From: twomack@chiark.greenend.org.uk   
      
   Suppose you have a telescope designed for the thermal infra-red, so   
   surfaces machined to about 2.5um accuracy.   
      
   If you use a dichroic mirror rather than a long-pass filter at the IR   
   detector, it's clearly possible to direct the visible light to a focus   
   on a second detector.  Do you get any form of image at that point   
   given that the mirror accuracy is 5-lambda?   
      
   And, if so, assuming that everything is rigid and correctly aligned   
   for perfect images on the IR detector, is the _centroid_ of the image   
   at the visible detector going to be reasonably well-defined?  Can this   
   be used at least to help out the orientation-determination system?   
      
   Thanks in advance for any help you can offer   
      
   Tom (wishing he had checked that the university had Zemax licenses   
   before taking on an MSc project involving optic design ...)   
      
   --- SoupGate-Win32 v1.05   
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