Data cleaning and explosion epoch estimation example for SN 2021zby. The panels, labelled as a, b, c, and d from top to bottom, present different stages of the analysis (we detail a and b in Section 2 and c and d in Section 3). Red and grey dots represent photometry in the o and c bands, respectively. Panel (a) shows the raw ATLAS forced photometry. Panel (b) presents the photometry after applying the ATClean methodology detailed in Section 2. Panel (c) shows the pre-SN photometry, where the red region highlights the relevant data, and the sky blue and brown dashed lines represent the 3 and 1.5 σ_{pre − SN} values, respectively. In Panel (d), green and black star symbols represent the last non-detection and discovery, which we use to estimate the explosion time.