Activation maximisation maps corresponding to three different mock datasets with different S/N for the four different classes of nebulae. For ease of visualisation, we show in colour the spectral channels where the absolute value of the activation is larger than 15% of the maximum value of the activation spectrum. The key emission lines are labelled. Panel a: activation map for the network trained with mock spectra with infinite S/N. Panel b: The high S/N mock spectra case. Panel c: the low S/N spectra case.