Example of a selection of stars near the edge of our considered area, containing only about 22 000 stars. The sample contains stars with 8 < R/kpc < 12 and 150° < ϕ < 155°. Upper left: phase plane showing strong extinction by dust. Upper right: background produced by the model. Lower left: spiral perturbation produced by the model (this panel does not share the colour bar with the rest). Lower right: best fit. Bottom: residuals computed as Data – Best fit. We can see that even without a clear spiral pattern in the data, the model still produces a convincing spiral and fit.