Astrophotography

The sky is faint and it moves. Those two facts set every number in the genre. Faint means a fast lens, a high ISO and a long exposure. Moving means the exposure has a ceiling, because the Earth turns fifteen degrees every hour and the stars smear.

That ceiling produces the one real fork in the genre. Stay under it and the stars are hard points. Go far past it, to twenty minutes or four hours, and every star draws an arc around the pole. Both are correct, and the prompt has to say which one it wants.

Name the lens and the aperture: 14mm to 24mm at f/2.8 or faster, focused manually at infinity. A slow lens cannot do this job at all.

Give the ISO and the exposure as numbers, and say which result you want. ISO 3200 for 20 seconds gives pinpoint stars. ISO 400 for an hour gives star trails.

Say what lights the ground. Starlight and airglow only, or a rising moon, or one distant town glow on the horizon. Say no torch and no street light.

Use it for a dark sky over a ridge, a ruin, a lake, a lone tree, a desert road. The land is a silhouette and the sky is the subject.

Leave it alone anywhere near a town. A sodium or LED glow twenty miles away is brighter than the galaxy, and it takes the whole lower sky.

Night photography wants artificial light and treats the city as the subject. Astrophotography needs no artificial light at all, and it fails wherever there is any.

Landscape photography puts the sun near the horizon. Astrophotography waits until the sun is eighteen degrees below it, which is the definition of astronomical darkness. Cinema publishes Blue Hour, and blue hour ends where this begins.

Deep sky imaging runs the same idea for hours on a tracking mount and stacks hundreds of frames into one. A single wide frame of the Milky Way over a ridge is the version which works as a picture on its own.

The Milky Way core only stands above the horizon at night between about March and October in the northern hemisphere, so most of this work carries the same months. The International Dark-Sky Association has certified dark sky places since 2001, and those certificates exist for one reason: the land is now the easy half of the picture and an unlit sky is the rare half.

The stars come back as short streaks. Either shorten the exposure and say pinpoint stars, or lengthen it hard and say star trails circling the pole.

The foreground comes back lit, usually by an invented torch or lamp. Say the foreground is lit only by starlight, and say no artificial light anywhere in the frame.