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Euclid: Nonparametric point spread function field recovery through interpolation on a graph Laplacian

M. A. Schmitz 1, * J.-L. Starck 1 Fred Maurice Ngolè Mboula 2 N. Auricchio 3 J. Brinchmann 4, 5 R. I. Vito Capobianco 3 R. Clédassou 6 L. Conversi 7 L. Corcione 3 N. Fourmanoit 8 M. Frailis 3 B. Garilli 3 F. Hormuth 9 D. Hu 10 H. Israel 11 S. Kermiche 8 T. D. Kitching 10 B. Kubik 12 M. Kunz 13 S. Ligori 3 P. B. Lilje 14 I. Lloro 15, 16 O. Mansutti 17 O. Marggraf 18 R. J. Massey 19 F. Pasian 17 V. Pettorino 1 F. Raison 20 J. D. Rhodes 21 M. Roncarelli 22, 23 R. P. Saglia 20, 11 P. Schneider 18 S. Serrano 24, 15 A. N. Taylor 25 R. Toledo-Moreo 26 L. Valenziano 23 C. Vuerli 17 J. Zoubian 8
Abstract : Context. Future weak lensing surveys, such as the Euclid mission, will attempt to measure the shapes of billions of galaxies in order to derive cosmological information. These surveys will attain very low levels of statistical error, and systematic errors must be extremely well controlled. In particular, the point spread function (PSF) must be estimated using stars in the field, and recovered with high accuracy. Aims. The aims of this paper are twofold. Firstly, we took steps toward a nonparametric method to address the issue of recovering the PSF field, namely that of finding the correct PSF at the position of any galaxy in the field, applicable to Euclid. Our approach relies solely on the data, as opposed to parametric methods that make use of our knowledge of the instrument. Secondly, we studied the impact of imperfect PSF models on the shape measurement of galaxies themselves, and whether common assumptions about this impact hold true in an Euclid scenario. Methods. We extended the recently proposed resolved components analysis approach, which performs super-resolution on a field of under-sampled observations of a spatially varying, image-valued function. We added a spatial interpolation component to the method, making it a true 2-dimensional PSF model. We compared our approach to PSFEx, then quantified the impact of PSF recovery errors on galaxy shape measurements through image simulations. Results. Our approach yields an improvement over PSFEx in terms of the PSF model and on observed galaxy shape errors, though it is at present far from reaching the required Euclid accuracy. We also find that the usual formalism used for the propagation of PSF model errors to weak lensing quantities no longer holds in the case of an Euclid-like PSF. In particular, different shape measurement approaches can react differently to the same PSF modeling errors.
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M. A. Schmitz, J.-L. Starck, Fred Maurice Ngolè Mboula, N. Auricchio, J. Brinchmann, et al.. Euclid: Nonparametric point spread function field recovery through interpolation on a graph Laplacian. Astronomy and Astrophysics - A&A, EDP Sciences, 2020, 636, pp.A78. ⟨10.1051/0004-6361/201936094⟩. ⟨cea-02550125⟩

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