Inferred flicker indexes as a function of the number of subseries (L) used to compute the averaged periodogram in Eq. (3) based on VIRGO solar data. Left column: influence of the time sampling, Δt = 1, 5, 10 min (from top to bottom). Middle column: influence of the duration of the subseries, T = 5 day, 1 day, 8-hours (from top to bottom). Right column: influence of the high-frequency noise level, σ_W < 30 ppm (VIRGO data, top), σ_W = 82 ppm (level measured on CHEOPS HD 67458 periodogram), σ_W = 92 ppm (approximately the level measured on TESS HD 67458 periodogram). The red horizontal line indicates the flicker index value for large L (α_ɡ,∞).