RE: Fresnel lenses again

[Brian Mansur <bmansur@oc.edu>]

>From Brian,

>>Tim
>>Another problem that we haven't thought of when using mirrors or lenses is
>>that they absorb some of the energy of the beam, this changes the density
>of
>>the lens/mirror. The result is that unwanted bumps arise and that the
>>focus-distance changes.
>
>Brian
>Density change?  Why would the lense/mirror, have a change in density.  Or
>are you talking about changing the density that the design calls for?  Why
>would there be bumps?

>Tim
>When some of the energy of the beam is absorbed, that means the lens/mirror
>get a bit warmer (or hotter). When things get warm their density decreases
>and their shape changes. Chances are that the material where the lens is
>constructed from is not equal everywhere and probably neither is the 
density
>of the beam.
>So unequal amounts of expansion will occur making the problem even worse.

>From Brian
To group:  I am now officially abandoning _all_ hope of a retro mirror and 
am using robots to set up decelerating arrays (complete with huge solar 
power collection arrays around TC) to stop incoming ships.  Before I 
completely repeat the Argosy Mission Overhaul paper, I'll stop writing. 
 Read the paper.  It explains my new plan (somewhat).

P.S.  Thanks to all who have replied to my questions and helped me come to 
the conclusion that we just can't make this darn sail thing work (without 
automated robots that is).