Böhringer, H., Chon, G., Retzlaff, J., Trümper, J., Meisenheimer, K., & Schartel, N. (2017). The extended Northern ROSAT Galaxy Cluster Survey (NORAS II) - I. Survey construction and first results. The Astronomical Journal, 153(5): 220, pp. 1-11. doi:10.3847/1538-3881/aa67ed.