2): 981-1000, 2001. 517
4 R. Cayrel, et al., “First stars V – Abundance patterns from C to Zn and supernova yields in the early Galaxy,” Astronomy & Astrophys., 416(3): 1117-38, 2004. 477
5 R. Amanullah, et al., Spectra and Hubble Space Telescope light curves of six Type Ia supernovae at 0.511 < z < 1.12 and the Union2 compilation,” Astrophys. J., 716(1): 712-38, 2010. 446
6 R. Giacconi, et al., “Chandra Deep Field South: The 1 Ms catalog,” Astrophys. J. Suppl Ser., 139(2): 369-410, 2002. 426
7 F. Eisenhauer, et al., “Sinfoni in the Galactic center: Young stars and infrared flares in the central light-month,” Astrophys. J., 628(1): 246-59, 2005. 362
8 R. Giacconi, et al., “First results from the X-ray and optical survey of the Chandra Deep Field South,” Astrophys. J., 551(2): 624-34, 2001. 356
9 P. Demarque, et al., “Y(2) isochrones with an improved core overshoot treatment,” Astrophys. J. Suppl. Ser., 155(2): 667-74, 2004. 347
10 R.G. Gratton, et al., “The O-Na and Mg-Al anticorrelations in turn-off and early subgiants in globular clusters,” Astronomy & Astrophys., 369(1): 87-98, 2001. 338
11 S. Gillessen, et al., “Monitoring stellar orbits around the massive black hole in the Galactic Center,” Astrophys. J., 692(2): 1075-1109, 2009. 334
12 G.P. Szokoly, et al., “The Chandra Deep Field South: Optical spectroscopy. I.,” Astrophys. J. Suppl. Ser., 155(2): 271-349, 2004. 320
13 O. Ilbert, et al., COSMOS Photometric redshifts with 30-bands for 2-deg2,” Astrophys. J., 690(2): 1236-49, 2009. 305
14 O. Le Fevre, et al., “The VIMOS VLT deep survey – First epoch VVDS-deep survey: 11564 spectra with 1 7.5 <= I-AB <= 24, and the redshift distribution over 0 <= z <= 5,” Astronomy & Astrophys., 439(3): 845-62, 2005. 286
15 M. Franx, et al., “A significant population  of red, near-infrared-selected high-redshift galaxies,” Astrophys. J., 587(2): L79-L82< 2003. 286
16 P. Rosati, et al., “The Chandra Deep Field South: The 1 million second exposure,” Astrophys. J., 566(2): 667-74, 2002. 284
17 S. Lilly, et al., “zCOSMOS: A large VLT/VIMOS redshift survey covering 0 < z < 3 in the COSMOS field,”  Astrophys. J. Suppl. Ser., 172(1): 70-85, 2007. 283
18 O. Ilbert, et al., “Accurate photometric redshifts for the CFHT legacy survey calibrated using the VIMOS VLT deep survey,” Astronomy & Astrophys., 457(3): 841-56, 2006. 273
19 G. Piotto, et al., “Metallicities on the double main sequence of ω Centauri imply large helium enhancement,” Astrophys. J., 621(2): 777-84, 2005. 245
20 A. Fontana, et al., “The Galaxy Mass Function up to z=4 in the GOODS-MUSIC sample: Into the epoch of formation of massive galaxies,” Astronomy & Astrophys., 459(3): 745-57, 2006. 241
21 T. Bensby, et al., “α-, r-, and s-process element trends in the Galactic thin and thick disks,” Astronomy & Astrophys., 433(1): 185-203, 2005. 235
22 R. Genzel, et al., “From rings to bulges: Evidence for rapid secular galaxy evolution at z ~ 2 from integral field spectroscopy in the SINS survey,” Astrophys. J., 687(1): 59-77, 2008. 233
23 G. Chauvin, et al., “A giant planet candidate near a young brown dwarf – Direct VLT/NACO observations using IR wavefront sensing,” Astronomy & Astrophys., 425(2): L29-32, 2004. 233
24 L. Fu, et al., “Very weak lensing in the CFHTLS wide: Cosmology from cosmic shear in the linear regime,” Astronomy & Astrophys., 479(1): 9, 2008. 218
25 A. Grazian, et al., “The GOODs-MUSIC sample: a multicolour catalog of near-IR selected galaxies in the GOODS-South field,” Astronomy & Astrophys., 449(3): 951, 2006. 208
SOURCE: Thomson Reuters Web of Science


