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    PR # 31226                                                                   
     We propose to investigate the origin of the rapid luminosity evolutio       
     n of clusters of galaxies through observations of very bright cluster       
     s at redshift around 0.2. Flux of the proposed clusters, A854 and A12       
     46, are more than 0.2 mCrab and high enough for studying kT, iron abu       
     ndance, and the total amount of the hot cluster medium. In order to o       
     btain a meaningful statistics, we propose to observe each cluster for       
      60 ksec.                                                                   
                                                                                 
                                                                                 
                                                                                 
                                                                                 
                                                                                 
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