The Northern ROSAT All-Sky (NORAS) Galaxy Cluster Survey I: X-ray   Properties of Clusters Detected as Extended X-ray Sources
  In the construction of an X-ray selected sample of galaxy clusters for cosmological studies, we have assembled a sample of 495 X-ray sources found to show extended X-ray emission in the first processing of the ROSAT All-Sky Survey. The sample covers the celestial region with declination $\delta \ge 0\deg $ and galactic latitude $|b_{II}| \ge 20\deg $ and comprises sources with a count rate $\ge 0.06$ counts s$^{-1}$ and a source extent likelihood of 7. In an optical follow-up identification program we find 378 (76%) of these sources to be clusters of galaxies. ... 
H. Boehringer, W. Voges, J.P. Huchra, B. McLean, R. Giacconi, P.   Rosati, R. Burg, J. Mader, P. Schuecker, D. Simic, S. Kom...