Spectral index maps between different frequencies. Top-left panel: (LOFAR/LBA – LOFAR/HBA) spectral index map. Shown are LBA contours at levels of [−5, 5, 9, 12, 15]σ and only pixels with values above 5σ in both maps are taken into account. LBA: σ = 1.32 mJy PSF⁻¹, HBA: σ = 0.13 mJy PSF⁻¹. Assumed flux scale accuracy is 20%. PSF size: 15″. Top-right panel: (LOFAR/HBA – APERTIF) spectral index map for pixels with values above 3σ. Shown are HBA contours at [−5, 5, 9, 12, 15]σ. HBA: σ = 0.6 mJy PSF⁻¹, WSRT: σ = 0.1 mJy PSF⁻¹. Assumed flux scale accuracy: HBA – 20%, WSRT – 5%. PSF: 33″. Bottom-left panel: (LOFAR/HBA – GMRT) spectral index map. Shown are HBA contours at [−5, 5, 9, 12, 15, 27, 36.]σ. for pixels with values above 3 σ in both maps. Bottom-right panel: (GMRT) spectral index map for pixels above 3σ. Shown are GMRT 385 MHz contours at [−5, 5, 9, 12, 15]σ. HBA: σ = 64.5 μJy PSF⁻¹, GMRT: σ = 26.7 μJy PSF⁻¹and 15 μJy PSF⁻¹. Assumed flux scale accuracy: HBA – 20%, GMRT – 5%. PSF size: 9″. The corresponding error maps are shown in Fig. B.1.