CSPNe validation. Top: extinction–distance profile of NGC 2899. The sources, reference distances, and lines follow the colour coding in Figure 2. The two published CSPNe are shown in grey with error bars. Bottom: discordant CSPNe. The black line represents the fitted R_PN–distance relation with the shaded area marking the ±1σ range. All PNe from Table 4 are included. The filled symbols indicate the adopted CSPN, and the open symbols indicate other published CSPNe. NGC 2899 is shown in grey circles, whereas CSPNe in other PNe are shown as coloured triangles.