Similar to Fig. 2 but for NGC 6397. The green crosses in panels a, b, and c refer to a sample of background galaxies selected from the NIRISS stacked image (see Appendix A for details). The clump of points in panel c at about (μ_α cos δ,μ_δ) ~ (10, 20) mas yr⁻¹ is caused by mismatches between residual PSF artifacts in the NIRISS data and real field stars in the HST catalog that moved by 11 pixels over 20 years.