Limiting magnitude M (V-band equivalent) for the optical precursor emission from GRB080319B as a function of time before the burst t − t₀. Limits to the precursor luminosity were calculated assuming no signal (thick lines) and taking the actual signal measured for each frame (thin lines). Limits were calculated at the 3σ level (solid lines) and 95% confidence level (dashed lines) and are based on combined data from two cameras of the “Pi of the Sky” prototype. The points with error bars represent the GRB080319B “Pi of the Sky” flux (Racusin et al. 2008).