A way to mine the Moon for water

It is said that the Moon has significant deposit of water in permanently shaded craters at the poles.

How to get at that water?

In the book about Space Sailing, it mentioned that a sail could hover in position, like a polar position vis-a-vis the Moon, or Mars, or what have you.

Now, if you were to deploy a large mirror from the sail, and beam the sunlight down on a portion of the crater's bottom, you could melt that water and collect it.  You could place a dome over that portion of the surface that you want to illuminate.  The transparent dome could allow the sunlight to heat the surface and melt the water.

The water would evaporate in the vacuum, but the pressure would build under the dome until it could support water in liquid form.

That's the theory anyway.