Re: Practically achievable beam divergence for white, non-coherent light

From: Ian Stirling (root_at_mauve.demon.co.uk)
Date: 02/28/05 

Date: 28 Feb 2005 15:21:48 GMT

Joe D. <joe@nospam.invalid> wrote:
> "Ian Stirling" <root@mauve.demon.co.uk> wrote in message
> news:4220f3fc$0$77057$ed2619ec@ptn-nntp-reader01.plus.net...
>> There is nothing magic about a Cassegrain (or any other system).
>> You end up focussing one point on the light source to one angle of output
>> from the main mirror/optic.
>
> In the Archimedes "burning mirror" myth, he focused light on ships
> hundreds of meters away.
>
The key word is 'myth'.
<snip>
> Given a 3 meter parabolic primary, a 0.5 meter final beam diameter
> that diverged over 500 meters to 1.0 meter beam diameter, that
> would produce sufficient flux to ignite wood in less than 30 seconds
> (about 40kw / m^2).

The fundamental limit is that you can't make an image of the sun that's
brighter than the sun.
If this was true, then you could run a heat engine between the sun, and its
image, and this is impossible.

In practice, 40Kw/m^2 needs an image of the sun that's (about) 40 times its
normal visible area at the target.

Let's say your target is 200m out.
The image off a flat mirror at 200m is (about) 2m.
So, we need 40 2m mirrors, or 160 1m diameter mirrors.
In theory, you could do this with 4*40 men on a framework, all
aiming their own mirror.

> However the required beam divergence would be 0.06 degrees (!!)
>
> Can any mirror-based optic system TODAY produce a 0.06 degree
> beam divergence using sunlight? What would the optic design be?
Yes, as I answered earlier.

Relevant Pages