Background derivation by median filtering. Shown are the images ℐ_λ of a simulated star-forming region (upper row), median-filtered background (middle row), and background-subtracted images (bottom row) at selected Herschel wavelengths (after M = 30 iterations). The maximum source sizes X_λ of 25, 100, and 100′′ for the getimages method were estimated directly from ℐ_λ (cf. Sect. 3.2). In panel a, small holes are starless cores seen in absorption at 70 μm, and white emission peaks are protostellar sources. Intensities (in MJy/sr) are limited in range, and their color scaling is linear.