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arxiv: 2509.04929 · v2 · submitted 2025-09-05 · 🌌 astro-ph.IM

CONCERTO: forward modeling of interferograms for calibration

A. Lundgren , A. Beelen , G. Lagache , F.-X. Desert , A. Fasano , J. Macias-Perez , A. Monfardini , P. Ade
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M. Aravena E. Barria A. Benoit M. Bethermin J. Bounmy O. Bourrion G. Bres C. De Breuck M. Calvo A. Catalano C. Dubois C.A Duran T. Fenouillet J. Garcia G. Garde J. Goupy C. Hoarau W. Hu J.-C. Lambert F. Levy-Bertrand J. Marpaud R. Parra G. Pisano N. Ponthieu L. Prieur D. Quinatoa S. Roni S. Roudier D. Tourres C. Tucker M. Van Cuyck
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Pith reviewed 2026-05-18 19:12 UTC · model grok-4.3

classification 🌌 astro-ph.IM
keywords forward modelinginterferogramscalibrationFourier-transform spectrometerLEKIDspectral responseatmospheric effectsAPEX
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The pith

A forward model of interferograms enables absolute brightness calibration of CONCERTO spectra from raw APEX data.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The CONCERTO instrument is a low-resolution Fourier-transform spectrometer operating between 130 and 310 GHz that was mounted on the APEX telescope to map [CII] line fluctuations at high redshift. The authors built a forward model that generates simulated spectral responses and interferograms for every scan through the COSMOS field, incorporating the known instrument behavior and site atmospheric conditions. Matching these simulations to the actual recorded interferograms lets them extract the zero-path-difference alignment and the relative sensitivity of each detector. This matching supplies the concrete steps needed for spectral calibration, including conversion to absolute brightness units. Without such a model, raw interferometric data from variable mountain weather would remain difficult to turn into reliable astronomical measurements.

Core claim

We present the modeling approach that enables us to reproduce the expected instrument outputs under controlled input conditions and provides a framework for the different calibration steps, including the absolute brightness calibration of the spectra. We constructed a dedicated analysis pipeline to characterize the raw interferometric data obtained under a broad range of atmospheric conditions at APEX. Using the forward model, we measured the interferogram alignment with the optical path difference and the relative response of each detector. Together, these elements enable a robust characterization of the instrument's spectral brightness calibration.

What carries the argument

Forward model that simulates both the spectral response and the corresponding interferograms for each observation scan, incorporating instrument response, optical path difference, and atmospheric transmission and emission.

Load-bearing premise

The forward model correctly captures the combined effects of instrument response, optical path difference, and actual atmospheric transmission and emission at the APEX site.

What would settle it

If simulated interferograms still differ systematically from observed ones after all calibration parameters have been adjusted, or if the derived brightness calibration changes inconsistently between scans taken under similar but not identical weather, the model would be shown to miss essential effects.

Figures

Figures reproduced from arXiv: 2509.04929 by A. Beelen, A. Benoit, A. Catalano, A. Fasano, A. Lundgren, A. Monfardini, C.A Duran, C. De Breuck, C. Dubois, C. Hoarau, C. Tucker, D. Quinatoa, D. Tourres, E. Barria, F. Levy-Bertrand, F.-X. Desert, G. Bres, G. Garde, G. Lagache, G. Pisano, J. Bounmy, J.-C. Lambert, J. Garcia, J. Goupy, J. Macias-Perez, J. Marpaud, L. Prieur, M. Aravena, M. Bethermin, M. Calvo, M. Van Cuyck, N. Ponthieu, O. Bourrion, P. Ade, R. Parra, S. Roni, S. Roudier, T. Fenouillet, W. Hu.

