We Saw to the Edge

We saw to the edge of all there is; when it happened. 

The flick of a switch; in truth, a major engineering operation. What started its life as a disaster became the most advanced aid to the human eye. It allowed us to look out to our backyard, looking for what we know to be there, just like a householder looking into their backyard at night for intruders. They could be there, but we cannot see them. The visitor - or the threat?