Standard deviations of the detrended Spitzer photometry binned per 30 min (open circles) and the corresponding formal errors (triangles) as a function of the K-magnitude of the targets. Blue = 3.6 μm, red = 4.5 μm. The dashed red line shows the best-fit linear relationship between the standard deviations measured at 4.5 μm and the K-magnitudes, its equation being σ_{30 min} = 32.5 + 11.97 × (K_mag−5) ppm.