(vertical, $z$) surface brightness profiles of four galaxies, binned radially. The effects of oversubtraction can be seen clearly in the minor-axis surface brightness profiles: they show artificial cut-offs, and the negative background values result in undefined surface brightnesses at these $z$ heights.

![image](14506fg1.ps){width="17cm"}

We used a special software package for our NIR data reduction, customised to reduce and analyse very deep data of extended objects observed in the IR, as a set of <span style="font-variant:small-caps;">iraf</span>[^2] tasks. The program removes spurious stars in sky frames, subtracts cleaned, scaled sky frames from object frames taken immediately prior to or after the relevant object frame, and flatfields the differences. It uses a median sky frame to detect and remove stars in the sky frames.

We observed several standard stars from the SAAO/ESO/ISO Faint Standard Stars Catalogue (Carter & Meadows 1995) to calibrate our targets and used the Wainscoat & Cowie (1992) correction to convert $K_{\rm s}$ to $K$ magnitudes. These stars were observed at least three times per night, at different airmasses, to calculate the prevailing atmospheric extinction with sufficient accuracy. We observed a total of 21 standard stars. These observations reached a surface brightness of approximately 23 mag arcsec$^{-2}$ in $K_{\rm
s}$.

