Paul,
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I respectfully disagree that there is always signal every where we look in space.
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Hi Paul,
Isn't there such a thing as sky noise? - the atmosphere glows.
There is thermal noise & shot noise etc
If the signal is below that noise you won't see any signal.
What is the signal from a dark area of open space?
The Hubble Deep field would tell us that at our resolution
it is a the sum of light from all those 1000's of galaxys.
In which case we can't resolve them & they just turn into a smudge of background noise.
I believe the correct term is
signal to noise ratio.
cheers
Allan