  High contrast imaging at optical wavelengths is limited by the modest
correction of conventional near-IR optimized AO systems.We take advantage of
new fast and low-readout-noise detectors to explore the potential of fast
imaging coupled to post-processing techniques to detect faint companions to
stars at small separations. We have focused on I-band direct imaging of the
previously detected brown dwarf binary HD130948BC,attempting to spatially
resolve the L2+L2 benchmark system. We used the Lucky-Imaging instrument
FastCam at the 2.5-m Nordic Telescope to obtain quasi diffraction-limited
images of HD130948 with ~0.1" resolution.In order to improve the detectability
of the faint binary in the vicinity of a bright (I=5.19 \pm 0.03) solar-type
star,we implemented a post-processing technique based on wavelet transform
filtering of the image which allows us to strongly enhance the presence of
point-like sources in regions where the primary halo dominates. We detect for
the first time the BD binary HD130948BC in the optical band I with a SNR~9 at
2.561"\pm 0.007" (46.5 AU) from HD130948A and confirm in two independent
dataset that the object is real,as opposed to time-varying residual speckles.We
do not resolve the binary, which can be explained by astrometric results
posterior to our observations that predict a separation below the NOT
resolution.We reach at this distance a contrast of dI = 11.30 \pm 0.11, and
estimate a combined magnitude for this binary to I = 16.49 \pm 0.11 and a I-J
colour 3.29 \pm 0.13. At 1", we reach a detectability 10.5 mag fainter than the
primary after image post-processing. We obtain on-sky validation of a technique
based on speckle imaging and wavelet-transform processing,which improves the
high contrast capabilities of speckle imaging.The I-J colour measured for the
BD companion is slightly bluer, but still consistent with what typically found
for L2 dwarfs(~3.4-3.6).
