Comparison of residuals. Blue and orange lines represent the distribution of residuals for the FASTAR and MILES interpolators when predicting the stellar spectra of the MILES library. The behavior of both interpolation schemes is similar, as shown by the vertical dashed lines marking the 20^th and 80^th percentiles. However, the 2^nd and 98^th percentile vertical lines indicate that extreme errors are less common in FASTAR. For comparison, the gray histogram shows the error associated to the PCA compression.