Light curve correction steps demonstrated on RR Lyrae star V20, which is close to the core and hence strongly affected by systematics. Top: Raw differential-image photometry (light gray) and after we applied the K2SC method (dark gray). The red and blue lines are the initial 20th degree polynomial and the final, optimized with through PDM. Middle: Corrected and detrended light curve, in terms of the relative flux. Bottom: Frequency spectra of the light curves at each step.