# A magnum opus on the Chajnantor plateau

Alessandro Monfardini, Guilaine Lagache

► To cite this version:

Alessandro Monfardini, Guilaine Lagache. A magnum opus on the Chajnantor plateau. Nature Astronomy, 2021, 5 (9), pp.970. 10.1038/s41550-021-01482-1. hal-03562541

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# A magnum opus on the Chajnantor plateau

The CONCERTO instrument paves the way to large field-of-view spectro-imagers operating at (sub-)millimetre wavelengths, write Instrument Scientist Alessandro Monfardini and Principal Investigator Guilaine Lagache.

CONCERTO (CarbON C11 line in post-rEionisation and ReionisaTiOn epoch), installed in April 2021 on the Atacama Pathfinder EXperiment (APEX) 12-m telescope and covering the frequency range 100–300 GHz, inaugurates a class of instruments. The newest-born on the Chajnantor plateau in northern Chile combines a large instantaneous field-of-view (20 arcminutes), diffraction-limited angular resolution (30 arcseconds) and moderate spectral capabilities ($R = \lambda/\Delta\lambda = 1–300$, settable). In order to achieve this trifecta without sacrificing sensitivity, a 4,300-pixel cryogenic camera has been coupled to a sophisticated optics system including a Martin-Puplett interferometer (MPI). A fast-moving (2.5 Hz) mirror allows for the separation of the spectral components of the incoming radiation in the Fourier domain for each individual pixel of the camera<fnref n="1" />. When CONCERTO is observing, more than 20,000 full interferograms, to be turned into spectra by means of powerful data processors, are generated every second.

The camera makes use of a dilution refrigerator, exploiting a mixture of ³He and ⁴He to achieve a base temperature of 0.06 K. Under these conditions the aluminium thin (20 nm) films, the building blocks of our detectors, are driven deeply into the superconducting state. The incoming quanta of millimetre radiation, despite their fractions-of-an-meV energies, are able to perturb the superconductivity and generate a detectable signal in the Kinetic Inductance Detectors (KID)²,³. A distinctive advantage of the KID is their built-in multiplexing ability, that is, the ability to read out a large number of pixels using a single wire. In CONCERTO, 400 detectors share a single readout line. The camera, detectors, and the readout electronics have been conceived, designed, fabricated and tested in our laboratories⁴. The data acquisition and analysis software have also been developed by our collaboration.

The project, funded by a European Research Council grant, started in April 2018 with our first visit to APEX. This observatory combines optimal performance (telescope) and location (5,100 m above sea level). In addition, it is operated by an excellent team, ensuring smooth operations,

maintenance and organization. After the ritual Critical Design Review, the fabrication of the thousands of mechanical pieces, several mirrors, valves, actuators, cables, pumps and so on, started in February 2019.

A year later, a pre-installation was carried out. The hydraulic, electrical and network cables linking the instrument sub-systems in the APEX telescope tower were prepared. Despite a deadly virus attacking Chile, we managed to fully integrate the instrument in the laboratory by the end of 2020.

CONCERTO was dismounted and then shipped to Chile in March, 2021. On 10 April 2021, after five days of intense work under an atmosphere of only 550 mbar (Fig. 1), the first cooldown of the camera at the high site was launched. The base temperature of 0.06 K was achieved 40 hours later, and the first instrumental checks were successful, granting the green light to the second team to travel from Santiago to APEX. CONCERTO was immediately moved into the commissioning phase⁵.

In July 2021, CONCERTO, operated remotely, started its ambitious primary

science programme consisting of studying the birth of the first generation of stars via the so-called line intensity mapping technique. Among the observational targets are also galaxy clusters, the largest gravitationally bound structures in the Universe, and Galactic star-forming regions.

<figure><img src="image_3.png" /><figcaption>**Fig. 1** | The CONCERTO 'chassis' flying to APEX. The CONCERTO chassis, including the sub-kelvin camera (left), being lifted to the telescope platform. Credit: CONCERTO team/ESO</figcaption></figure>

Alessandro Monfardini ID <fnref n="1" /> and
Guilaine Lagache ID ²

Published online: 15 September 2021
https://doi.org/10.1038/s41550-021-01482-1

## References

1. Martin, D. H. & Puplett, E. *Infrared Phys.* **10**, 105–109 (1970).

2. Day, P. K. et al. *Nature* **425**, 817–821 (2003).

3. Adam, R. et al. *Astron. Astrophys.* **609**, A115 (2018).

4. The CONCERTO collaboration. *Astron. Astrophys.* **642**, A60 (2020).

5. Monfardini, A. et al. Preprint at https://arxiv.org/abs/2106.14028 (2021).

<footnote n="1">¹Institut Néel – CNRS and Université Grenoble Alpes, Grenoble, France. ²Laboratoire d'Astrophysique de Marseille – CNRS and Université Aix-Marseille, Marseille, France.</footnote>

<footnote>✉e-mail: monfardini@neel.cnrs.fr</footnote>