present in the field for straightforward PSF determination. For most sources, the core of the PSF was derived from faint field stars, while the wing was extrapolated using a suitable Moffat (1969) function obtained from fitting bright stars in other frames of similar seeing during the same night. We stress that this extrapolation does not affect the fit for the low redshift, often host–dominated BL Lacs in our sample. The luminosity profiles were fitted into point source and galaxy components by an iterative least-squares fit to the observed profile. For low redshift objects, we attempted both elliptical (de Vaucouleurs law) and exponential disc models to represent the galaxy. However, consistently with results from optical images, in no case did the disc model give a better fit than the elliptical one for the observed sources. For higher redshift objects, data quality does not allow one to discriminate between the morphologies and for them the elliptical model was assumed. We estimate the uncertainty of the derived host galaxy magnitudes to be $\sim$$\pm$0.3 mag.

Results and discussion
======================

In Fig. 1 we show the H band contour plots of all the BL Lacs, after smoothing the images with a Gaussian filter of $\sigma$ 