Second Workshop on Numerical and Observational Astrophysics From the First Structures to the Universe Today, 2011 M. E. De Rossi, S. E. Pedrosa and L. J. Pellizza, eds. Differential CCD photometry of globular clusters R. Figuera Jaimes1, A. Arellano Ferro1, D. M. Bramich2 and S. Giridhar3 1Instituto de Astronomía, UNAM, Mexico. 2ESO, Garching bei München, Germany. 3IIA, Bangalore, India. Since 2003 we have been carrying a program of time series CCD photometry of globular clusters with the 2.0 m telescope in Hanle, in the indian Himalayas at 4500 m above sea level. The analysis is made via Differential Imaging Ap- proach (DIA). The goal has been to estimate physical parameters of stars from specific empirical calibrations. With the aim of highlighting the usefulness of the program, we summarize some of the major results encountered for numerous globular clusters and point to representative publications by our team. The DIA approach allows precise measurements of star fluxes even in highly crowded central regions of a globular cluster, and allows to fully complete the variable star census of a given cluster. This has lead to the discovery of 22 RR Lyrae, 18 semi-regular red giants, 1 anomalous cepheid, 1 eclipsing binary and 27 SX Phe in the clusters M53, M72, NGC 5053, NGC 6366, NGC 5466, M2, and NGC 4147 (e.g., Arellano Ferro et al. 2011, and references therein). The program also allowed to produce good colour-magnitude and colour-colour diagrams to estimate the reddening and age of the clusters. The Fourier decomposition of the light curves of RR Lyrae stars allows to obtain good estimates of the mean value of the cluster [Fe/H], the mean distance and the structure of the horizontal branch. The accuracy of the photometry produces high quality light curves, which has allowed the discovery of numerous RR Lyrae stars with the Blazhko effect, in particular the largest population known of 22 Blazhko RR1 stars in a given cluster (M53; Arellano Ferro et al. 2012). These results suggest that stellar evolution in the horizontal branch takes place towards the red and that the Blazhko modulations may be associated with a change of pulsating mode. The numerous SX Phe stars have lead to an empirical calculation of the P-L relation, and a general discussion of these short-period pulsators as distance indicators (Arellano Ferro et al. 2012). A new statistical index was given that helps the detection of stars with amplitude and phase modulations. The program continues on numerous clusters in both hemispheres, and expects to significantly extend our knowledge of the variable stars in globular clusters. References Arellano Ferro, A., Bramich, D. M., Figuera Jaimes, et al. 2012, MNRAS, 420, 1333 Arellano Ferro, A., Figuera Jaimes, R., Giridhar, S., et al. 2011, MNRAS, 416, 2265 144.