From: tkoenig@netcologne.de   
      
   Wolffan  schrieb:   
   > On 14Feb 2018, Thomas Koenig wrote   
   > (in article ):   
   >   
   >> First, lasers spread. Their radial beam divergence (look that up)   
   >> is at least lambda/(pi * w), where lambda is the wavelength and   
   >> w the radius of the aperture.   
   >>   
   >> Assuming green light of 540 nm, 10 cm diamter (5 cm radius)   
   >> laser beam will spread to twice its radius over 14 km, energy   
   >> density will be one quarter then.   
   >>   
   >> Of course, you could try to focus your beam using some   
   >> really fancy adaptive optics.   
   >   
   > three solutions to beam spreading:   
   >   
   > 1 make the aperture bigger.   
      
   This makes your beam bigger to start with, so the area which gets   
   the energy might also be bigger.  The optimum aperture radius is   
   sqrt(lambda * D/pi) (with the distance D), the size of the blob is   
   twice that. So, for 10 km distance and 400 nm, the optimum blob   
   size would be around 0.07 m, or 70  mm.   
      
   > 2 use higher freqs (blue light, ultraviolet...) Ultraviolet has problems in   
   > air, but not in a vacuum.   
      
   > 3 both. A one meter aperture hard ultraviolet laser would kill at sizable   
   > fractions of a light second,   
      
   The optimum blob size radius at 100 000 km and 100 nm is around   
   3.6 m, with an aperture radius of 1.8 m, or a 3.6 m "caliber".   
   Still a pretty big patch to beam your laser on if you're planning   
   to kill your target with heat.   
      
   What sort of power budget are you proposing?   
      
   A laser that big is going to have issues with vibration, with   
   targeting, etc   
      
   > a.k.a. so far out that you’ll have a problem   
   > holding the beam on target.   
      
   Most definitely.   
      
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