Azimuthally averaged power spectrum of both fluctuation contributions: the stellar population luminosity variability ($PS{({\mathrm{Gal}}_{\mathrm{fluc}}/\sqrt{{\mathrm{Gal}}_{\mathrm{mean}}\otimes \mathrm{PSF}})}_{\mathrm{r}}$) is in black and the instrumental noise ($PS{(R_{\mathrm{approx}}/\sqrt{{\mathrm{Gal}}_{\mathrm{mean}}\otimes \mathrm{PSF}})}_{\mathrm{r}}$) is in grey. In this case we consider a mask, introduced as in Eq. (21), with the same data used in Fig. 4. Note that the input SBF value (${\overline{N}}_{\mathrm{ref}}=22.59$ counts) coincides with the average value of the fluctuation term.