UVB flat field performance analysis. The top panel shows a zoomed-in section of the continuum-normalized SED of the D2 flat field lamp (echelle order four). The line emission is removed with an additional median along the cross-dispersed direction (second panel). In the third panel, we show the pixel-to-pixel quantum efficiency variations (solid black curve) at the central slit position and the corresponding line emission spectrum (dashed red curve). The bottom panel shows the SED of BD+174708 without flat fielding (solid black curve), flat fielded with the continuum-normalized D2 lamp SED (dot-dashed red curve), and flat fielded with its emission-line-free counterpart (dashed blue curve). The wavelength scales of the four panels are aligned so that the positions of the emission lines are the same for images and plots.