Perseus filament (from right to left and top to bottom: NGC 1333, IC 348, L 1448, and B 5) dust opacity maps at 850 µm by Zari et al. (2016), convolved at an angular resolution of 36″. Contours are (0.056, 0.13, 0.24, 0.56, 1.01, and 1.6) × 10²¹ cm⁻², which, according to Eq. (7) from Zari et al. (2016), corresponds to visual extinctions of ~5, 7.5, 10, 15, 20, and 25 mag, respectively. Positions observed with the 30 m telescope are indicated with white triangles. Red circles represent the positions of the starless cores.