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   52177
   We are proposing to complete our program to measure the X-ray
   temperatures (Tx) of X-ray luminous, distant (z>0.5) clusters of
   galaxies. The final cluster in our sample, MS1137+66 (z=0.78), is just
   as luminous but optically much poorer than the other 3 clusters. The
   distant clusters observed thus far in our sample are hotter than
   standard structure formation models predict. Cluster temperatures at
   high redshift constrain predictions of Lx-Tx evolution. Because
   MS1137+66 is chosen from a well-defined X-ray selected sample, its Tx
   also constrains the z=0.8 cluster temperature function. It is bright
   and compact enough for 1-CCD SIS observations; the temperature break is
   redshifted to ~3-5 keV. HST WFPC2 and ROSAT HRI images are scheduled
   for MS1137+66.
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