XPost: sci.astro, sci.physics, sci.space.policy   
   From: fjmccall@gmail.com   
      
   willner@cfa.harvard.edu (Steve Willner) wrote on Mon, 18 Jun 2018   
   20:36:35 -0000 (UTC):   
      
   >> >At least three observatories with seven telescopes in active use will   
   >> >be surprised to learn [that making a 6.5-m primary mirror is impossible]   
   >   
   >That should have been five observatories and ten telescopes.  I   
   >forgot some.  I won't swear I'm not still forgetting others.   
   >   
      
   And you're still talking about earthbound scopes.   
      
   >In article <1q38id1rbuqh4k46gpk8o1m70o294nn3k8@4ax.com>,   
   > Fred J. McCall  writes:   
   >> You can do things with earthbound scopes that you cannot do with   
   >> something you're going to shoot into space.   
   >   
   >How does that apply to the current discussion?  Launching a 6.5-m   
   >mirror monolithic should in principle be easier than having the same   
   >size mirror deploy to the required precision.  The problem is making   
   >it fit into the payload fairing.   
   >   
      
   Oversize fairings are easy.   
      
   >> >  I wouldn't be surprised if there are "black" programs with the   
   >> >same difficulty.   
   >   
   >> Nope.  They use a mirror roughly the size of Hubble's.   
   >   
   >The ones we know about used mirrors that size.  Anyone who actually   
   >knows the current situation -- I don't -- wouldn't be allowed to say.   
   >   
      
   Of course they're allowed to say.  You can see the bloody things from   
   Earth, after all.  Past a certain point a bigger mirror doesn't help   
   you for Earth observation.  Atmosphere speckle becomes the driving   
   parameter and a bigger mirror doesn't help that.   
      
   >   
   >>   
   >>  Remember, they're looking at something relatively close as such   
   >> things go.   
   >>   
   >   
   >6.5-m mirrors would have advantages over smaller ones.   
   >   
      
   No, they wouldn't.  The next generation of recce satellites will use a   
   mirror right around 2.4 meters; the same size used since KH-11.   
      
   >   
   >(I don't see   
   >what distance has to do with anything.)  I've seen hints that some   
   >have been built and deployed, but that may be salemanship.  Companies   
   >vying for the JWST contract would have had an incentive to drop such   
   >hints whether true or not.   
   >   
      
   Don't let them kid you.  Distance has a lot to do with everything when   
   it comes to telescopes.   
      
   >>  The point is that a balloon does NOT replace a 'first stage'.   
   >   
   >We agree on that.   
      
   --   
   "The reasonable man adapts himself to the world; the unreasonable   
    man persists in trying to adapt the world to himself. Therefore,   
    all progress depends on the unreasonable man."   
                                         --George Bernard Shaw   
      
   --- SoupGate-Win32 v1.05   
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