With a two-metre mirror, the Bernard Lyot Telescope (TBL), located at Pic du Midi, cannot compete in size with today’s large instruments. Its relevance lies instead in the equipment capable of breaking down light and measuring its properties precisely. This specialisation enables it to contribute to research on stars and planets, combining long-term observations with updated instrumentation.
A modest mirror, specialised instruments
The TBL is at the Pic du Midi Observatory, in the Hautes-Pyrénées, at an altitude of 2 877 metres. Its two-metre mirror is much smaller than those of the ten-metre telescopes mentioned in the Radio France report, and the mirror of the future 39-metre European telescope referred to in the same programme. But mirror diameter does not sum up what an observatory can do: the instruments installed behind it also determine which properties of light can be measured.
The report attributes a technical challenge to the teams at Pic du Midi: separating visible and infrared components while preserving the polarisation of the light. Polarisation describes a property of the light wave; measuring it can provide information about the magnetic fields of stars. This separation and spectropolarimetric analysis are two complementary functions: directing the components of light towards the instruments, then extracting physical information from them.
The documentation for the SPIP project, or Pyrénées Infrared Spectropolarimeter, makes it possible, however, to identify an instrument currently associated with the TBL. This spectropolarimeter analyses light in the near-infrared, between 0,95 and 2,5 micrometres, in the Y, J, H and K bands. It shares the telescope with Neo-Narval, which works at optical wavelengths.
From light to the properties of stars
A spectropolarimeter does not simply produce an image of the sky. It breaks down light according to its wavelengths and examines its polarisation. Scientists can then interpret these measurements to study phenomena that are not directly visible as surface details.
SPIP is intended for several purposes: characterising planets beyond the Solar System using the radial velocity method, studying the magnetic fields of stars, particularly red dwarfs, and analysing their atmospheres. The radial velocity method involves looking for small periodic movements in a star, which may be caused by the gravitational pull of a planet. It provides information about the possible presence of a companion; it is not the same as photographing that planet directly.
This distinction matters: the instrument collects a light signal, and models and analyses are then used to draw conclusions from it. Measurement and interpretation are related, but not interchangeable, stages. The search for exoplanets and the study of magnetic activity complement each other: characterising the star helps to interpret the variations measured in its light.
A Pyrenean instrument in coordinated work
The TBL does not work in isolation. SPIP’s page states that its observations can be coordinated with those of ESPaDOnS and with SPIRou, a near-infrared instrument installed since 2018 on the Canada-France-Hawaii Telescope, whose mirror measures 3,6 metres. SPIRou and SPIP thus cover the near-infrared from two different sites, while Neo-Narval provides the visible component at Pic du Midi.
Coordination between observatories makes it possible to compare or complement observations made with different instruments. It does not mean that these instruments are identical, or that they necessarily observe the same object at the same time. The wavelengths accessible to each instrument and the location of each observatory are therefore criteria for complementarity, just like mirror diameter.
The case of Pic du Midi thus shows why an old telescope can retain a scientific role: a modest-sized mirror can be paired with instruments designed for specialised measurements and integrated into collaborations with other sites. What matters is not only collecting more light, but also knowing what information the instrument can extract from that light.
References
- At Pic du Midi, a historic telescope remains at the forefront of star observation — Radio France
- Pyrénées Infrared Spectropolarimeter — SPIP
The Bernard Lyot Telescope at Pic du Midi, photographed by AirSThib. Archive image; the instruments have changed since the photograph was taken.
Photograph: AirSThib — Wikimedia Commons — CC BY-SA 4.0.
