This is not much of a question, just local wisdom.   
   I use Thor labs lens tubes to couple a diode laser onto   
   a diffraction grating, single bounce, Littrow?   
   I've been having problems with getting the coupling,   
   "just right".  I'm setting a lens position with a thread,   
   as I'm adjusting the lens position the "spot"   
   (of lasing) is moving around, (In the phase space of the   
   grating angle)  Some tubes are better   
   some worse... I've been trying to figure it out.   
      
   Anyway a single layer of teflon tape..   
   (on the thread of the lens tube)   
   seems to have made the whole operation much better.   
   I'm adding teflon to every lens from now on.   
      
   Should I send an email to Thor labs?   
      
   George H.   
      
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