r (orange) and i (red) absolute magnitude light curves of the SN 2011fe model used in the simulations in the ZTF bands as a function of phase from rest-frame B-band peak (Mazzali et al. 2014). The bumpiness in the models is because the underlying SNCOSMO model class interpolates in flux space but fails to find an exponential decay. The added rest-frame CSM interaction model based on Hα emission (starting at a phase of 100 d) is shown with dotted lines for the r-band. Once the interaction becomes the dominant source, it smooths out the bumps from the underlying tail. The black line shows a radioactive decay with t_1/2 = 50 d, typical of a declining normal SN Ia tail.