  In spite of major advances in both ground- and space-based instrumentation,
the ultra-low-surface brightness universe (ULSB) still remains a largely
unexplored volume in observational parameter space. ULSB observations provide
unique constraints on a wide variety of objects, from the Zodiacal light all
the way to the optical cosmological background radiation, through dust cirri,
mass loss shells in giant stars, LSB galaxies and the intracluster light. These
surface brightness levels (>28-29 mag arcsec^-2) are observed by maximising the
efficiency of the surveys and minimising or removing the systematics arising in
the measurement of surface brightness. Based on full-system photon Monte Carlo
simulations, we present here the performance of a ground-based telescope aimed
at carrying out ULSB observations, with a curved focal surface design. Its
off-axis optical design maximises the field of view while minimising the focal
ratio. No lenses are used, as their multiple internal scatterings increase the
wings of the point spread function (PSF), and the usual requirement of a flat
focal plane is relaxed through the use of curved CCD detectors. The telescope
has only one unavoidable single refractive surface, the cryostat window, and
yet it delivers a PSF with ultra-compact wings, which allows the detection, for
a given exposure time, of surface brightness levels nearly three orders of
magnitude fainter than any other current telescope.
