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However, the source extent and smearing through the point spread function (PSF) cause each spaxel to integrate light from a region larger than itself. The usually adopted geometric spaxel centre is therefore not representative of where the absorbed flux originates. Informed by high-resolution imaging, we introduce a flux-contribution formalism that reconstructs the spatial origin of the flux in each spaxel by defining flux-weighted coordinates and impact parameters. Using a data-driven simulation built on a real gravitational-arc configuration, we show that spaxel centres are displaced from the flux-weighted positions by |\u0394\n                    <jats:bold>x<\/jats:bold>\n                    |\u221d\n                    <jats:italic>\u03b8<\/jats:italic>\n                    <jats:sub>PSF<\/jats:sub>\n                    \u2006(1\u2005\u2212\u2005\n                    <jats:italic>f<\/jats:italic>\n                    ), with\n                    <jats:italic>f<\/jats:italic>\n                    the spaxel\u2013source overlap fraction, and that the resulting impact-parameter bias scales with the lensing magnification as\n                    <jats:italic>\u03bc<\/jats:italic>\n                    <jats:sup>\u22121\/2<\/jats:sup>\n                    . These offsets are largest for off-arc spaxels and flatten the radial column-density profiles. They also inject a coherent geometry-driven scatter that mimics intrinsic CGM structure and that is removed neither by a tighter spaxel selection nor by a finer spatial binning. For identical uncertainties, the spaxel-centre coordinates leave residuals of about twice the noise level (\n                    <jats:italic>\u03c7<\/jats:italic>\n                    <jats:sub>\n                      <jats:italic>\u03bd<\/jats:italic>\n                    <\/jats:sub>\n                    <jats:sup>2<\/jats:sup>\n                    \u2004\u223c\u20044), whereas the flux-weighted coordinates recover the input profile to within the uncertainties (\n                    <jats:italic>\u03c7<\/jats:italic>\n                    <jats:sub>\n                      <jats:italic>\u03bd<\/jats:italic>\n                    <\/jats:sub>\n                    <jats:sup>2<\/jats:sup>\n                    \u2004\u223c\u20041) and remain robust to a PSF misspecification of \u00b110%. The method we propose provides a better-motivated and readily adoptable framework for tomographic studies, whether as the primary analysis or as a robustness check on spaxel-centre results.\n                  <\/jats:p>","DOI":"10.1051\/0004-6361\/202661590","type":"journal-article","created":{"date-parts":[[2026,8,27]],"date-time":"2026-08-27T18:45:01Z","timestamp":1787856301000},"page":"L10","source":"Crossref","is-referenced-by-count":0,"title":["Recovering the effective impact parameters in integral-field absorption-line tomography"],"prefix":"10.1051","volume":"713","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-0966-1203","authenticated-orcid":false,"given":"J. 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