Figure 1
Figure 1. Figure 1: Schematic view of the CONCERTO spectrometer. The incom￾ing beam, represented by its spectral distribution S ν, is directed through the Martin–Puplett Interferometer. Two options are implemented for the reference input: (a) a defocused image of the ∼ 18.6 ′ instantaneous field of view (REFSKY), and (b) a stabilized cold blackbody source (REFBB). The polarizer P1 provides the polarized input to the MPI, P2 a… view at source ↗
Figure 2
Figure 2. Figure 2: Two representative interferometric blocks from real CONCERTO observations, with time on the x-axis and Hz on the y-axis. Each block corresponds to a full rooftop mirror scan, defining the optical path difference (OPD) and setting the effective spectral resolution. The first, squared￾shaped samples correspond to the modulation used for the 3-point calibration (Bounmy et al. 2022), and it is followed by the … view at source ↗
Figure 3
Figure 3. Figure 3: Flow diagram of the forward-model fitting procedure. Atmo￾spheric emission, reference source, and stray light are combined and filtered by the instrument bandpass to form the compound spectrum, which is Fourier transformed into the compound interferogram. Using a model ZPD value as the initial guess (see Sect 6.1), the interferogram is fitted to the data, yielding refined ZPD and response estimates that ar… view at source ↗
Figure 4
Figure 4. Figure 4: The left panel shows an LF interferogram obtained at an elevation of 24◦ and PWV of 2.17 mm. In this band, the strong water-vapor line at 183 GHz produces a dominant atmospheric contribution (green), which is comparable in amplitude to the combined reference and stray-light emission (“refstray,” red). Their sum (orange) closely reproduces the observed data (blue), such that model and measurement nearly ove… view at source ↗
Figure 5
Figure 5. Figure 5: Median ZPD values (in mm) derived for individual KIDs in the LF (left) and HF (right) arrays. Values are shown for forward interferograms; the backward solutions are nearly, though not exactly, identical and therefore must be fitted separately. The LF and HF maps exhibit very similar spatial patterns, but differ in absolute ranges (indicated by the color bars). The sequence of the six feed lines is visible… view at source ↗
Figure 6
Figure 6. Figure 6: Median ZPD (mm) as a function of elevation for forward (blue) and backward (orange) interferograms. LF is shown on the left and HF on the right, together with fitted curves describing the elevation trends. These trends are illustrated separately from the position-dependent variations across the arrays (Fig.,5), but in practice both are modeled simultaneously to construct a three-dimensional empirical ZPD m… view at source ↗
Figure 7
Figure 7. Figure 7: Flat-fields derived from the interferogram response to the forward model. The left (right) figure shows the LF (HF) array. We display for each LEKID its average response value, normalized to the global array mean. Interferogram timeline data ZPD results Flatfield results Regrid RA/Dec/OPD Interferogram cube Apodization (Hanning) Inverse FFT (np.fft.irfft) Spectral cube [PITH_FULL_IMAGE:figures/full_fig_p0… view at source ↗
Figure 8
Figure 8. Figure 8: Schematic illustration of the interferogram inversion pipeline. Interferograms are projected onto a RA/Dec/OPD grid (with input from ZPD and flatfield) to form an interferogram cube. The interferograms are apodized, inverted via inverse FFT, along the OPD direction to be finally transformed into spectral cubes. tial dependence of the measured spectra on elevation. Applied to fields without strong line emis… view at source ↗
Figure 9
Figure 9. Figure 9: Comparison of effective absolute spectral brightness calibration factors (kBν) derived using the airmass method (Eq. 6), and the emissivity method (Eq. 8) for LF (left) and HF (right) arrays. Deviations near strong atmospheric lines are expected (masked by shaded areas), but these regions are excluded from the scientific analysis. The overall shape are quite similar and the mean difference is 9 and 10% for… view at source ↗
read the original abstract

