of stars in the overlaping region. For each final mosaic an astrometric solution was found using stellar positions derived from the Guide Star Catalog 2.0. The absolute coordinate accuracy for each mosaic is better than 0.4 arcsec, although the typical relative uncertainty in the registration between images has been reduced to less than 0.1 pixels.

The surface brightness calibration process was  performed using narrow-band images of Pis24 obtained with the Advanced Camera for Surveys (ACS) and the Wide Field and Planetary Camera 2 (WFPC2) on board the Hubble Space Telescope (HST). The datasets correspond to those obtained by Observing Programs No 9857 (PI: O. De  Marco) and No 9091 (PI: J. Hester) for ACS and WFPC2 instruments, repectively. The ACS/WFC filter F658N ($\lambda_c \sim6584$, FWHM$\sim$73Å) has similar characteristics as the  filter used in the Curtis-Schmidt camera. The WFPC2 filters F673N ($\lambda_c\sim6732$, FWHM$\sim$65Å) and F502N ($\lambda_c\sim5012$, FWHM$\sim$37Å) have also similar characteristics as the  and filters used in the Curtis-Schmidt Camera. The relative surface brightness zero points were obtained using two areas of about 30 arcsec in common with ACS and WFPC2 images. Uncertainties in the relative calibrations for the Curtis-Schmidt filters are below 20%. This figure is derived from the comparison 