Time series of the MOPTOP instrumental zero points (q_inst, u_inst) for the (a) MOP-L and (b) MOP-I filters. The unpolarised stars used for the determination of these zero points are GD 319, BD +32 3739, and HD 14069. The grey lines correspond to the mean instrumental offset and the shaded grey regions to its standard deviation. The S/N panel corresponds to the target average signal-to-noise ratio.