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   PR # 21147
    To obtain a detailed account of the cluster population in the Univers
    e is  one of the prime interests in observational cosmology.  Of part
    icular interest in this respect is the study of most massive, galaxy-
    rich clusters. This proposal is to observe a sample of distant, very 
    X-ray luminous clusters detected in the ROSAT All Sky Survey for     
    which optical identification and z-determination are completed. The  
    ASCA observation of these clusters will  allow determination of the  
    characteristics, in particular total mass, of these very X-ray lumino
    us clusters for the first time. Mass determination of these distant, 
    massive clusters is also important with respect to the gravitational 
    lensing of the background galaxies.                                  
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