Re: Beam Weapon Targeting / Aiming

On Jul 7, 2:03 am, Mike Williams <nos...@xxxxxxxxxxxxxxxxxx> wrote:

It can't get smaller because of the diffraction limit. OWL is designed
be diffraction limited.

The same diffraction limit that limits the dispersion of the weapon
beam, so, if you're using an optical laser, with a 100m telescope your
beam can't possibly be narrower than 0.001 seconds of arc.

If I've done my sums correctly, at a distance of 1 light second,
0.001SoA gives you a resolution of 1.5 metres, and the weapon beam
dispersal is about the same.

To put some numbers on this - the resolution of a telescope with a
circular (or nearly so) aperture is taken to be the first radial
minimum in the Airy disk diffraction pattern. For a beam profile that
has a Gaussian falloff, with the characteristic falloff length
parameter equal to the primary mirror's radius, but clipped to zero at
distances greater than the primary mirror's radius, you can get 95% of
the beam power within the spot given by the first minimum of the Airy
diffraction disk. (Although for a clipped Gaussian beam the first
diffraction minimum is a bit larger than for a pure Airy disk). I've
got more details here, along with pretty pictures of what the
diffraction patterns look like.
http://panoptesv.com/SciFi/Diffraction.html

You can use the same mirror for both
purposes, just make sure you stop looking before you start firing.

If you bracket the primary mirror with three or four smaller aiming
mirrors, you can use them to get the same resolution as the primary
mirror while the laser is firing. Your sensitivity goes down because
of the smaller area, of course, but since your target will be
illuminated by megawatts of laser light, you don't need amazing
sensitivity (and anyway, 1 meter mirrors should be quite adequate for
seeing spacecraft, and a few 1 meter mirrors around the edge of a 100
meter OWL-like scope will not add much cost or complexity).

Luke
.

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