Sky background test. Results of the King (panel a); tidal radius, r_t, vs. core radius, r_c) and EFF (b); power-law index, n, vs. scale radius, r_e) model fitting for artificial clusters placed in the real M31 Suprime-Cam CCD mosaic from Narbutis et al. (2008). Black dots indicate initial parameters. For each node series of 100 frames were simulated. Three concentric 1σ ellipses, representing the derived parameter distributions around each node, correspond to the artificial clusters of V = 17.5, 18.5, and 19.5 mag. Arrows show systematic biases of derived parameter distributions, when stars and variable sky background are not taken into account in model fitting for clusters of V = 17.5 (red/shorter) and 18.5 mag (blue/longer).