Christopher Merchant | Searcharxiv

arXiv2026

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

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.

astro-ph.IM↗

arXiv2022

Pixel centroid characterization with laser speckle and application to the Nancy Grace Roman Space Telescope detector arrays

The Nancy Grace Roman Space Telescope will use its wide-field instrument to carry out a suite of sky surveys in the near infrared. Several of the science objectives of these surveys, such as the measurement of the growth of cosmic structure using weak gravitational lensing, require exquisite control of instrument-related distortions of the images of astronomical objects. Roman will fly 4kx4k Teledyne H4RG-10 infrared detector arrays. This paper investigates whether the pixel centroids are located on a regular grid by projecting laser speckle patterns through a double slit aperture onto a non-flight detector array. We develop a method to reconstruct the pixel centroid offsets from the stochastic speckle pattern. Due to the orientation of the test setup, only x-offsets are measured here. We test the method both on simulations, and by injecting artificial offsets into the real images. We use cross-correlations of the reconstructions from different speckle realizations to determine how much of the variance in the pixel offset maps is signal (fixed to the detector) and how much is noise. After performing this reconstruction on 64x64 pixel patches, and fitting out the best-fit linear mapping from pixel index to position, we find that there are residual centroid offsets in the x (column) direction from a regular grid of 0.0107 pixels RMS (excluding shifts of an entire row relative to another, which our speckle patterns cannot constrain). This decreases to 0.0097 pix RMS if we consider residuals from a quadratic rather than linear mapping. These RMS offsets include both the physical pixel offsets, as well as any apparent offsets due to cross-talk and remaining systematic errors in the reconstruction. We comment on the advantages and disadvantages of speckle scene measurements as a tool for characterizing the pixel-level behavior in astronomical detectors.

astro-ph.IM↗