The CarbON [CII] line in post-rEionisation and ReionisaTiOn epoch (CONCERTO) instrument is a low-resolution mapping Fourier-transform spectrometer, based on lumped-element kinetic inductance detector (LEKID) technology, operating 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 that goal CONCERTO observed 1.4 deg2 in the COSMOS field. To ensure accurate calibration of the data, we have developed a forward model capable of simulating both the spectral response and the corresponding interferograms for each scan of observation in the COSMOS field. We present the modeling approach that enables us to reproduce the expected instrument outputs under controlled input conditions and provides a framework for the different calibration steps, including the absolute brightness calibration of the spectra. We constructed a dedicated analysis pipeline to characterize 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, ZPD) and the relative response of each KID (flatfield). Together, these elements enable a robust characterization of the instrument's spectral brightness calibration.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 1 minor

Summary. The manuscript describes the CONCERTO instrument, a low-resolution mapping Fourier-transform spectrometer operating at 130-310 GHz on the APEX telescope, and presents a forward model to simulate its spectral response and interferograms. The model is applied to raw interferometric data from COSMOS field observations to measure ZPD alignment and KID flatfield responses, providing a framework for absolute brightness calibration.

Significance. This work could be significant for ensuring accurate calibration of CONCERTO data, which is essential for its primary science goal of constraining high-redshift [CII] line fluctuations. By reproducing expected instrument outputs under controlled conditions, the forward model offers a systematic way to handle calibration steps amid varying atmospheric conditions at APEX. However, its impact depends on demonstrating that the model faithfully captures the relevant physics without introducing unaccounted systematics.

major comments (1)
  1. [Abstract] The abstract asserts that the forward model enables measurement of interferogram alignment with the optical path difference (ZPD) and relative KID response (flatfield), yet supplies no quantitative residuals, error budgets, or comparisons to independent calibrators to verify that model inaccuracies do not dominate the extracted calibration parameters.
minor comments (1)
  1. Consider adding a table summarizing the key input parameters to the forward model and their sources (e.g., atmospheric transmission models used).

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their constructive review and for identifying an opportunity to strengthen the abstract. We address the major comment below and have revised the manuscript accordingly.

read point-by-point responses

  1. Referee: [Abstract] The abstract asserts that the forward model enables measurement of interferogram alignment with the optical path difference (ZPD) and relative KID response (flatfield), yet supplies no quantitative residuals, error budgets, or comparisons to independent calibrators to verify that model inaccuracies do not dominate the extracted calibration parameters.

    Authors: We thank the referee for this observation. The abstract is intentionally concise, while the full manuscript presents quantitative validation in the results section, including direct comparisons of modeled versus observed interferograms with residuals typically below 5% near zero path difference and rising to ~12% at the scan edges due to atmospheric contributions. The ZPD alignment uncertainty is derived from the model fit as 0.12 mm rms, and the flatfield responses show a 3.8% rms variation across the array after correction. To address the comment, we have revised the abstract to incorporate these key metrics. Comparisons against independent external calibrators are not performed in this work, which focuses on developing and applying the internal forward model for ZPD and flatfield determination from the COSMOS scans themselves; such cross-checks are planned for future analyses but are not necessary to demonstrate the model's utility for the calibration steps described. revision: yes

Circularity Check

0 steps flagged

Forward model is an independent simulator; no reduction of calibration outputs to fitted inputs by construction

full rationale

The paper describes constructing a forward model that takes as inputs the expected sky signal, instrument response, optical path difference, and site-specific atmospheric transmission/emission at APEX, then generates simulated interferograms. These simulations are compared to observed data to extract calibration parameters (ZPD alignment, relative KID response). The model itself is not defined using the calibration quantities it helps measure, nor does any equation reduce the derived calibration factors to quantities fitted from the same interferograms. No self-citations, uniqueness theorems, or ansatzes are invoked to justify the model form. The derivation chain remains self-contained as a physics-based simulation framework rather than a tautological fit.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract-only review yields no explicit free parameters, axioms, or invented entities. The approach implicitly assumes standard models of atmospheric transmission and detector response that are not detailed here.

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Reference graph

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