Calibration and performance of the NIKA2 camera at the IRAM 30-m Telescope

¹ Univ. Grenoble Alpes, CNRS, Grenoble INP, LPSC-IN2P3, 53 avenue des Martyrs, 38000 Grenoble, France
e-mail: perotto@lpsc.in2p3.fr
² Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
³ LLR (Laboratoire Leprince-Ringuet), CNRS, École Polytechnique, Institut Polytechnique de Paris, Palaiseau, France
⁴ Centro de Estudios de Física del Cosmos de Aragón (CEFCA), Plaza San Juan 1, Planta 2, 44001 Teruel, Spain
⁵ Astronomy Instrumentation Group, University of Cardiff, Cardiff, UK
⁶ AIM, CEA, CNRS, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, 91191 Gif-sur-Yvette, France
⁷ Institut d’Astrophysique Spatiale (IAS), CNRS and Université Paris Sud, Orsay, France
⁸ Institut Néel, CNRS and Université Grenoble Alpes, France
⁹ Institut de RadioAstronomie Millimétrique (IRAM), Grenoble, France
¹⁰ Dipartimento di Fisica, Sapienza Università di Roma, Piazzale Aldo Moro 5, 00185 Roma, Italy
¹¹ Chinese Academy of Sciences South America Center for Astronomy, National Astronomical Observatories, CAS, Beijing 100101, PR China
¹² Instituto de Astronomía, Universidad Católica del Norte, Av. Angamos 0610, Antofagasta 1270709, Chile
¹³ Centro de Astrobiología (CSIC-INTA), Torrejón de Ardoz, 28850 Madrid, Spain
¹⁴ Instituto de Radioastronomía Milimétrica (IRAM), Granada, Spain
¹⁵ Aix Marseille Univ., CNRS, CNES, LAM (Laboratoire d’Astrophysique de Marseille), Marseille, France
¹⁶ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, 75014 Paris, France
¹⁷ Institut d’Astrophysique de Paris, CNRS (UMR7095), 98 bis Boulevard Arago, 75014 Paris, France
¹⁸ Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
¹⁹ School of Earth and Space Exploration and Department of Physics, Arizona State University, Tempe, AZ 85287, USA

Received: 1 July 2019
Accepted: 15 January 2020

Abstract

Context. NIKA2 is a dual-band millimetre continuum camera of 2 900 kinetic inductance detectors, operating at 150 and 260 GHz, installed at the IRAM 30-m telescope in Spain. Open to the scientific community since October 2017, NIKA2 will provide key observations for the next decade to address a wide range of open questions in astrophysics and cosmology.

Aims. Our aim is to present the calibration method and the performance assessment of NIKA2 after one year of observation.

Methods. We used a large data set acquired between January 2017 and February 2018 including observations of primary and secondary calibrators and faint sources that span the whole range of observing elevations and atmospheric conditions encountered by the IRAM 30-m telescope. This allowed us to test the stability of the performance parameters against time evolution and observing conditions. We describe a standard calibration method, referred to as the “Baseline” method, to translate raw data into flux density measurements. This includes the determination of the detector positions in the sky, the selection of the detectors, the measurement of the beam pattern, the estimation of the atmospheric opacity, the calibration of absolute flux density scale, the flat fielding, and the photometry. We assessed the robustness of the performance results using the Baseline method against systematic effects by comparing results using alternative methods.

Results. We report an instantaneous field of view of 6.5′ in diameter, filled with an average fraction of 84%, and 90% of valid detectors at 150 and 260 GHz, respectively. The beam pattern is characterised by a FWHM of 17.6″ ± 0.1″ and 11.1″ ± 0.2″, and a main-beam efficiency of 47%±3%, and 64%±3% at 150 and 260 GHz, respectively. The point-source rms calibration uncertainties are about 3% at 150 GHz and 6% at 260 GHz. This demonstrates the accuracy of the methods that we deployed to correct for atmospheric attenuation. The absolute calibration uncertainties are of 5%, and the systematic calibration uncertainties evaluated at the IRAM 30-m reference Winter observing conditions are below 1% in both channels. The noise equivalent flux density at 150 and 260 GHz are of 9 ± 1 mJy s^1/2 and 30 ± 3 mJy s^1/2. This state-of-the-art performance confers NIKA2 with mapping speeds of 1388 ± 174 and 111 ± 11 arcmin² mJy⁻² h⁻¹ at 150 and 260 GHz.

Conclusions. With these unique capabilities of fast dual-band mapping at high (better that 18″) angular resolution, NIKA2 is providing an unprecedented view of the millimetre Universe.