Charge diffusion and modulation transfer function in a Nancy Grace Roman Space Telescope detector

TL;DR

This paper analyzes charge diffusion and the modulation transfer function in a Roman Space Telescope detector, fitting models to laser speckle data to improve weak lensing measurement accuracy.

Contribution

It introduces a hyperbolic secant model for charge diffusion that outperforms the Gaussian model and informs survey strategies for the Roman mission.

Findings

Sech model fits charge diffusion data well without extra parameters.

No wavelength dependence observed from 850 to 2000 nm.

Measured diffusion standard deviation is approximately 0.328 pixels.

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.