I don't understand what you are trying to say. There is no such thing as a focal line. More generally, it's not possible for light from several points to focali

I don't understand what you are trying to say. There is no such thing as a focal line. More generally, it's not possible for light from several points to focalize at the same point because you can't revert those rays. I guess Randal Monroe explains it better than me here: https://what-if.xkcd.com/145/ https://what-if.xkcd.com/145/ However, I can see one way to read what they mean. If you take the Sun's "focal point", the point where parallel lines from one side intersect after being bent by its gravity, and move a tiny away from the Sun, then you form the image of a point that resides on the line going from you through the Sun all the way to infinity on the other side. You need to move very little for the opposite to move a lot, so for measurement purposes you are still at 590 AU from the Sun. This way with very tiny moves you could survey the planets in a different solar system, and in particular you could resolve them to multi-pixels.