In two previous articles, I wrote about capturing photographs during civil twilight and nautical twilight. After nautical twilight in the evening (and before it in the morning) is a period known as astronomical twilight. This stage of twilight occurs from the time the sun is 12 to 18 degrees below the horizon.
During astronomical twilight, the sky will appear almost completely dark to the human eye, but there will still be some light from the sun refracted through the atmosphere that can be picked up by a camera. At the beginning of astronomical twilight in the evening, the sky will still have a bluish color to it. If there is no moon out, the sky will gradually fade to purple and then to black at the end of twilight. There may also be some reds or yellows in the direction the sun set (or is about to rise).
If there is no moon or a thin crescent moon, the skies will be sufficiently dark that you will be able to capture many stars and the Milky Way in your images. If there is a brighter moon out, the light from it will likely overpower any light refracted through the atmosphere from the sun, and your shots will look like shots taken under a moon in the dark of night (the sky will appear blue, the foreground will be lit up, and some stars will be obscured).
Even if there is no moon, you can still get a little light on the foreground if you are facing directly away from the spot the sun set (or is about to rise). However, you’ll probably need to illuminate the foreground with a flashlight or combine multiple exposures – one or more for the foreground and one for the sky – if you want to get good detail throughout the image during astronomical twilight.
You will need to use manual exposure to capture images during astronomical twilight. You should generally use the widest aperture on your lens, and you will need to start raising the ISO significantly to get good exposures with data extending to the right side of the histogram. To determine shutter speed, you can use the rule of 500. Take 500 divided by the focal length of your lens to get the number of seconds for your exposure.
Fortunately, you won’t have to stop shooting after the end of astronomical twilight in the evening. There will no longer be light refracted through the atmosphere, but you can often still get some green and red color in the sky from airglow. The skies will also be at their darkest, making it a great time for shooting the Milky Way.
There really is no bad time to capture images at night. You can take shots as soon as the sun sets and keep shooting as long as you want, while the light of day fades into the blackness of night.