with the well-established technique of simple PSF subtraction.

In each case, the PSF was scaled to the central flux of the AGN, integrated inside a circular aperture of 4 pixel (012) diameter centered on the nucleus. This radius encircles approximately 34% of the total energy of a point source. A smaller radius (e.g. 1 pixel) would become too sensitive to shot noise and PSF mismatch, while larger radii contain a significantly higher fraction of the total flux (50%/60%/70% at 3/4/5 pixel radius) and thus would make a detection of any host galaxy component successively harder.

This procedure somewhat oversubtracts the nuclear component by an amount corresponding to the underlying host galaxy contribution inside the encircled region. However, this method yields strictly conservative estimates the host galaxy flux, i.e. always unserestimating it. We used extensive simulations to determine correction factors for this oversubtraction (see Section \[sec:errors\]). The peak-scaled PSF subtraction method has the advantage of being independent of any assumption about the host galaxy morphology.

Two-dimensional deblending {#sec:galfit}
--------------------------

As a second method we employed the modelling package [[galfit]{}]{} in Version 1.7a [@peng02] that allows the simultaneous fitting of several two-dimensional components to an image, convolved with a given PSF. We describe our application of [[galfit]{}]{} to quasar images in @sanc04a in detail.

In total we ran [[galfit]{}]{} in three configurations, always fitting two components, which were the point-source nucleus and either an exponential disk [@free70], a $r^{1/4}$ de Vaucouleurs spheroid [@deva48], or a @serc68 model with the Sérsic index as a free parameter. However, simulations (Sect. \[sec:errors\]) showed that with the present data, the [[galfit]{}]{} version used[^1] was operating near its limit, due to the very high contrast between nuclei and host galaxies. In the context of this paper we thus used [[galfit]{}]{} only as a cross-check on t