This might seem like a backwards question, but I'm interested in what wavelength to select in a (hypothetical) ground-based radio telescope observation to expect detecting as little as possible! :)

It should still be reachable from space, i.e. a wavelength with a high atmospheric absorption is probably a bad candidate.

I guess this could be answered by looking at a broad averaged spectrum collected by a radio telescope, and comparing against an atmospheric absorption spectrum chart.


1 Answer 1


You may be looking for the "water hole". See http://www.setileague.org/general/waterhol.htm.

  • $\begingroup$ Interesting! Yes that is more or less exactly like the summary I was hoping for. It had been optimal if there was some link to an actual background spectrum (composite, if needed) I guess but this is a great start :) $\endgroup$
    – BjornW
    Commented Jan 2, 2015 at 11:15
  • $\begingroup$ See my answer regarding observing the 21cm hydrogen line @1420MHz at astronomy.stackexchange.com/questions/26139/… $\endgroup$ Commented Jul 3, 2018 at 1:55

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