Details of the [O iii] emission of Ou4 near the ridge of mid-infrared emission. The bright star is HD 239597. The colour scale of the [O iii] image is linear. The contour map is the 22 μm emission from the W4-filter image from 89.375 to 92.5 digital numbers (DN) with linear steps of 0.223 DN (i.e., from 2.471 to 2.557 mJy/arcsec² with linear steps of 0.006 mJy/arcsec²). The linear scale and the angular resolution of the W4-filter image (12″-FWHM) are shown in the bottom-right corner.