A new measurement technique found at the frontier between Physics and Astronomy
2019 August 07

The huge diffraction grating of ESPRESSO pictured here undergoing testing at ESO Headquarters. Credits: ESO/M. ZamaniThe intense laser beams that enabled an alternative way of calibrating the future hunter of other Earths. Créditos: ESO/G. HüdepohlThe Very Large Telescope, with the unit UT4 using the laser guide stars during the testing phase of ESPRESSO. Credits: Alexandre Cabral / IA.
The search for exoplanets will get to the next level with the detection and characterization of Earth-like planets. A new instrument in this quest is the ESPRESSO1 spectrograph. With Portuguese participation, it is installed in one of the best telescopes in the world, the Very Large Telescope (VLT), of the European Southern Observatory (ESO), in Chile. It will enable the detection, in the light of the stars, of the tiny effects induced by planets as small as Earth.

One of the effects of the presence of exoplanets is on the motion of the star. For this reason, ESPRESSO will have to measure the velocity in the movement of the stars with deviations from the real value not exceeding ten metres per second. An alternative technique was now discovered that is capable of calibrating the data of this spectrograph in order to attain such an accuracy. It uses a system already available at the VLT, although created for other purposes, and is announced in a paper2 published in the Physical Review Letters, the outcome of a study led by Frédéric Vogt, of ESO, with the participation of Pedro Figueira, of ESO and Instituto de Astrofísica e Ciências do Espaço (IA3), and Alexandre Cabral, of IA and Faculdade de Ciências da Universidade de Lisboa (FCUL).

Modern observatories use a system of adaptive optics to correct the effects of atmospheric turbulence on the real images of the sky. This system employs lasers to produce artificial stars in the upper atmosphere. It is the observation of this reference “star” that allows the characterization of the atmospheric turbulence at each moment. Now, according to this new study, these lasers can also be used to calibrate data collected by instruments installed at the observatory.

“This is the first time that such a method is proposed and demonstrated,” says Frédéric Vogt, of ESO and the first author of this paper. “ESPRESSO is a state-of-the-art, ultra-stable, high-resolution spectrograph, and thus provides the ideal means to demonstrate the potential of this new complementary calibration technique.

The new unanticipated application of these lasers was first assessed on the 2nd February, 2018, on one of the first testing nights of the instrument observing the sky. One of the lasers was pointed to the field of view of VLT's UT4 telescope, so that ESPRESSO could receive the laser's light that had crossed the atmosphere.

One of the advantages of this new technique is the fact that it takes into account the full course of the light from the sky to the spectrograph,” says Alexandre Cabral, member of the instrument's consortium. “The light passes not only through the telescope, but also through the Portuguese component of ESPRESSO, the optical system ʻCoudé Trainʼ, which carries the light over a track of about 60 metres, from the telescope to the spectrograph.” Frédéric Vogt adds: “While other methods typically used rely on reference lamps, which light is artificially injected into the instrument, the lasers' light comes from the sky and goes through the telescope and all the optics of the instrument in the exact same way that the signal of scientific targets will.

Although the laser light has a narrow spectrum, of known wavelength, this is spread out into several other wavelengths when the laser photons hit molecules of the atmosphere (being molecular nitrogen and oxygen the most abundant). As a consequence of affecting certain states of the molecules, photons can loose or gain energy, and their wavelength is changed by a little.

These scattering effects, named in Physics as inelastic scattering, such as the Raman effect, produce then a range of wavelengths that can be predicted theoretically. When comparing the theoretic prediction with the observations done with an instrument, it is possible to assess the accuracy of the wavelength values measured by that instrument.

The observations conducted with ESPRESSO captured all the wavelengths predicted by the theoretical models. They captured also some additional signals, which justify in-depth future observations. “The next step will be to detect even weaker signals in the spectrum in order to constrain the molecular parameters yet unknown,” says Pedro Figueira, ESO's scientist responsible for ESPRESSO and member of the instrument's consortium.

ESPRESSO might thus reveal new facts in molecular physics. “With this study, we arrived at an extraordinary result in fundamental science right in the first testing nights of the instrument,” states Pedro Figueira.

Laser guide stars are today present at all large professional astronomical observatories, and were up to now seen only as a component of adaptive optics,” says Frédéric Vogt. “But astronomers are experts at collecting and exploiting all forms of light. This work highlights some of the yet-untapped potential of these laser beams, which bright light sources are very precisely characterized.

  1. The ESPRESSO spectrograph is a high-resolution instrument that will enable the discover of Earlh-like exoplanets. The Instituto de Astrofísica e Ciências do Espaço (IA) had a strong participation in the consortium, and so will have privileged access to the scientific exploitation of this instrument. ESPRESSO will also be the most powerful tool in the world to test if the physical constants of Nature have changed since the early ages of the Universe, as predicted by some fundamental theories of Physics.
  2. The article “Rotational and rotational-vibrational Raman spectroscopy of air to characterize astronomical spectrographs”, by Frédéric Vogt at al, was published on 6th August, 2019, in the journal Physical Review Letters, Volume 123 (DOI: 10.1103/PhysRevLett.123.061101).
  3. The Instituto de Astrofísica e Ciências do Espaço (Institute of Astrophysics and Space Sciences – IA) is the reference Portuguese research unit in space sciences and integrates researchers from the University of Porto and the University of Lisbon. The institute encompasses most of the field’s national scientific output and it was evaluated as Excellent in the last evaluation Fundação para a Ciência e Tecnologia (FCT) commissioned from the European Science Foundation (ESF). IA's activity is funded by national and international funds, including Fundação para a Ciência e a Tecnologia (UID/FIS/04434/2019).

Frédéric Vogt
Pedro Figueira
Alexandre Cabral

Science Communication Group
Sérgio Pereira
Ricardo Cardoso Reis
João Retrê (Coordination, Lisboa)
Daniel Folha (Coordination, Porto)

Faculdade de Ciências da Universidade de Lisboa Universidade do Porto Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia