Spectral index maps of the halo in MACS J0717.5+3745 at 8″ resolution. Top: low-frequency spectral index, between 144 and 1500 MHz, with the corresponding spectral index uncertainty. Bottom: high-frequency spectral index between 1.5 and 5.5 GHz, with the corresponding spectral index uncertainty. Contour levels are drawn at [1, 2, 4, 8, …] × 4σ_rms and are from the VLA L-band image. The errors are based on the individual rms noise values in the maps and an absolute flux calibration uncertainty of 2.5%, 4%, and 10% for the S/C-band, L-band and LOFAR (144 MHz), respectively. The beam size is indicated in the bottom left corner of each image. It is evident that the spectral index variations across the halo are different at low and high frequencies.