Can I create star trails by stacking many short exposures?
Asked 2/1/2011
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I’m shooting the night sky as a timelapse using continuous 15-second exposures so the stars stay as points in each frame. I’d also like to make a single image that looks like one long exposure, with star trails, using those same short exposures. Is it possible to combine the frames to simulate a long exposure, and are there any tips to avoid gaps or other issues?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
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If you have Photoshop, you can create an image stack. This automatically aligns the layers, so this works hand-held, too. It's a nifty trick if you're shooting a static scene without a tripod and have some extra memory space.
(I wonder if the auto-alignment would be fooled by star trails, as a significant part of the image will be moving in unison.)
Here's the before/after on a set of 12 pics I took:
On top, a scaled down version of the whole thing. On the bottom, a 100% crop of the image. Left is one of the 12 original frames, and the right is the averaged picture.
Although the original images were only 1/250s, making a total exposure time of about 1/20s, the wave motion is almost completely averaged away. You can also see some horizontal blurring in the clouds as they move across the sky. This is because the images were taken with about 1s delays in between.
Originally by user378. Source · Licensed CC BY-SA 4.0
user378
15y ago
0
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Yes. For star trails, you can combine the short exposures into a stack and blend them so the brightest pixels from each frame are kept. In editors like Photoshop, this is commonly done with a stack of layers using a Lighten/Lighten Only blend mode; dedicated tools like Startrails can do the same automatically, and GIMP can do it with scripting.
A few practical tips:
- Shoot continuously with as little delay between frames as possible, or the trails may show visible gaps.
- This works best when the camera is fixed on a tripod and the foreground is static.
- You can also use image stacking/alignment tools for other long-exposure-like effects, but for star trails specifically, keeping the bright star positions from each frame is the key.
- A dark frame taken at the end can be subtracted to help reduce noise/hot pixels.
So, while it’s not identical to one single long exposure in every respect, stacking your 15-second frames is a standard and effective way to create a star-trail image from timelapse shots.
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