Voronoi diagram of E(B–V) based on the extended nebular Hα/Hβ ratio, binned to a signal-to-noise ratio of 200 per polygon. The color coding is linear between E(B–V) = 0 mag (white) and E(B–V) = 1.8 mag (black). The extinction is highest close to MWC 137 and along the rim of the nebula, tracing the dust. It decreases toward the outskirts of the cluster. The white spots are stars and MWC 137 is saturated. This diagram serves as illustration. For absolute extinction values, individual spectra were examined (Sect. 3.2).
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