Three of the top five papers, #1, #2, and #5 account for 3,911 citations. Papers #1 by Adam G. Reiss and colleagues, and #2 by P. Astier and the Supernova Legacy Survey, are on supernova cosmology. The two research teams used ESO’s telescopes at La Silla and Paranal to show that the expansion of the universe is accelerating. The VLT was the most sensitive telescope deployed for follow-up spectroscopy of newly discovered Type 1a supernovae in the distant universe.

The 2011 Nobel Prize in Physics was awarded "for the discovery of the accelerating expansion of the Universe through observations of distant supernovae.” And, incidentally, the three winners—Riess, along with Brian P. Schmidt and Saul Permutter—were all correctly predicted beforehand in the annual Thomson Reuters selection of Citation Laureates, highlighting researchers whose work and influence unmistakably merit science’s top prize.

Paper #5 is from the Supernova Cosmology Project and it presents photometric measurements of six high-redshift supernovae. The VLT provided data for the most distant supernovae for which ground-based measurements in the near-infrared have been obtained.

The spectroscopy data on Lyman break galaxies given in paper #3 includes observations by the VLT in late 1999. This paper is the earliest example of the 10-m Keck Telescope in Hawaii and an 8.2-m Unit Telescope at Paranal working in unison on a spectroscopic campaign.


Papers #6, 8, 12, and 16 take a look at the patch of the X-ray sky known as the Chandra Deep Field South, where there is much less interstellar dust and gas obscuring distant sources. In 1999-2000, through this clear window the Chandra X-ray Observatory obtained 11 exposures for a cumulative exposure time of 942 ks, and it discovered over 300 very distant X-ray sources The X-ray source catalog is published in #6, the second part of which has 349 identifications made by the VLT and the ESO 2.2m telescope

Thanks to the efficiency of the VLT, working to the faintest magnitudes among the deepest X-ray surveys, a successful spectroscopy campaign led to 137 reliable optical identifications and redshifts for active galactic nuclei and X-ray galaxies.

Paper #13 reports on more recent work involving the Hubble Space Telescope’s COSMOS survey combined with spectroscopic follow-up from ground based telescopes. The same theme is found in #15 reporting on VLT spectroscopy of high-redshift galaxies in the Hubble Deep Field South.

The early chemical evolution of our Galaxy is investigated in #4. The VLT provided very high quality spectra of 70 stars, selected because they are very metal poor, and therefore likely to have descended from the first generation of stars formed after the Big Bang. The theme of stellar population history continues in paper #19, dealing with chemical abundances in the globular cluster Omega Centauri.


The VLT is the world’s finest telescope for peering into the center of the Galaxy. Paper #7 reported on the first observations (in 2004) with the SINFONI spectrometer of the central 30 light days of the galactic center. Paper #11, published in 2009,  presents the results of 16 years of monitoring stellar orbits around the massive black hole at the heart of the Milky Way. VLT data greatly aided the task of determining the orbits of 28 stars, one of which has been followed for a full circuit. The mass of the central object is 4.31 x 106 solar masses, while the distance to the galactic center is R0 = 8.33 kpc.


 VLT Papers per Year - 1999 to 2012

Graph 1 illustrates the annual count of papers in which VLT data contribute significantly to an observational database. In 2003 the number of papers, 212, was double that of 2002, 106. The rapid increase is testament to the immediate impact of the VLT. In 2003 there were 14 institutions that contributed 10 or more papers. Back then, ESO headed the rankings with 79 papers, followed by the Max Planck institutes at 43, then Leiden University and the Observatoire de Paris with 20 each.

 Collective citations to VLT Papers by year, 2001 to 2012

Graph 2 charts the citation rate of these papers by year. The steady increase year on year reflects the growth of databases for spectroscopy and magnitudes of astronomical objects of all kinds that VLT has spawned.


