Upper panel: rms of the smoothed granulation RV time series (red curve) versus scale. The rms RV of the corresponding residuals are in green. The horizontal dot-dashed line indicates a typical level of the noise induced by future instruments. Middle panel: rms RV of the granulation versus observing time during one night for a different number of observing periods n during each night (1: solid line; 2: dotted line; 3: dashed line; 4: dot-dashed line; 5: dot-dot-dot-dashed line). The exposure time is the observing time divided by n. Lower panel: same for the rms RV of the granulation versus the number of nights over which the signal is averaged, in the case of a 26-min observing time per night.