is only 6% of the sky brightness ($\mu_{B,{\rm sky}}=23$ mag arcsec$^{-2}$), so that we had to be particularly careful in measuring the sky brightness. The sky background in NIR passbands is much higher than in the optical ($\mu_{K,{\rm sky}}= 13$ mag arcsec$^{-2}$) and can change significantly on temporal and spatial scales the order of two minutes and two arcmin, respectively. It was therefore necessary to obtain as many sky frames as object frames to perform sky subtraction with sufficiently high accuracy. We took sky and object frames alternately, using the same exposure time for each individual frame (60 s) and employing interframe shifts of $\sim2$ arcmin. An additional reason we needed to obtain separate sky frames is that the IR arrays used were not large enough to properly sample the sky on the science frames. Incorrect background subtraction can cause the sky level to be either underestimated (so that sky flux will contribute to the surface brightness at large galactic radii) or overestimated (resulting in a fictitious radial cut-off). Figure \[mag\] illustrates the importance of careful background subtraction in the $K_{\rm{s}}$ band, where the sky contribution is greater than in optical passbands. The figure shows the major- and the minor-axis 