- The history of infrared astronomy, from the perspective of esteemed French astrophysicist, Pierre Léna
- How the VLT makes use of two state-of-the-art techniques — interferometry and adaptive optics — to be the world’s most advanced optical and infrared telescope
- How Pierre Léna was instrumental in the design of the VLT, especially as an infrared telescope that uses interferometry
Q. You started studying astronomy in the late 1950s, when infrared astronomy was hardly conceivable. What first piqued your interest in astronomy, and how did you become a professional astronomer?
A. My relationship with astronomy is probably similar to that of many of my colleagues. My curiosity began at a very young age, at six or seven. The mystery of the stars and the phases of the Moon attracted me enormously. I had an infinite curiosity for nature, for light, for the colours of butterflies and beetles. All this seemed mysterious, full of attraction, full of questions that I could not even formulate for myself, let alone for an adult.
My father, a self-taught man, was very curious about science, and the construction of my first telescope, which followed the microscope he had given me, fascinated him. Then in high school, the discovery of physics and mathematics excited me. At the time, to pass the high school exams, the mathematics course taught cosmography, an elementary celestial mechanics, but also the basics of astrophysics. While still in high school, I discovered the Société Astronomique de France. Joining amateur astronomers in its small observatory, I observed the sky every Thursday night. At that time, the Paris night sky had fewer stray lights, but more aerosols than today, and terrible seeing.
In high school, we created an astronomy club. We built small refractors with achromatic objective lenses recovered from German submarine periscopes and sold in Paris. But most of all, with my small refractor, I loved summer nights at our holiday home in Burgundy, far from the city lights. There, I would travel through the sky and its nebulae. Physics attracted me by its precision, the beauty of its experiments — which I willingly repeated at home — and the mathematical tools that made it so precise, so I devoted my higher education to it.
I started my first research project at the Haute-Provence Observatory. There, during a whole summer, I was introduced to the mysteries of Fourier spectroscopy. In 1960, I obtained my Agrégation in physics, which qualified me to become a high school teacher, but also to begin preparing a doctorate. Passionate about teaching, I immediately obtained a position at the Sorbonne, in the new modern campus that was opening south of Paris (Orsay). I hesitated for a while, suffering from serious health problems, before really getting involved in research.
Q. Then you became fascinated by infrared astronomy. Why did you become interested in this area of astronomy, and how have you seen it advance throughout your career?
A. In 1958, de Gaulle, Head of the French government and soon to be President of the French Republic, asked two eminent scientists for a report on the future of space research. Regarding astronomy, this report focused on the two spectral domains that seemed accessible as soon as instruments could go into space: ultraviolet and infrared. I was one of the few young researchers who got involved in the second field, which was infinitely more difficult. Hardly any measurements or spectra existed of infrared radiation except from the Sun and Mars and the experimental difficulty was also considerable. The British military use of infrared detection during the war had led to some progress, but the detectors, operating at room temperature, were deplorably unsensitive and, moreover, consisted of only a single pixel. Obtaining an image was therefore a hazardous experimental venture.
The four decades that followed were punctuated by successive advances on these limitations, which improved detection sensitivity by a factor of more than a billion and multiplied the number of pixels in the cameras by more than a million. These advances required instruments to work at increasingly lower temperatures, first cooled with liquid nitrogen, then with liquid helium, and even with helium-3. The difficulties did not stop there. A lot of near-infrared radiation is absorbed by Earth’s atmosphere. As for the far-infrared, the atmosphere is totally opaque to it. The need to place telescopes on high mountains, then on board aircraft, stratospheric balloon-borne nacelles, rockets and later in space added to the difficulties.
Q. So how did you personally contribute to developing and implementing instruments that could carry out infrared astronomy?
A. I have been an actor or user of each of these steps in the discovery of the immense and fertile field of infrared astronomy. Between 1966 and 1969, I began grappling with the determination, by observation, of the variation in brightness between the centre and edge of the solar disc in the near- and mid-infrared, to improve the model of the Sun’s atmosphere. The McMath-Pierce Solar Telescope at Kitt Peak Observatory, Arizona, was the only telescope with sufficient altitude and spatial resolution in the near-infrared. When it appeared necessary to complete these measurements with photometry of the Sun in the far infrared I used NASA’s CV-990 Galileo-I aircraft. A stay of a few months at the High Altitude Observatory in Boulder, Colorado, allowed us to install a Fourier spectrometer on the aircraft and obtain the necessary measurements, made in the stratosphere. I then had all the material to defend my Doctorat d’Etat in 1969 when I returned to France.
