Schirmer M.Schirmer M.Okumura K.Venemans B.Jahnke K.2026-06-012026-06-012026Paterson, K., Schirmer, M., Okumura, K., Venemans, B., Jahnke, K., Aghanim, N., Altieri, B., Amara, A., Andreon, S., Baccigalupi, C., Baldi, M., Balestra, A., Bardelli, S., Battaglia, P., Biviano, A., Bonchi, A., Branchini, E., Brescia, M., Brinchmann, J. and Camera, S. (2026). Euclid. Astronomy & Astrophysics, 707, p.A226. https://doi.org/10.1051/0004-6361/202555106. ‌https://doi.org/10.1051/0004-6361/202555106https://hdl.handle.net/10566/22961The Near-Infrared Spectrometer and Photometer (NISP) on board Euclid includes several optical elements in its path that introduce artefacts into the data from non-nominal light paths. To ensure uncontaminated source photometry, these artefacts must be accurately accounted for. This paper focuses on two specific optical features in NISP's photometric data (NISP-P): ghosts caused by the telescope's dichroic beamsplitter, and the bandpass filters within the NISP fore-optics. Both ghost types exhibit a characteristic morphology and are offset from the originating stars. The offsets are well modelled using 2D polynomials; only stars brighter than approximately 10 magnitudes in each filter produce significant ghost contributions. The masking radii for these ghosts depend on both the source-star brightness and the filter wavelength, ranging from 20 to 40 pixels. We present the final relations and models used in the near-infrared (NIR) processing function (PF) to mask these ghosts for Euclid's Quick Data Release (Q1).enBandpass FiltersInfrared SpectrometersSource-StarPhotometric DataDichroic BeamsplitterArticle