Investigation of the angular expansion of HuBi 1. (left) Stellar-continuum subtracted [N II] spatial profiles of the inner region of HuBi 1 along PAs 0° (top left), 45° (top right), 90° (bottom left), and 135° (bottom right) extracted from the 2008 (red line) and 2020 (black line) NOT images. Their differences are shown by a dark blue line and those of the best-fit expansion by a cyan line. The location of the inner region is marked by vertical black dotted lines and the zero level by a horizontal red dashed line. (middle) Normalized residuals between the spatial profiles and polar images of the inner region of HuBi 1 (diamonds) and least-squares best fits (solid lines). (right) Stack of the [N II] 2008 and 2020 polar image differences of the inner region of HuBi 1. The angular offset applied to the 2020 polar images is overlaid on the corresponding polar image difference.