The CarbON CII line in post-rEionisation and ReionisaTiOn epoch (CONCERTO) instrument was a low-resolution mapping Fourier-transform spectrometer based on lumped-element kinetic inductance detector (LEKID) technology that operated at 130--310,GHz. It was installed on the 12-meter APEX telescope in Chile in April 2021 and operated until December 2022. CONCERTO's main science goal is to constrain the CII line fluctuations at high redshift. To reach this goal, CONCERTO observed 1.4 deg2 in the COSMOS field. To ensure accurate calibration of the data, we developed a forward model capable of simulating the spectral response and the corresponding interferograms for each scan of observation in the COSMOS field. We present the modelling approach that enabled us to reproduce the expected instrument outputs under controlled input conditions and that provided a framework for the different calibration steps, including the absolute brightness calibration of the spectra. We constructed a dedicated analysis pipeline to characterise the raw interferometric data (interferograms) obtained under a broad range of atmospheric conditions at APEX. Using the forward model, we measured the interferogram alignment with the optical path difference (zero path difference) and the relative response of each KID (flatfield). Together, these elements enabled us to characterise the instrument’s spectral brightness calibration reliably. We demonstrate that the zero path difference systematically varies with elevation and across detectors, with variations that are consistent with small optical misalignments and elevation-dependent mechanical effects in the optical structure. The full measurement of these variations allowed us to construct a data base that is used to determine the zero path difference for each measured individual interferogram accurately. The flatfield shows systematic variations with detector position, but is extremely stable with time and atmospheric contribution. The accurate determination of the zero path differences and flatfields allowed us to construct spectral cubes that combine all detectors and all blocks of data. Finally, we present a novel method for calibrating the absolute brightness of these spectral cubes, which is agnostic to the exact knowledge of the bandpasses and directly applicable to extended emission. Our analysis establishes a framework for precise calibration directly from on-sky data. This approach ensures a reliable performance for cosmological and astrophysical applications and can readily be adapted to future Martin–Puplett interferometer–based Fourier-transform spectrometers.
Lundgren et al. (Mon,) studied this question.