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We analyse three clusters of galaxies, Abell 901 (z=0.17), Abell 1437(z=0.13) and Abell 3570 (z=0.037). They have low to intermediate X-ray fluxesand an irregular morphology in the ROSAT All-Sky Survey (RASS). These clustersare chosen to test the abilities and limitations of the RASS in terms ofcluster fluxes and cluster morphologies. Therefore some ``worst'' cases areused here. X-ray observations with the ROSAT/HRI and optical spectroscopicobservations are carried out. The ROSAT/HRI observations, which have a muchbetter spatial resolution than the RASS, reveal in two of the three cases asignificantly different morphology than seen in the RASS. The reasons are pointsources which could not be resolved in the RASS and were therefore confusedwith the cluster emission. For A3570 we could confirm the relaxed state of thecluster by the optical determination of a small velocity dispersion. In thecluster with the lowest flux (Abell 901) the countrate measurement is stronglyaffected by point sources, in the two other cases the countrate measurements ofthe RASS are reliable, i.e. they are reproduced by the ROSAT/HRI measurement.We conclude that for clusters with a flux of a few times 10^{-12} erg/cm^2/s orsmaller, which show at the same time a non-relaxed morphology, the fluxmeasurement of the RASS can be seriously affected by fore- or backgroundsources. We point out that an all-sky survey of a second ABRIXAS mission wouldprovide a much clearer source distinction for low-flux clusters and thus a muchimproved countrate determination.Comment: 8 pages (incl. 6 figures), accepted for publication in A&A (Suppl.

X-ray and optical observations of three clusters of galaxies: Abell 901, Abell 1437, and Abell 3570