Photographing Meteor Showers By Stacking Multiple Images
So how do you make a photograph that captures the feeling of watching an active meteor shower? The answer, in brief, is to shoot back-to-back 30-second exposures all night with the widest lens you own. Once you get home, comb through your images to locate those containing meteors, then stack all of those frames as layers in a single Photoshop file. Choose one image as the background star and land layer, then mask out everything but the meteor streak from all the remaining layers. The result will be a composite image containing all the meteors that fell within your lens’ field of view during the entire night.
The most photogenic meteor showers of the year are the Perseids, which peak every year around Aug. 12 to 13, and the Geminids, which peak around Dec. 13 to 14. The Perseids rain down most frequently between midnight and dawn; Geminid meteors start streaking across the sky around 9 or 10 p.m. and continue all night.
You’ll capture a lot more meteors if you can find a dark location well away from city lights. For help locating a dark-sky location near you, check out jshine.net/astronomy/dark_sky. You’ll also see more meteors when the moon is below the horizon. The 2016 Geminids coincide with a full moon, which will significantly reduce the number of meteors you see but will also make it easier to hold detail in the land. The moon will rise at about 11 p.m. during the peak night of the 2017 Perseids and will be about 70 percent illuminated. You’ll enjoy moon-free skies nearly all night during the peak of the 2017 Geminids. Clouds, of course, will shut down the show. Check out cleardarksky.com/csk for a forecast of cloud cover at your chosen location.
Meteor showers have radiants, a point in the sky from which the meteors appear to originate. Meteor showers are named for the constellations containing their radiants. Most meteors travel roughly 30 degrees from their radiant before becoming bright enough to see. You don’t have to find the radiant to see meteors. During an active shower, meteors will appear in all parts of the sky, which means you can point your camera in any direction and capture meteors. To capture images like my shots of the Perseids and Geminids, however, you’ll need to compose so that the radiant is within the frame some time during the night.
Like all celestial objects, radiants appear to move across the sky as the earth rotates. The radiant for the Perseids, which is near the star Al Fakhbir, is in the northeast sky during the peak of the shower. The radiant for the Geminids, which is near the star Castor, rises to the northeast, is nearly straight overhead at 2 a.m., and is setting to the west at astronomical dawn.
Gear And Exposure Basics
I shot both the Perseids and Geminids with a Canon EF 16-35mm f/2.8L II USM lens set to 16mm. Even better would have been a 14mm, which can cover about 30 percent more sky than a 16mm. My exposure for each frame was 30 seconds, ƒ/2.8, ISO 6400.
For the Perseids, I positioned the camera to look northeast, started shooting at midnight, and shot about 540 frames before astronomical dawn. Only 39 frames contained bright meteors.
For the Geminids, I set up looking south, so the radiant would be near the top of the frame during the peak of the shower. Out of 900 frames, only 51 contained a bright meteor.
For both images, I also shot several frames at 2 minutes, ƒ/2.8, ISO 6400 to get better detail in the land.
Processing Your Meteor Shower Images
After locating the meteor-containing images in Lightroom, I selected all of them and opened them as layers in a single Photoshop document (Photo > Edit In > Open as Layers in Photoshop). I chose one two-minute exposure with good detail in the land and dragged it to the bottom of the layer stack. Then I chose a layer containing the radiant to be the background star layer and dragged it to just above the good-land layer. Then I selected each meteor on all the other layers with the Pen tool, added a layer mask, filled the path with black (which hid the meteor), then inverted the mask (Control+I) to reveal the meteor and hide everything else. In a final step, I masked out the dark land from the background star layer to reveal the properly exposed land underneath.
When you first composite your meteor shower image, the meteors will appear to be crisscrossing the sky at random. Almost all of the meteors in both images actually originated at the radiant (a few were strays), but that pattern is hidden because the radiant appears at a different place in each meteor-containing layer, since I shot the images over a period of hours. To reveal that pattern, I used two different methods to rotate each meteor-containing layer so that the meteor streaks appeared to originate at the radiant.
The radiant for the Perseids is circumpolar, which means it makes a giant circle around Polaris. That makes it possible to use Polaris as the center of rotation while using Free Transform to rotate each meteor-containing layer to align its stars with the stars in the background layer. Target the meteor-containing layer, invoke Free Transform, then drag the point of rotation to Polaris. To identify Polaris, use a star-stacking program like StarStaX to create a temporary star-trails image. The star trails will form concentric circles around Polaris. In the final image, each Perseid meteor will appear to originate at the radiant, as if all the meteors had been captured in one exposure.
This technique is impractical for the Geminid meteor shower in mid-December, since the radiant crosses almost the entire sky. Trying to align the stars in each meteor-containing layer with those in the background star layer would push many meteor streaks entirely out of the frame. To create my Geminid image, I used Free Transform to rotate each meteor streak around its center so it would appear to originate at the radiant.
I certainly didn’t see all of the meteors in either image fall simultaneously, but I did watch them fall one-by-one as I stood under the moonless sky, awed by the celestial fireworks display. The techniques I described in this article are the best way I know to make single images that capture that experience.