Astronomical twilight ends once the sun drops below 18° below the horizon, the point at which the sky is considered fully, astronomically dark.
Astronomical twilight ends when the sun drops to 18 degrees below the horizon, and that moment is the one Milky Way and deep-sky photographers actually wait for. Until then, sunlight scattered high in the atmosphere still adds a faint glow that lifts the sky's brightness and cuts contrast, washing out the fainter stars and the dust lanes of the galactic core even though the sun set hours ago.
The gap between sunset and full darkness is bigger than most people expect. At mid latitudes in summer it can run close to two hours, and above about 48 degrees latitude astronomical twilight may not end at all near the solstice, which is why Milky Way season there is limited to spring and autumn. Planning a night shoot means counting from the end of astronomical twilight, not from sunset.
A photographer drives to a dark site for a Milky Way shot, arriving at sunset at 8:30 pm. The core is technically above the horizon, but every test frame shows a milky grey sky with weak star contrast. Civil twilight ends at 9:05, nautical at 9:45, and the sky keeps improving but is still not right.
Only after astronomical twilight ends at 10:28 do the dust lanes snap into clear contrast against a genuinely black sky, and the same 20-second, f/2.8, ISO 3200 exposure that looked flat at 9:00 now shows structure across the core. In June at that latitude astronomical twilight never fully ends, so the photographer checks a calendar and books the trip for late August instead, when the same site gives more than four hours of true darkness before astronomical twilight starts again at dawn.
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