Re: Invisibility
"Stray radiation is undetectable beyond about 1 light year, even by a hubble-class telescope or an Arecibo."
I agree that non collimated difuse optical radiation (city lights, etc.) is undetectable even at close interstellar distance with practical scopes, despite what "some" have claimed here.
However, I believe I've read credible analysis that state that stray signals from BMEWS Ballistic Missile Early Warning radar Systems are detectable by an Arecibo sized dish at a few LY.
-philw (maybe they just don't work upside down in Australia?)