  We propose a new, simple, dedicated X-ray mission to measure the power
spectrum of density fluctuations in the Universe, by accurately mapping the
X-ray background on the whole sky on scales of arounf one sq deg. Since the
method relies on the detection of excess fluctuations produced by the
clustering of X-ray sources at intermediate redshifts above the confusion noise
produced by bright foreground sources, counting noise and systematic noise need
to be kept to a minimum. We propose a proportional counter with collimated
fields of view of various sizes as the optimal instrument for this mission.
