arXiv subjects

Katherine Laliotis

Publications and source records attributed to Katherine Laliotis.

10 recordsLinked to original sources

arXiv2026

PSFSim: PhySics First Simulations for the Point Spread Function of the Roman Space Telescope

We present a new tool for generating simulated point spread functions (PSFs) in the Wide Field Instrument (WFI) of the Nancy Grace Roman Space Telescope. The simulation tool is physics forward, and performs a raytrace of the optical system of the instrument to compute the PSF, while also accounting for various detector effects. The tool also has the capability to add perturbations to the optical system and detectors. We explore several applications of the tool that may arise during the commissioning of the telescope, including optical ghosts, measuring the optical aberrations from diffraction spikes, and studying the effect of mirror polarization.

astro-ph.IM↗

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↗

arXiv2026

Robust Photometry for Roman High-Latitude Imaging Survey Cosmology Using Roman and Rubin Imaging

Accurate and precise photometry is essential for Roman cosmology because it affects photometric-redshift performance, galaxy and cluster selection, tomographic binning, and redshift-distribution characterization. Achieving robust photometry is challenging because Roman's depth leads to substantial source blending, particularly in joint Roman--Rubin analyses, where the two surveys have different spatial resolutions. In this work, we develop and validate slimfarmer, a model-fitting photometry pipeline designed for Roman High-Latitude Imaging Survey (HLIS) cosmological analyses. Building on The Farmer, slimfarmer introduces treatments of the correlated noise present in Roman images and of astronomical shot noise, together with model-fitting configurations tuned for Roman imaging. We validate the pipeline using Roman coadded simulated images and perform forced photometry on matched Rubin image simulations. We find that slimfarmer recovers galaxy colors with the mode of the color residuals within 20 millimag for Roman-based colors and within 70 millimag for Rubin-based colors. Properly accounting for correlated noise is essential for uncertainty quantification, as photometric uncertainties are otherwise underestimated by up to a factor of ~3. We also find that applying multi-object fitting consistently to both Roman and Rubin imaging substantially mitigates blending-induced color systematics. In crowded regions, single-object fitting produces environment-dependent color biases of up to ~0.5 mag, leading to environment-dependent degradation of photometric-redshift performance. Together, these results establish slimfarmer as a prototype photometric pipeline for Roman HLIS cosmology applications and identify the correlated-noise corrections, astronomical shot-noise treatment, and joint Roman--Rubin multi-object fitting required for robust photometric-redshift characterization.

astro-ph.IM↗

arXiv2026

Simulating Image Coaddition with the Nancy Grace Roman Space Telescope. IV. Hyperparameter Optimization and Experimental Features

For weak gravitational lensing cosmology with the forthcoming Nancy Grace Roman Space Telescope, image coaddition, or construction of oversampled images from undersampled ones, is a critical step in the image processing pipeline. In the previous papers in this series, we have re-implemented the {\sc Imcom} algorithm, which offers control over point spread functions in coadded images, and applied it to state-of-the-art image simulations for Roman. In this work, we systematically investigate the impact of {\sc Imcom} hyperparameters on the quality of measurement results. We re-coadd the same $16$ blocks ($1.75 \times 1.75 \,{\rm arcmin}^2$, $2688 \times 2688$ pixels each) from OpenUniverse2024 simulations with $26$ different configurations in each of $5$ bands. We then compare the results in terms of $12$ objective evaluation criteria, including internal diagnostics of {\sc Imcom}, properties of coadded noise frames, measurements of injected point sources, and time consumption. We demonstrate that: i) the Cholesky kernel is the best known linear algebra strategy for {\sc Imcom}, ii) for our measurements, a wide Gaussian target output PSF outperforms a smoothed Airy disk or a narrow Gaussian, iii) kernel-specific settings are worth considering for future coaddition, and iv) {\sc Imcom} experimental features studied in this work are either inconsequential or detrimental. We end this paper by discussing current and next steps of {\sc Imcom}-related studies in the context of Roman shear and clustering measurements.

astro-ph.IM↗

arXiv2025

Removing correlated noise stripes from the Nancy Grace Roman Space Telescope survey images

