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As the largest, clearly defined building blocks of our Universe, galaxyclusters are interesting astrophysical laboratories and important probes forcosmology. X-ray surveys for galaxy clusters provide one of the best ways tocharacterise the population of galaxy clusters. We provide a description of theconstruction of the NORAS II galaxy cluster survey based on X-ray data from thenorthern part of the ROSAT All-Sky Survey. NORAS II extends the NORAS surveydown to a flux limit of 1.8 x 10^(-12) erg s^-1 cm^-2 (0.1 - 2.4 keV)increasing the sample size by about a factor of two. The NORAS II clustersurvey now reaches the same quality and depth of its counterpart, the SouthernREFLEX II survey, allowing us to combine the two complementary surveys. Thepaper provides information on the determination of the cluster X-rayparameters, the identification process of the X-ray sources, the statistics ofthe survey, and the construction of the survey selection function, which weprovide in numerical format. Currently NORAS II contains 860 clusters with amedian redshift of z = 0.102. We provide a number of statistical functionsincluding the logN-logS and the X-ray luminosity function and compare these tothe results from the complementary REFLEX II survey. Using the NORAS II sampleto constrain the cosmological parameters, sigma_8 and Omega_m, yields resultsperfectly consistent with those of REFLEX II. Overall, the results show thatthe two hemisphere samples, NORAS II and REFLEX II, can be combined withoutproblems to an all-sky sample, just excluding the Zone-of-Avoidance.

The extended Northern ROSAT Galaxy Cluster Survey (NORAS II) I. Survey Construction and First Results