Full details about the observing strategy and data reduction can be found in Ripepi et al. (2014). Here we briefly recall the main facts.

• Data pre-reduction (corrections for the instrumental signature, including e.g. bias, flat-field, image illumination concentration, fringing) and astrometry have been carried out with the VSTTube pipeline (Grado et al. 2012).

• PSF photometry was obtained with Stetson's DAOPHOT/ALLSTAR packages.

• Photometric calibration is based on the "The AAVSO Photometric All-Sky Survey" (APASS). The average photometric accuracy of the STEP survey is of the order of 0.02 mag in both g and i. Deviations from these values are possible at the border of each tiles (and consequently in the overlapping regions) due to the non-perfect correction for image illumination concentration and PSF variation.