How far away can the oxygen signature in our atmosphere be detected? Indicative of photosynthesis. How far away can optical flashes from orbiters/satellites in Moon orbit be detected? A sign of technological civilization from afar? Is this much more effective than radio wave detection? TMM
Perhaps enhanced detection of non-repeating signals embedded in noise, might be useful.
Ataie V., Esman D. , Kuo B.P.-P. , Alic N. , Radic S., Subnoise detection of a fast random event, Nature vol 350 6266, Dec 11 2015.