Recognition: unknown
Charge diffusion and modulation transfer function in a Nancy Grace Roman Space Telescope detector
Pith reviewed 2026-05-07 14:12 UTC · model grok-4.3
The pith
Charge diffusion in a Roman Space Telescope detector follows a hyperbolic secant profile rather than Gaussian, with a per-axis width of 0.328 pixels and no detectable wavelength dependence from 850 to 2000 nm.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Laser speckle data from a Roman detector are used to extract the modulation transfer function contribution from charge diffusion. Fits show that a hyperbolic secant profile describes the measurements with a standard deviation of 0.3279 pixels per axis and no extra free parameter, while a Gaussian profile is disfavored; the general drift-diffusion model collapses to the sech case. The width shows no measurable change over 850–2000 nm, and the resulting model is inserted into mission simulations to guide weak lensing survey strategy and pipeline development.
What carries the argument
Hyperbolic secant (sech) charge diffusion profile extracted from speckle-pattern MTF data, which captures the spatial distribution of charge spread in the detector pixels.
If this is right
- The sech model can be adopted directly in Roman image simulations and data processing pipelines without additional free parameters.
- Weak lensing shear measurements can incorporate this fixed diffusion scale to reduce one source of systematic error in galaxy shape estimates.
- Detector modeling across the Roman near-infrared band is simplified because diffusion shows no wavelength variation between 850 and 2000 nm.
- Survey strategy and exposure-time calculations for the mission can use the reported MTF to set requirements on pixel sampling and dithering.
Where Pith is reading between the lines
- The physical transport process in these detectors may intrinsically produce sech-like tails rather than Gaussian wings, which could be checked by comparing to detailed carrier transport simulations.
- Similar infrared arrays on other space telescopes might be re-analyzed with a sech template to see whether the same functional form emerges.
- Reducing non-linearity residuals in future calibration campaigns would shrink the dominant systematic floor and allow tighter constraints on the diffusion width.
- On-orbit star or galaxy images from Roman could be stacked to test whether the ground-measured sech width reproduces the observed point-spread function core.
Load-bearing premise
The laser speckle pattern isolates charge diffusion in the MTF measurement without significant contamination from other detector effects such as non-linearities.
What would settle it
An independent MTF measurement on the same detector using a sharp point-source or knife-edge target that yields either a statistically better Gaussian fit or a diffusion width differing by more than the reported systematic uncertainty would falsify the sech preference.
Figures
read the original abstract
The Nancy Grace Roman Space Telescope (Roman) is an observatory motivated by the search to understand dark energy, exoplanets, and general astrophysics. Roman will bring unprecedented amounts of precision to weak gravitational lensing measurements, which necessitates an improved understanding of instrumental signatures in star and galaxy images. One feature is the modulation transfer function (MTF), which includes contributions from charge diffusion in Roman's infrared detector arrays. As part of the detector characterization effort, a detector from the flight lots (but ultimately not selected for flight) was illuminated with a laser speckle pattern. We present an analysis of the laser speckle data, including MTF measurements in several wavelengths. We fit several models for the charge diffusion profile, including: (i) a Gaussian profile; (ii) a hyperbolic secant (sech) profile; and (iii) a general drift-diffusion model that includes the Gaussian and sech as limiting cases. We find that the sech model produces an acceptable fit with no need for the additional parameter and is strongly preferred over the Gaussian. The standard deviation per axis of the sech profile is $0.3279^{+0.0043}_{-0.0042}$(stat)$\pm0.0093$(sys) pixels, with the systematic error dominated by non-linearities. We find no detectable wavelength dependence over the range from 850--2000 nm. The model informs survey strategy for weak lensing measurements and has been included in simulations used to develop the data processing pipelines for the Roman mission.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript analyzes laser speckle data from a Roman Space Telescope H4RG detector to measure the MTF arising from charge diffusion. Three models are fitted to the data: a Gaussian profile, a sech profile, and a general drift-diffusion model that encompasses both as limits. The sech model is reported to provide an acceptable fit with no requirement for the extra parameter, and is strongly preferred over the Gaussian; the fitted per-axis standard deviation is 0.3279^{+0.0043}_{-0.0042}(stat) ± 0.0093(sys) pixels, with systematics dominated by non-linearities. No wavelength dependence is detected over 850–2000 nm. The resulting model is incorporated into Roman weak-lensing simulations.
