28th September 2008 - 11:53 PM
Answers to questions like "How fast can a man run?" generally make the implicit assumption that the man isn't locked up in a small room with rubber walls.
True, I was assuming an idealized (diffraction-limited) telescope, so the spot size on-target could easily be bigger than that if you built the telescope wrong (like the Hubble was, originally).
The thing to remember about optics is that light goes both ways through the optics just as easily. If you want to look at a small crater on the moon, you need a big-aperture telescope. That's the main reason the serious telescopes (e.g., Palomar) have big mirrors. (They also collect more light, but that's icing on the cake.)
So if you want to put a megawatt laser on an ICBM after it's launched, you want a big mirror for your telescope so you can see the target clearlyfrom a long way off, then, where you'd see the image of the weakest point of the missile (at the small end of the telescope, like where you'd put film if you were taking a picture of it), you focus your megawatt laser right there and the laser goes in the opposite direction as the light coming *from* the target and is focussed on the aimpoint. The diameter of the focus on the target is calculated using the equation posted prior. Buy a smaller-aperture telescope, and the spot size will be bigger, and at some point, you're just warming up the whole missile rather than burning a hole in its side.
There's always the trade-off where the telescope guys don't want to build a bigger mirror, and the laser guys don't want to build a more powerful laser, so you have to "distribute the misery" equally between them.
It shouldn't come as a surprise, if you think about it, that bigger telescopes are needed to make smaller spots on-target. Bigger telescopes are more expensive. Would you really expect nature to cut us a break, making cheaper telescopes work better than more expensive ones?