Spectral index maps of 4C43.15 obtained from BRATS and considering only pixels with values > 5σ_RMS. Left: 0.9″ resolution map made from the LBA, HBA, and VLA data. The same additional signal-to-noise constraint as when fitting the ageing model was used. Right: 0.38″ pixel-by-pixel map made from the HBA and VLA data. No additional signal-to-noise constraint was used in BRATS. The grey contours are drawn based on the 4.71 GHz map, starting at 0.29 and 0.38 mJy beam⁻¹ for the left and right images, respectively. Each contour represents a factor of two increase in intensity.