From: mikko.syrjalahti@nospam.fi   
      
   RichD  writes:   
      
   > At conclusion, he forecast a schedule of long term performance   
   > improvements.  One of these, 10 years out, is the use of silicon,   
   > or silicon oxide, mirrors.   
   >   
   > What's the advantage of this material?  And what are they using now?   
      
   A fellow amateur astronomer working in silicon wafer business did a   
   Si-mirror. I think it was about d=200-250mm.   
      
   He commented that the material is very easy grind and polish,   
   except for the slurry is really dark and messy stuff. Being a single   
   crystal, it does not have any voids or density variations making it easy   
   to reach a perfect shape.   
      
   Mechanically Si is also more rigid than, say, Zerodur. This makes the   
   mirror support easier in a movin telescope. Not an issue with LIGO, I think!   
      
   The thermal expansion coefficient is higher than Zerodur (about the   
   same as borosilicate?), but he commented that the good thermal   
   conductivity causes the mirror to reach thermal balance really fast   
   compared to a glass mirror. Again, I would guess temperature changes are not   
   an issue inside LIGO :-)   
      
   Reflectivity was reasonably good even without coating and bare Si-mirror   
   made a nice moon telescope as such!   
      
   --   
   mikko OH2HVJ   
      
   --- SoupGate-Win32 v1.05   
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