The observations that are needed in order to build the model are the
number density, angular size and surface brightness distribution of the
galaxies observed on the sky.
Observations in the 
or 
band are preferable because the very broad 
band, in which galaxies will be detected, is intermediate between these two
bands, and the broad bands that will be implemented in the BBP, and in which
galaxy observations will be carried out, will approximately cover this
wavelength range.
Recent literature, and more specifically the Medium Deep Survey (MDS,
[Ratnatunga et al. 1999]) and Hubble Deep Field North (HDF-N, [Williams et al. 1996])
databases, offer a substantial amount of data in both bands, extending
to large sky regions and faint magnitudes. The model's results, however, are
here expressed in the band, since mostly -band data were used.
A magnitude limit of 
mag, much fainter than it was in principle
necessary for our purposes, was chosen, so as to provide an estimation of the
average disturbance from galaxies to the observation of stars.
It turns out that, to a first approximation, the galaxy statistical properties
that are relevant for our purposes can be conveniently expressed as function
of two parameters only, namely the galaxy total magnitude and morphological
type.
It must be emphasized how, under our assumptions, these two parameters
completely characterize the photometric properties of a galaxy.
The predictions of our model are thus much different in nature from the results
of conventional galaxy surface photometry analysis.
While in the latter a set of parameters is fitted to galaxy images in order to
obtain information about galactic structural properties, in our model the
results of this analysis are combined to derive analytical expressions
predicting the photometric properties of typical galaxies.
While this model obviously cannot do justice to the strong individuality
displayed by many galaxies, it is believed to yield sufficiently reliable
results when, as in our case, only statistical properties, i.e. properties
averaged over large samples, are of interest.