Left panel: 1.5 GHz VLA image of A2390 with a beam of 15′′ × 15′′ and an rms noise of 26 μJy beam⁻¹. The first 3σ contour is shown in white. Right panel: spectral index map between the 1.5 GHz VLA and 144 MHz LOFAR images of A2390. The contour levels, overlaid in grey, are from the LOFAR high-resolution image in Fig. 12 with (4, 8, 12, 24)  ×  σ where σ = 400 μJy beam⁻¹. Pixels below 3σ are blanked. The error map is shown in Fig. A.4.