Weak gravitational lensing has emerged as a powerful tool for investigating the matter distribution in the Universe and how it has evolved over cosmic time. The Wide Field Instrument (WFI) on the Nancy Grace Roman Space Telescope (Roman) will deliver some of the highest precision measurements of weak lensing ever made. Since weak lensing is based on statistics of faint sources, it can be biased by even tiny instrument systematics, including correlated read noise. Previous works have shown the infrared detectors used in the Roman WFI show correlations in their noise fields at a level significant for weak lensing measurements, even after application of standard reference pixel corrections; of particular concern is 1/f noise, which appears as horizontal banding in the detector frame. In this paper, we present imDestripe: a new Python module utilizing the multiple roll angles in Roman's observing strategy and linear algebra techniques to remove correlated noise stripes from observed images. We test imDestripe in a hybrid simulation by combining real noise realizations (from darks taken during ground testing) with simulated images of the astronomical scene, and find that the power spectrum of the banding can be suppressed by factors of 10--30 on large scales. We briefly discuss plans for further development of imDestripe in the context of the WFI pipeline.

astro-ph.IM↗

arXiv2025

Simulating image coaddition with the Nancy Grace Roman Space Telescope: III. Software improvements and new linear algebra strategies

The Nancy Grace Roman Space Telescope will implement a devoted weak gravitational lensing program with its High Latitude Wide Area Survey. For cosmological purposes, a critical step in Roman image processing is to combine dithered undersampled images into unified oversampled images and thus enable high-precision shape measurements. IMCOM is an image coaddition algorithm which offers control over point spread functions in output images. This paper presents the refactored IMCOM software, featuring full object-oriented programming, improved data structures, and alternative linear algebra strategies for determining coaddition weights. Combining these improvements and other acceleration measures, to produce almost equivalent coadded images, the consumption of core-hours has been reduced by about an order of magnitude. We then re-coadd a $16 \times 16 \,{\rm arcmin}^2$ region of our previous image simulations with three linear algebra kernels in four bands, and compare the results in terms of IMCOM optimization goals, properties of coadded noise frames, and measurements of simulated stars. The Cholesky kernel is efficient and relatively accurate, yet its irregular windows for input pixels slightly bias coaddition results. The iterative kernel avoids this issue by tailoring input pixel selection for each output pixel; it yields better noise control, but can be limited by random errors due to finite tolerance. The empirical kernel coadds images using an empirical relation based on geometry; it is inaccurate, but being much faster, it provides a valid option for "quick look" purposes. We fine-tune IMCOM hyperparameters in a companion paper.

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arXiv2024

Analysis of biasing from noise from the Nancy Grace Roman Space Telescope: implications for weak lensing

The Nancy Grace Roman Space Telescope, set to launch in 2026, will bring unprecedented precision to measurements of weak gravitational lensing. Because weak lensing is an inherently small signal, it is imperative to minimize systematic errors in measurements as completely as possible; this will ensure that the lensing measurements can be used to their full potential when extracting cosmological information. In this paper, we use laboratory tests of the Roman detectors, simulations of the Roman High Latitude Survey observations, and the proposed Roman image combination pipeline to investigate the magnitude of detector read noise biasing on weak lensing measurements from Roman. First, we combine lab-measured detector noise fields with simulated observations and propagate these images through the Roman image combination pipeline, IMCOM. We characterize the specific signatures of the noise fields in the resultant images and find that noise contributes to the combined images most strongly at scales relevant to physical characteristics of the detector including PSF shape, chip boundaries, and roll angles. We then measure shapes of simulated stars and galaxies and determine the magnitude of noise-induced shear bias on these measurements. We find that star shape correlations satisfy the system noise requirements as defined by the Roman Science Requirements Document. However, for galaxies fainter than $m_{\rm AB}\simeq24$, correction for noise correlations will be needed in order to ensure confidence in shape measurements in any observation band.

astro-ph.IM↗

arXiv2024

Simulating image coaddition with the Nancy Grace Roman Space Telescope: II. Analysis of the simulated images and implications for weak lensing