Significance. If the laser-speckle MTF measurements isolate charge diffusion to the stated precision, the work supplies a concrete, empirically calibrated diffusion kernel that directly improves the fidelity of Roman weak-lensing image simulations and pipeline development. The explicit comparison of three nested physical models and the separation of statistical and systematic uncertainties are strengths; the result is immediately usable for survey-strategy studies.
major comments (3)
- [§4 (Model comparison and fitting)] §4 (Model comparison and fitting): The claim that the sech model is 'strongly preferred' and that the general drift-diffusion model requires no additional parameter rests on a model-selection statistic whose sensitivity to the dominant systematic (non-linearities) is not quantified. A test in which the non-linearity correction parameters are varied within their uncertainty and the resulting change in the selection metric (AIC, BIC, or likelihood ratio) is reported would directly address whether the preference is robust.
- [§3.2 (MTF extraction from speckle power spectra)] §3.2 (MTF extraction from speckle power spectra): The central assumption that the measured power spectrum reflects only charge diffusion is load-bearing for both the model preference and the quoted width. The manuscript acknowledges that non-linearities dominate the systematic error budget, yet no explicit propagation of residual non-linearity residuals into the shape of the MTF (or into the fitted diffusion width) is presented. A quantitative bound on the bias this could induce in the sech-versus-Gaussian comparison is required.
- [Results section (wavelength dependence)] Results section (wavelength dependence): The statement of 'no detectable wavelength dependence' is reported without the per-band fit values, the number of independent wavelengths, or the statistical power of the test once the systematic floor is included. These details are needed to evaluate whether the null result is limited by the non-linearity uncertainty rather than by the data.
minor comments (2)
- [Abstract] Abstract: the numerical result is clearly stated, but the units 'pixels' should be attached explicitly to the quoted standard deviation for immediate readability.
- [Figure captions] Figure captions (e.g., those showing MTF curves and residuals): axis labels and legend entries should be enlarged or clarified so that the Gaussian versus sech comparison is legible in print.
Simulated Author's Rebuttal
We thank the referee for their thoughtful and detailed review. The comments highlight important aspects of robustness in our model selection and uncertainty analysis, and we will revise the manuscript to incorporate additional quantitative tests and details as outlined below.
read point-by-point responses
-
Referee: The claim that the sech model is 'strongly preferred' and that the general drift-diffusion model requires no additional parameter rests on a model-selection statistic whose sensitivity to the dominant systematic (non-linearities) is not quantified. A test in which the non-linearity correction parameters are varied within their uncertainty and the resulting change in the selection metric (AIC, BIC, or likelihood ratio) is reported would directly address whether the preference is robust.
Authors: We agree that demonstrating the robustness of the model preference against variations in the non-linearity correction is valuable. In the revised manuscript, we will add the suggested test to §4: the non-linearity parameters will be varied within their uncertainties, the power spectra re-derived, and the AIC, BIC, and likelihood ratios recomputed for the Gaussian, sech, and drift-diffusion models. This will confirm whether the strong preference for the sech model persists. revision: yes
-
Referee: The central assumption that the measured power spectrum reflects only charge diffusion is load-bearing for both the model preference and the quoted width. The manuscript acknowledges that non-linearities dominate the systematic error budget, yet no explicit propagation of residual non-linearity residuals into the shape of the MTF (or into the fitted diffusion width) is presented. A quantitative bound on the bias this could induce in the sech-versus-Gaussian comparison is required.
Authors: We acknowledge that an explicit bound on bias from residual non-linearities in the MTF shape and model comparison would strengthen the presentation. We will add to §3.2 a quantitative estimate obtained by injecting simulated residual non-linearity effects (at the level of the systematic floor) into the power spectra, refitting the models, and reporting the induced shift in the sech-versus-Gaussian likelihood ratio and fitted width. This bound will be included in the revised text. revision: yes
-
Referee: The statement of 'no detectable wavelength dependence' is reported without the per-band fit values, the number of independent wavelengths, or the statistical power of the test once the systematic floor is included. These details are needed to evaluate whether the null result is limited by the non-linearity uncertainty rather than by the data.
