  We present a new technique to quantify the light contribution coming from the
faint high redshift ($z\sim6$) galaxies below the detection threshold of
imaging data, set conventionally at S/N=4.5. We illustrate the technique with
an application to Hubble Space Telescope Advanced Camera for Surveys images in
the F775W and F850LP filters of the Ultra Deep Field parallel field NICP12. The
aim of this analysis is to extend by a few magnitudes the faint end of the
luminosity function at $z\sim6$. After masking all the detected sources in the
field we apply a Fast Fourier Transform to obtain the spatial power spectrum of
the background signal. The power spectrum permits us to separate the background
noise signal, the residuals due to the data reduction of the wide field, and
the overall signal produced by faint galaxies. The ratio of the signal in the
i_775 and z_850 bands is used to estimate the contribution of the faint
i-dropout objects. We rely on extensive Monte Carlo simulations to characterize
various sources of uncertainty and quantify the number of faint i-dropout
galaxies in the field. The analysis allows us to put constraints on the
luminosity function at $z\sim6$ down to z_850= 30 mag, 2.5 mag fainter than
with standard techniques on the same data. The data are consistent with a faint
end slope of the luminosity function of $\alpha = -1.9$. Assuming a specific
set of values for the clumping factor, escape fraction, and spectral energy
distribution, we find that the $z\sim6$ undetected galaxies down to z_850=30
mag could have driven cosmic reionization.
