You need a dark sky, away from city or town lights, to be able to expose a wide angle lens wide open for 15 secs. Living in the suburbs of Brisbane, I find that 3 to 4 secs is sufficient to easily reveal light pollution, at 800ASA.
Pointing at the zenith (up) lessens the effects of light pollution and I can expose for 5 to 8 secs before it become objectionable.
Cheers
Dennis
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