One of the eight goals of WEAVE is to study quasi-stellar objects, galaxy nuclei that harbor a black hole and are powerful sources of electromagnetic radiation. These objects, observable at very large distances, are used to probe the large-scale structure of the universe, the properties of galaxies, and those of the intergalactic medium in the first three billion years since the Big Bang.

Another objective is to study very close objects like white dwarfs, extremely dense stars. With the WEAVE spectra, we will determine the masses and temperatures of more than 100 000 white dwarfs, and thus their ages thanks to the white dwarf cooling law.

But it is the "galactic archaeology" survey that will occupy most of WEAVE’s observing time : the spectra of the observed stars will allow us to better understand the structure of the Galaxy, its formation and its evolution over the long 13.5 billion years of its history.

Spectroscopy, or how to break down starlight to learn more about the cosmos

At the end of the 19th century, astronomers such as Jules Janssen (1824-1907) and Angelo Secchi (1818-1878) began to systematically study the light of the Sun, planets and stars. They dispersed the light coming from these stars thanks to "prisms", and by analyzing the obtained colors, studied the physical properties of stars distant of millions of kilometers from the Earth.

Spectroscopy marked the birth of astrophysics. Since then, it is our main tool to study the cosmos, because we can determine from the spectrum of a cosmic object its radial velocity (if it is approaching or moving away from us), its chemical composition and its physical state (temperature, density, electronic pressure, etc.).

Analyze several stars simultaneously

In the last thirty years, multi-object spectrographs have appeared, capable of studying simultaneously the individual properties of very distant objects. These spectrographs allow to select different objects in the field of view of the telescope and to send their light into the analyzer to produce separate spectra.

They are particularly important for studies that require a large number of measurements, such as the study of our galaxy, clusters of galaxies, or oscillations in baryonic matter. The latter are fluctuations in the density of matter (protons and neutrons) that were already present in the primordial universe, just after the Big Bang. Galaxies in particular were formed from these inhomogeneities and the measurement is complementary to the study of the anisotropies of the cosmic microwave background, whose observation was awarded the 2006 Nobel Prize in Physics.

Meeting the scientific needs of astronomers

A dozen years ago, the European astronomical community identified as a priority need a large field of view instrument (one to three degrees in diameter, i.e. several times the size of the full moon) capable of simultaneously acquiring the spectra of several thousand objects on a telescope of the order of 4 meters in diameter. Indeed, such an instrument makes it possible to approach several fields of research in astrophysics, in particular the structure and the evolution of the Milky Way, the structure and the evolution of the clusters of galaxies, the large-scale structure of the universe and cosmology.

WEAVE, for "WHT Enhanced Area Velocity Explorer", was born. It is the product of an international collaboration involving the countries that own the telescope (United Kingdom, Netherlands, Spain), but also France, Italy and Mexico, as well as several organizations and researchers.
Schematic of WEAVE and its implementation on the William Herschel telescope. WEAVE consortium

One of the largest lenses ever made

From a technological point of view, WEAVE was a challenge at the limit of current technologies. In particular, it requires an optical system composed of six lenses. The largest of these six lenses has a diameter of 1.1 meters - one of the largest lenses ever made.

La danse des robots-positionneurs.

We have also designed and developed two "positioning robots" that move up to 960 optical fibers to the positions needed on the focal plane of the telescope to analyze the stars. The robots weave a veritable web of fibers, which are expected to be made and unmade thousands of times, in a dance that is not without charm.
A fully configured WEAVE field, with 700 of the 950 or so fibers placed by two robots (out of frame), on site in the William Herschel telescope. WEAVE consortium, provided by the author

The fiber optics then carry the light 32 meters along the telescope structure to the spectrograph. WEAVE has several types of fiber connections, which allow it to image differently to study different types of objects. For example, 960 individual fibers can be used, each designed to collect light from a point object. Another option is an assembly of more than five hundred fibers covering a large field of view for the observation of large extended objects. Finally, with 20 assemblies of 37 fibers, one can study several extended objects, such as galaxies, and study their properties, such as their rotation curve, their chemical composition and the differences in chemical composition, in different parts of the galaxy.

An international collaboration with multiple skills

We used a wide network of expertise to manufacture the parts and assemble them. For example, the collimator and the fourteen spherical lenses of the cameras were polished in Mexico after the glass disks were manufactured in Europe, Japan and the United States. These components designed and built specifically for WEAVE were then sent to the Netherlands where the integration of the optical and mechanical elements of the spectrograph took place. The assembly of the fiber optic links relied on the contributions of three different manufacturers in France, Canada and the United States. The fiber links were finally tested at the Paris Observatory, before being sent to Oxford to be integrated into the positioner.

The spiral galaxy Messier 74 observed with a test camera to check the optical quality of the field corrector. Once in operation, WEAVE will not take images, but only spectra. Darío González Picos, Lara Monteagudo, Chris Benn and Ovidiu Vaduvescu (Isaac Newton Group of Telescopes, Roque de Los Muchachos Observatory, La Palma, Spain)

After more than a decade of work involving about 100 people in a dozen countries, the WEAVE components have now arrived at the William Herschel Telescope site. The optical system has been tested on the telescope and has demonstrated excellent image quality. Still undergoing integration, WEAVE is expected to make its first observations - spectra, not images - in December 2021.

In France, the construction of WEAVE was financed by the CNRS, the Observatoire de Paris-PSL, the regions Île-de-France and Franche-Comté ; The United Kingdom (STFC), the Netherlands (NOVA and NWO), Spain (IAC, International Isaac Newton Telescope Group, Ministry of Economic Affairs and Digital Transformation), Italy (INAF), Mexico (INAOE), Sweden (Lund Observatory, Uppsala University), Germany (AIP, MPIA), the United States (University of Pennsylvania) and Hungary (Konkoly Observatory) also participated.

The Île-de-France Region funds research projects in areas of major interest and is committed to the development of doctoral programs and training through research through the Paris Region Phd program, which will co-finance 100 doctoral contracts by 2022. For more information, visit iledefrance.fr/education-recherche.