Signal value at the GRB080319B position relative to the nearby reference star, obtained from the PSF profile fit to the data from the k2a camera of the “Pi of the Sky” prototype as a function of time before the burst t − t₀ (vertical bars). We also indicate the limits on the precursor luminosity calculated assuming no signal (thick lines) and taking the actual signal measured for each frame (thin lines). Limits are calculated at both the 3σ level (solid lines) and 95% confidence level (dashed lines).