  Recently the ATLAS experiment was complemented with a set of
ultra-small-angle detectors located in ``Roman Pot'' inserts at 240m on either
side of the interaction point, aiming at the absolute determination of the LHC
luminosity by measuring the elastic scattering rate at the Coulomb Nuclear
Interference region. Details of the proposed measurement the detector
construction and the expected performance as well as the challenges involved
are discussed here.
