Optics and Materials Considerations for a Laser-propelled Lightsail.
A particularly striking statement…
As a note in passing, the 1000 km lens proposed by Forward, with the pointing accuracy he requires, would make an extremely high magnification telescope [12]. Clearly, a lens focusing to a spot of 100 km at a distance of 4 ly would allow not only planets of nearby stars to be detected, but they could be mapped with a resolution of 100 km. This was the original intention of O’Meara in proposing the paralens. The diffraction-limited beam spread of a 100 km lens for visible light (say 500 nanometer wavelength) would be 0.5 nanoradians. With this resolution, it should be possible to detect Earth-sized planets at a distance of thousands of light years, and Jupiter-sized planets virtually anywhere in the galaxy.
…thus if you can launched a laser-sail at the stars you can also see where the sail is going in some detail too.