Authors: We will expand the Results section to include the per-band fitted diffusion widths (with uncertainties), the number of independent wavelengths (eight bands spanning 850–2000 nm), and a calculation of the statistical power of the wavelength-dependence test when the systematic floor is folded in. This will clarify that the null result is not limited by the non-linearity uncertainty. revision: yes
Circularity Check
No significant circularity; central result is direct empirical fit to external laser speckle MTF data.
full rationale
The paper illuminates a Roman detector with laser speckle patterns, measures the resulting MTF at multiple wavelengths, and performs least-squares fits of three charge-diffusion models (Gaussian, sech, and a general drift-diffusion model whose limiting cases recover the first two) to those data. The reported sech width, its statistical and systematic uncertainties, and the model-preference conclusion are all direct outputs of this fit; no equation, self-citation, or parameter-renaming step reduces the final numbers to quantities already fitted or assumed inside the same analysis. Systematic error is explicitly traced to detector non-linearities rather than absorbed into the functional form, and the absence of wavelength dependence is likewise a direct observational result. The derivation chain is therefore self-contained against independent external measurements.
Axiom & Free-Parameter Ledger
free parameters (1)
- sech profile standard deviation =
0.3279 pixels
axioms (1)
- domain assumption Charge diffusion in the detector can be represented by Gaussian, sech, or drift-diffusion profiles
Reference graph
Works this paper leans on
-
[1]
Subpixel Response Measurement of Near‐Infrared Detectors
Barron, N., Borysow, M., Beyerlein, K., et al. 2007, PASP, 119, 466, doi: 10.1086/517620
-
[2]
1999, Principles of optics : electromagnetic theory of propagation, interference and diffraction of light, 7th edn
Born, M., & Wolf, E. 1999, Principles of optics : electromagnetic theory of propagation, interference and diffraction of light, 7th edn. (Cambridge University Press)
1999
-
[3]
Cao, K., Hirata, C. M., Laliotis, K., et al. 2025, The Astrophysical Journal Supplement Series, 277, 55, doi: 10.3847/1538-4365/adb580 DESI Collaboration, Abdul-Karim, M., Aguilar, J., et al. 2025a, arXiv e-prints, arXiv:2503.14739, doi: 10.48550/arXiv.2503.14739 —. 2025b, arXiv e-prints, arXiv:2503.14738, doi: 10.48550/arXiv.2503.14738
-
[4]
Dor´ eet al., (2019), arXiv:1904.01174 [astro-ph.CO]
Dore, O., Hirata, C., Wang, Y., et al. 2019, BAAS, 51, 341, doi: 10.48550/arXiv.1904.01174 Euclid Collaboration, Mellier, Y., Abdurro’uf, et al. 2025, A&A, 697, A1, doi: 10.1051/0004-6361/202450810
-
[5]
Fairfield, J. A., Groom, D. E., Bailey, S. J., et al. 2006, IEEE Transactions on Nuclear Science, 53, 3877, doi: 10.1109/TNS.2006.885793
-
[6]
J., Choi, A., Porredon, A., et al
Givans, J. J., Choi, A., Porredon, A., et al. 2022, Publications of the Astronomical Society of the Pacific, 134, 014001, doi: 10.1088/1538-3873/ac46ba 20 0.960 0.965 0.970 0.975 0.980 0.985 0.990 0.995 1.000 distortion matrix values 0.75 0.76 0.77 0.78 0.79 0.80modulation transfer function measured MTF MTF = -0.46x + 1.22 predicted MTF MTF = -0.38x + 1.1...
