An example of sky-ring subtraction tests described in the text. Panel a: one selected raw image of the CCD2 corresponding to a central wavelength of 9130 Å, with 500 artificial stars added. Arc patterns (bluer as the curvature radius grows) corresponding to equal wavelengths of the airglow band spectrum are noticeable. Panels b, c, and d: sky-ring subtracted and defringed images by using, respectively, the ringsub algorithm (adopted in this work), tringSub, and the azimuthally averaged approach. All images are scaled to the median. Below each panel is shown the mean radial spectrum (1 pixel sampling) of the original image background or residual, as appropriate.