Table 2
Reports from the Very Large Telescope: Prolific Institutions, 1999-2012

(Listed by number of papers)

Rank Institution Papers Percent
1 European Southern Observatory 1,020 31.3
2 Observatoire de Paris 297 9.1
3 Max Planck Institute for Astronomy, Heidelberg 275 8.4
4 Max Planck Institute for Extraterrestrial Physics, Garching 251 7.7
5 Caltech 218 6.7
6 Space Telescope Science Institute 204 6.3
7 CNRS, France 150 4.6
8 Astronomical Observatory of Rome 144 4.4
9 Max Planck Institute for Radio Astronomy, Bonn 138 4.2
  Astronomical Observatory of Trieste 138 4.2
10 University of Padua 135 4.1
SOURCE: Thomson Reuters Web of Science

Table 2 shows the number of VLT papers from the top ten institutions. Not surprisingly ESO’s record of 1020 papers (31.3% of the sample) is well ahead of national research organizations and universities.


Table 3
VLT Reports: Prolific Nations, 1999-2012

(Listed by number of papers)

Rank Country Papers Percent
1 Germany * 1,628 50.0
2 USA 1,395 42.8
3 France * 1,128 34.6
4 UK ** 933 28.6
5 Italy ** 876 26.9
6 Chile 744 22.8
7 Netherlands * 436 13.4
8 Spain ** 429 13.1
9 Switzerland** 264 8.1
10 Australia 199 6.1
11 Canada 188 5.8
12 Belgium * 169 5.2
SOURCE: Thomson Reuters Web of Science   *Founding member of ESO
**Member of ESO

Table 3 ranks the top 12 countries by the number of VLT papers having an author affiliated to an institution in that country, scoring a percentage of 5% or more. ESO, the European Organisation for Astronomical Research in the Southern Hemisphere, is an inter-governmental organization with 15 member states that is based in Germany. The telescopes are in Chile. The largest contributors to the ESO budget are Germany 22.78%, France 16.39%, United Kingdom 16.28% and Italy 11.68%. These member states are ranked in the table in the same order as their relative contributions. The USA is ranked #2 in this analysis because many large telescope collaborations are partnerships of VLT, the Hubble Space Telescope and the Keck Telescope, Hawaii. Chile is ranked #6 because, as noted, that’s where the telescopes are.


Our analysis highlights the revolutionary science impact of the VLT since the beginning of the third millennium. It is the most productive ground-based facility for observational astronomy, and ESO is the most productive observatory in the world. In 2011 and 2012 results from VLT led to an average publication rate of one peer-reviewed scientific paper every day. Graph 1 shows the publication rate rising steadily in 1999-2007, and then settling at 300+ per year. Graph 2 shows that citations have increased year on year by about 10%; in 2012 they jumped by an impressive 19.3%.

Users of the VLT come from a large number of universities, research institutes, and observatories. There are 100 universities that have contributed 25 papers or more in the first 15 years of operation. There are 31 countries represented in this sample. The VLT has given opportunities to a global population of astronomers. Even the sovereign microstate of Vatican City participated in the authorship of 11 papers.

The analysis shows a striking change in the places of employment of the VLT users compared to previous generations. Today large numbers of astronomers are teaching faculty based in world class universities, whereas many of their predecessors worked exclusively on research, and were not embedded in university departments.

The VLT instrumentation program is the most ambitious ever conceived for a single observatory. It includes large-field imagers, adaptive optics, and spectrographs spanning from mid-infrared (24 µm) to deep ultraviolet (300 nm) wavelengths. The impressive array of instruments underpins the high productivity of the VLT. Most of the instruments have been built by consortia of European laboratories, so the VLT has led to a strengthening of European expertise in astronomical instrumentation. In fact, development and construction of second generation instrumentation is already underway.

VLT will surely continue its transformational work for many years to come. The European Extremely Large Telescope will see first light by 2022, and it will take a decade to complete its instrumentation. A Thirty Meter Telescope is planned for Mauna Kea, Hawaii. In Chile site preparation has started for the Giant Magellan Telescope, for completion in 2020.

Dr. Simon Mitton is Vice-President (2012–2014) of the Royal Astronomical Society, and co-author with Jeremiah P. Ostriker of Heart of Darkness: Unraveling the Mysteries of the Invisible Universe (Princeton, 2013).

The data and citation records included in this report are from Thomson Reuters Web of ScienceTM. Web of ScienceTM is a registered trademark of Thomson Reuters. All rights reserved.