  We describe the construction of an X-ray flux-limited sample of galaxy
clusters, the REFLEX survey catalogue, to be used for cosmological studies. The
survey is based on the ROSAT All-Sky Survey and on extensive follow-up
observations conducted in the frame of an ESO key program. In the paper we
document the process by which the primary cluster candidate catalogue is
constructed prior to the optical follow-up observations. We describe the
reanalysis of the RASS source catalogue which enables us to impose a proper
flux limit cut to the X-ray source list without introducing a severe bias
against extended sources. We discuss the correlation of the X-ray and optical
(COSMOS) data to find galaxy density enhancements at the RASS X-ray source
positions and the further evaluation of the nature of these cluster candidates.
Based also on the results of the follow-up observations we provide a
statistical analysis of the completeness and contamination of the final cluster
sample and show results on the cluster number counts. The final sample of
identified X-ray clusters reaches a flux limit of 3e-12 erg s-1 cm-2 in the 0.1
- 2.4 keV band and comprises 452 clusters. The results imply a completeness of
the REFLEX cluster sample well in excess of 90% and a contamination by
non-cluster X-ray sources of less than 9%, an accuracy sufficient for the use
of this cluster sample for cosmological tests.