What to do next? I was very sceptical about using stratospheric balloons for infrared astronomy, which were unreliable without considerable resources. In the Paris Observatory, I formed a team to install an infrared telescope onboard a Caravelle aircraft — much more reliable indeed — that flew for several years in the northern and southern hemispheres and accumulated observations in the far infrared. We learned the trade, we could establish a fructuous cooperation over Europe, and make ‘infrared friends’ in the UK, Germany, the Netherlands and Sweden, who were building and flying with us. The young European Space Agency had us participate in Space Shuttle simulation missions with our telescope installed on a NASA aircraft. Infrared astronomy was beginning to be tamed.
Q. This led to you playing a big part in the conception of ESO’s Very Large Telescope (VLT). How exactly were you involved?
A. My relationship with ESO began in 1977 when I was appointed as a member of ESO’s Scientific Technical Committee (STC), and then as its president the following year. Faced with the financial difficulties of maintaining an airborne astronomical programme in the long term, I decided to change my research trajectory towards near-infrared interferometry, since with a large telescope the resolution achieved by beating the seeing would make it possible to resolve dim objects such as forming stars or the energetic active nuclei of galaxies. Using a novel revolutionary technique called “speckle interferometry”, a very simple instrument was installed first on the Mayall 4-meter telescope at Kitt Peak, then on the ESO 3.6-metre telescope at La Silla Observatory. It was quickly productive. Chairing the STC and observing at La Silla, I got to know and enjoy ESO, its then director Lo Woltjer, and its staff in Chile.
The prospect of a European telescope with a surface area equivalent to that of an instrument with a diameter of 16 metres, to succeed the La Silla facilities was raised in 1977. In the following years, in addition to equipping the ESO 3.6-metre telescope with instruments and commissioning ESO’s New Technology Telescope, the STC naturally became involved in this new project, which eventually became the VLT. The operation of this telescope as an optical interferometer had not been taken very seriously by many astronomers, but Lo Woltjer, always attentive to new developments, accepted my proposal to organise a conference at ESO Headquarters in Germany in 1981 on the scientific importance of high angular resolution in infrared and visible wavelengths. Soon after, ESO set up a working group to explore the feasibility of an interferometric mode for this future telescope. Should this be really feasible and succeed, the scientific harvest we could forecast was already outstanding. Thirty years later, reality has taken it far beyond our most dreamy expectations! These were incredibly exciting years that culminated in December 1987, when the interferometric mode of the VLT, including adaptive optics, was approved as an integral part of the project. I have described the details of this at length in my book “Une Histoire de flou”.
Q. You were then Scientific Representative of France to the ESO Council from 1986 to 1993. What exactly does this role involve and how did you continue to advocate for the VLT?
A. I was appointed by France to the ESO Council shortly before the official approval of the VLT. For each of the eight states that were members of ESO at that time, two representatives sat on the Council, one a scientist and the other a diplomat. Being on the Council swung me over to scientific policy, financial issues and sometimes to politics in general. The dictatorship in Chile, for example, posed all kinds of challenges for the functioning of ESO until the country’s return to democracy in 1990.
I was convinced that the future of research lay in a European organisation with minimal bureaucracy, and this was offered by the founding treaty of ESO, which I could observe working so well. I appreciated the complementarity of styles, characters and personalities within the Council between different nationalities and I admired the management style of the Director General Lo Woltjer. This period was critical for the approval of the VLT concept and budget. I was obviously identified by my Council colleagues as a stubborn advocate of the VLT interferometric mode. I must say that this required a great deal of trust on their part, because at the time we had very few very demonstrative results: we had to trust physics and engineering! But I should also add that our allies in the astronomical community, few but strong, and especially the team of engineers within ESO who were pursuing the detailed studies, played a big role so that after the approval of the VLT in 1987 and the choice of the Paranal site in 1989, interferometry could develop.