One challenge for applying current weak lensing analysis tools to the Nancy Grace Roman Space Telescope is that individual images will be undersampled. Our companion paper presented an initial application of Imcom - an algorithm that builds an optimal mapping from input to output pixels to reconstruct a fully sampled combined image - on the Roman image simulations. In this paper, we measure the output noise power spectra, identify the sources of the major features in the power spectra, and show that simple analytic models that ignore sampling effects underestimate the power spectra of the coadded noise images. We compute the moments of both idealized injected stars and fully simulated stars in the coadded images, and their 1- and 2-point statistics. We show that the idealized injected stars have root-mean-square ellipticity errors (1 - 6) x 10-4 per component depending on the band; the correlation functions are >= 2 orders of magnitude below requirements, indicating that the image combination step itself is using a small fraction of the overall Roman 2nd moment error budget, although the 4th moments are larger and warrant further investigation. The stars in the simulated sky images, which include blending and chromaticity effects, have correlation functions near the requirement level (and below the requirement level in a wide-band image constructed by stacking all 4 filters). We evaluate the noise-induced biases in the ellipticities of injected stars, and explain the resulting trends with an analytical model. We conclude by enumerating the next steps in developing an image coaddition pipeline for Roman.

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arXiv2024

Simulating image coaddition with the Nancy Grace Roman Space Telescope: I. Simulation methodology and general results

The upcoming Nancy Grace Roman Space Telescope will carry out a wide-area survey in the near infrared. A key science objective is the measurement of cosmic structure via weak gravitational lensing. Roman data will be undersampled, which introduces new challenges in the measurement of source galaxy shapes; a potential solution is to use linear algebra-based coaddition techniques such as Imcom that combine multiple undersampled images to produce a single oversampled output mosaic with a desired "target" point spread function (PSF). We present here an initial application of Imcom to 0.64 square degrees of simulated Roman data, based on the Roman branch of the Legacy Survey of Space and Time (LSST) Dark Energy Science Collaboration (DESC) Data Challenge 2 (DC2) simulation. We show that Imcom runs successfully on simulated data that includes features such as plate scale distortions, chip gaps, detector defects, and cosmic ray masks. We simultaneously propagate grids of injected sources and simulated noise fields as well as the full simulation. We quantify the residual deviations of the PSF from the target (the "leakage"), as well as noise properties of the output images; we discuss how the overall tiling pattern as well as Moiré patterns appear in the final leakage and noise maps. We include appendices on interpolation algorithms and the interaction of undersampling with image processing operations that may be of broader applicability. The companion paper ("Paper II") explores the implications for weak lensing analyses.

astro-ph.IM↗

arXiv2023

Doppler Constraints on Planetary Companions to Nearby Sun-like Stars: An Archival Radial Velocity Survey of Southern Targets for Proposed NASA Direct Imaging Missions

Directly imaging temperate rocky planets orbiting nearby, Sun-like stars with a 6-m-class IR/O/UV space telescope, recently dubbed the Habitable Worlds Observatory, is a high priority goal of the Astro2020 Decadal Survey. To prepare for future direct imaging surveys, the list of potential targets should be thoroughly vetted to maximize efficiency and scientific yield. We present an analysis of archival radial velocity data for southern stars from the NASA/NSF Extreme Precision Radial Velocity Working Group's list of high priority target stars for future direct imaging missions (drawn from the HabEx, LUVOIR, and Starshade studies). For each star, we constrain the region of companion mass and period parameter space we are already sensitive to based on the observational baseline, sampling, and precision of the archival RV data. Additionally, for some of the targets we report new estimates of magnetic activity cycle periods, rotation periods, improved orbital parameters for previously known exoplanets, and new candidate planet signals that require further vetting or observations to confirm. Our results show that for many of these stars we are not yet sensitive to even Saturn-mass planets in the habitable zone, let alone smaller planets, highlighting the need for future EPRV vetting efforts before the launch of a direct imaging mission. We present evidence that the candidate temperate super-Earth exoplanet HD 85512 b is most likely due to the star's rotation, and report an RV acceleration for delta Pav which supports the existence of a distant giant planet previously inferred from astrometry.

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