standard SDSS images have a relatively short exposure time of only 54 s per filter, the drift-scan mode in which the imaging survey was conducted (Gunn et al. 1998) ensures very accurate flat-fielding. Moreover, for most galaxies the field of view is quite large. Both factors are crucial for obtaining an accurate measurement of the sky background and its associated error, which determines the limiting surface brightness sensitivity. For most galaxies the azimuthally averaged surface brightness profile can be measured reliably down to $\mu_r \approx 27$ magarcsec$^{-2}$, which is deep enough to study the outer structure of galaxies (Pohlen & Trujillo 2006; Erwin et al. 2008; Li et al. 2011).

We chose to work with the $r$ band, which is closest to the [*HST*]{} F606W filter. We first mask all photometric objects in the field identified either by Sextractor (Bertin & Arnouts 1996) or by the SDSS photometric pipeline and then run the IRAF[^2] task [ellipse]{} to fit isophotes with a linear step of 2 pixels between successive steps, allowing the center, position angle, and ellipticity of each ellipse to vary. The sky background level is determined from the best-fit ellipses that have constant surface brightness with respect to radius, 