Semi-resolved index–index diagrams. Black lines correspond to the fully sampled H_βO–Mgb FASTAR predictions for different ages and metallicities, as noted by the labels. On top, green, red, and blue contours show FASTAR index–index distributions for N_stars = 10^5.5, 10^3.5, and 10^2.5, respectively, for a population with solar metallicity and an age of 10 Gyr. Colored marks indicate the median of the distributions.