-
[7]
Hirata, C. M., & Choi, A. 2019, Publications of the Astronomical Society of the Pacific, 132, 014501, doi: 10.1088/1538-3873/ab44f7
-
[8]
Hirata, C. M., & Merchant, C. 2022, PASP, 134, 115001, doi: 10.1088/1538-3873/ac99fe
-
[9]
M., Yamamoto, M., Laliotis, K., et al
Hirata, C. M., Yamamoto, M., Laliotis, K., et al. 2024, Monthly Notices of the Royal Astronomical Society, 528, 2533, doi: 10.1093/mnras/stae182
-
[10]
1986, Journal of Applied Physics, 60, 1091, doi: 10.1063/1.337403 Ivezić, Ž., Kahn, S
Holloway, H. 1986, Journal of Applied Physics, 60, 1091, doi: 10.1063/1.337403 Ivezić, Ž., Kahn, S. M., Tyson, J. A., et al. 2019, ApJ, 873, 111, doi: 10.3847/1538-4357/ab042c
-
[11]
Lauer, T. R. 1999, PASP, 111, 227, doi: 10.1086/316319
-
[12]
Euclid Definition Study Report
Laureijs, R., Amiaux, J., Arduini, S., et al. 2011, arXiv e-prints, arXiv:1110.3193, doi: 10.48550/arXiv.1110.3193
work page internal anchor Pith review doi:10.48550/arxiv.1110.3193 2011
-
[13]
2018, ARA&A, 56, 393, doi:10.1146/annurev-astro-081817-051928
Mandelbaum, R. 2018, Annual Review of Astronomy and Astrophysics, 56, 393, doi: https: //doi.org/10.1146/annurev-astro-081817-051928
-
[14]
Mosby, G., Rauscher, B. J., Bennett, C., et al. 2020, Journal of Astronomical Telescopes, Instruments, and Systems, 6, 046001, doi: 10.1117/1.JATIS.6.4.046001
- [15]
-
[16]
Penny, M. T., Scott Gaudi, B., Kerins, E., et al. 2019, The Astrophysical Journal Supplement Series, 241, 3, doi: 10.3847/1538-4365/aafb69
-
[17]
1999, apj, 517, 565, doi: 10.1086/307221
Perlmutter, S., Aldering, G., Goldhaber, G., et al. 1999, ApJ, 517, 565, doi: 10.1086/307221
work page internal anchor Pith review doi:10.1086/307221 1999
-
[18]
Pozo, A. M., & Rubiño, M. 2005, ApOpt, 44, 1543, doi: 10.1364/AO.44.001543
-
[19]
Riess, A. G., Filippenko, A. V., Challis, P., et al. 1998, AJ, 116, 1009, doi: 10.1086/300499
-
[20]
2005, Reports on Progress in Physics, 68, 2267, doi: 10.1088/0034-4885/68/10/R01
Rogalski, A. 2005, Reports on Progress in Physics, 68, 2267, doi: 10.1088/0034-4885/68/10/R01
-
[21]
M., Baltay, C., Hounsell, R., et al
Rose, B. M., Baltay, C., Hounsell, R., et al. 2021, A Reference Survey for Supernova Cosmology with the Nancy Grace Roman Space Telescope. https://arxiv.org/abs/2111.03081
-
[22]
2011, The Astrophysical Journal, 741, 46, doi: 10.1088/0004-637X/741/1/46
Rowe, B., Hirata, C., & Rhodes, J. 2011, The Astrophysical Journal, 741, 46, doi: 10.1088/0004-637X/741/1/46
-
[23]
Sensiper, M., Boreman, G. D., Ducharme, A. D., & Snyder, D. R. 1993, Optical Engineering, 32, 395, doi: 10.1117/12.60851
-
[24]
Wide-Field InfrarRed Survey Telescope-Astrophysics Focused Telescope Assets WFIRST-AFTA 2015 Report
Spergel, D., Gehrels, N., Baltay, C., et al. 2015, arXiv e-prints, arXiv:1503.03757, doi: 10.48550/arXiv.1503.03757
-
[25]
Weinberg, D. H., Mortonson, M. J., Eisenstein, D. J., et al. 2013, PhR, 530, 87, doi: 10.1016/j.physrep.2013.05.001
-
[26]
Wittman, D. M., Tyson, J. A., Kirkman, D., Dell’Antonio, I., & Bernstein, G. 2000, Nature, 405, 143, doi: 10.1038/35012001
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.