During these years, I had not abandoned my scientific activity, which was focused on the experimental demonstration of adaptive optics, an absolutely essential step if we wanted interferometry to work with the 8-metre VLT Unit Telescopes. The remarkable support of ESO, the partnership with French Aerospace Lab, ONERA, a dedicated team and the understanding of the French authorities to use certain capacities developed for military purposes led to the first worldwide astronomical demonstration of adaptive optics in October 1989 at the Haute-Provence Observatory. A close double star was resolved, the seeing was defeated. Extraordinary prospects for high angular resolution with the new giant telescopes loomed on the horizon, while a generation of young doctoral and postdoctoral students joined us to prepare for the decade that was to follow.
Q. You were involved in the VLT when it was still just a dream. How does it feel to now see it helping astronomers make incredible discoveries, including finding many exoplanets?
A. At the end of the 1990s, when the NACO adaptive optics system was built for the VLT, I was not really surprised by the avalanche of observations and results that followed. The first exoplanets had just been discovered and it seemed likely that NACO would be the first to directly image an exoplanet, which it did in 2003.
However, the technical wager made on the VLTI interferometric mode was of a completely different nature. It almost collapsed in 1993 and it took several years of joint effort to get it back on track. It is quite extraordinary that as early as 2001, two of the four 8-metre VLT Unit Telescopes could be successfully coupled and that the coupling of the whole four-telescope system was successful in 2011. By then I had retired and reduced my scientific activity. However, in 2005, joining Reinhard Genzel's vision, I contributed with some colleagues to drawing up a plan for a seemingly utopian new VLT instrument, GRAVITY, which has made incredible observations of the black hole at the centre of the Milky Way, SgrA*.
Needless to say, I am delighted to see that the VLTI, which is certainly unique in the world, is now capable of differential astrometric precision of the order of a few micro-arcseconds and has reached such a level of sensitivity that it is open to the ultrafine study of the orbits of exoplanets. I see in these successes the confirmation of my firm beliefs: confidence in young people, teamwork, and the strength of European cooperation, especially between research institutions and industry. The requirements of astronomers can push industry to its limit capabilities, or a bit beyond. Then, the immensity of the Universe becomes an endless source of discoveries.
Numbers in this article
|Number of VLT Unit Telescopes.
|Diameter of the VLT Unit Telescopes’ primary mirrors in metres.
|Year that Charles de Gaulle asked for a report on the future of space research, which included a focus on ultraviolet and infrared astronomy.
|Year that Pierre obtained his Agrégation in physics, which qualified him to become a high school teacher, but also to begin preparing a doctorate.
|Year that Pierre began grappling with determining the variation in brightness between the centre and edge of the solar disc to improve the model of the Sun’s atmosphere.
|Year that Pierre was appointed as a member of ESO’s Scientific Technical Committee (STC).
|Year that the prospect of a European telescope with a surface area equivalent to that of an instrument with a diameter of 16 metres was raised.
|Year that Pierre was appointed to the ESO Council.
|Year that the VLT was approved, with the interferometric mode, including adaptive optics, as an integral part of the project.
|Year that Cerro Paranal was selected as the site for the VLT.
|Year of the first worldwide astronomical demonstration of adaptive optics at the Haute-Provence Observatory.
|Year that the plan for the VLT’s interferometric mode almost collapsed.
|Year that two VLT Unit Telescopes were used together as an interferometer for the first time.
|Year that an exoplanet was directly imaged for the first time, using the VLT’s NACO adaptive optics instrument.
|Year that GRAVITY was planned.
|Year that all four VLT Unit Telescopes were used together as an interferometer.
|1 000 000
|Multiplication in the number of pixels on infrared cameras in the last four decades of the 20th century.
|1 000 000 000
|Factor of improvement in sensitivity of infrared detectors in the last four decades of the 20th century.
Biography Pierre Léna
Pierre Léna, aged 82, is Emeritus professor at Paris Diderot University and Paris Observatory. Engaged in research for over 40 years, he has accompanied the emergence of infrared astronomy since the 60s, focusing on star formation, with a special dedication to imaging. As a partner to ESO since the late 70s and later a Council member on behalf of France, he contributed to the conception of the VLT, especially the implementation of interferometry and adaptive optics, stimulating and training students to enter this new domain.