Weighing 40 X-ray Luminous Clusters of Galaxies with Weak Gravitational   Lensing
  Deep two-color imaging of 40 clusters of galaxies with the NOT and the University of Hawaii 2.24-m telescope is used to measure the weak gravitational shear acting on faint background galaxies. From this, maps of the projected cluster mass distribution are constructed, and the cluster masses are measured within circular apertures of up to ~3 Mpc/h. The results are used to derive the cluster mass function at z~0.2. The average mass-to-light ratio of the clusters indicate a low-density Universe with Omega_0 around 0.25. Additional mass concentrations are found along the lines of sight to the clusters, some of which are most likely associated with clusters at z > 0.5. In addition, the data are used to measure weak gravitational lensing due to large-scale structures along the line of sight. The future prospects for this kind of research with the NOT are briefly discussed. 
H. Dahle