(Left) A deep [Fe II]+[Si I] 1.64 μm image of Cas A (B.-C. Koo et al. 2018). QSFs with optical counterparts are highlighted with magenta contours. The filled red star symbol indicates the explosion center at (α, δ)_J2000 = (23^h23^m27.77^s, +58^◦48′49.4) (J. R. Thorstensen et al. 2001), while the yellow contour marks the outer boundary of the SNR in radio in September 2000 (T. A. DeLaney 2004). North is up, and east is to the left. The white boxes represent 11 fields used for measuring proper motion of QSFs (see Section 2). (Right) An Hα image of Cas A, taken with GMOS on the Gemini North telescope in 2019. The white contours show the location of the main ejecta shell in the deep [Fe II]+[Si I] image.