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Published December 2013 | Published + Submitted
Journal Article Open

Weak Gravitational Lensing Systematics from Image Combination


Extremely accurate shape measurements of galaxy images are needed to probe dark energy properties with weak gravitational lensing surveys. To increase survey area with a fixed observing time and pixel count, images from surveys such as the Wide Field Infrared Survey Telescope (WFIRST) or Euclid will necessarily be undersampled and therefore distorted by aliasing. Oversampled, unaliased images can be obtained by combining multiple, dithered exposures of the same source with a suitable reconstruction algorithm. Any such reconstruction must minimally distort the reconstructed images for weak lensing analyses to be unbiased. In this paper, we use the image combination (IMCOM) algorithm of Rowe, Hirata, and Rhodes to investigate the effect of image combination on shape measurements (size and ellipticity). We simulate dithered images of sources with varying amounts of ellipticity and undersampling, reconstruct oversampled output images from them using IMCOM, and measure shape distortions in the output. Our simulations show that IMCOM creates no significant distortions when the relative offsets between dithered images are precisely known. Distortions increase with the uncertainty in those offsets, but become problematic only with relatively poor astrometric precision; e.g., for images similar to those from the Astrophysics Focused Telescope Asset (AFTA) implementation of WFIRST, combining eight undersampled images (sampling ratio Q = 1) with highly pessimistic uncertainty in astrometric registration (σ_d ∼ 10^(-3) pixels) yields an rms shear error of O(10^(-4)). Our analysis pipeline is adapted from that of the Precision Projector Laboratory—a joint project between NASA Jet Propulsion Laboratory and Caltech that characterizes image sensors using laboratory emulations of astronomical data.

Additional Information

© 2013 Astronomical Society of the Pacific. Received 2013 August 07; accepted 2013 October 14; published 2013 November 25. We thank E. Jullo of Laboratoire d'Astrophysique de Marseille and V. Velur for their contributions to the analysis pipeline. This research was carried out at the Jet Propulsion Laboratory and California Institute of Technology, under a contract with the National Aeronautics and Space Administration. We are grateful to the following organizations and programs for their support of this effort: internal JPL Research and Technology Development (RTD) and Director's Research Development Fund (DRDF) programs; US Department of Energy's (DOE) Supernova Acceleration Probe (SNAP) and Joint Dark Energy Mission (JDEM) projects; the NASA Wide Field IR Survey Telescope (WFIRST) and Joint Dark Energy Mission (JDEM) project offices. CS was supported by a NASA Postdoctoral Program Fellowship from Oak Ridge Associated Universities. BR acknowledges support from European Research Council in the form of a Starting Grant with number 240672. JPL is run by the California Institute of Technology under a contract for NASA. Thanks also to our anonymous referee for improvements to this manuscript.

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August 19, 2023
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