Star trails can be a powerful way to compliment and accentuate a subject. In this image, they not only draw your attention towards the a moonlit focal point, they also echo the shape of the surrounding arches to help tie everything together. You can capture such a scene with a single long exposure, but you can often get a better image or simplify processing by taking a serious of short exposures to blend together.
There are several benefits to shooting star trails as a stack of short exposures rather than a single long exposure, including:
- You won’t lose hours of work work if something goes wrong – such as someone shining a headlamp into your image, lens flare if the moon moves into a problematic position, the camera getting bumped, batteries dying, etc.
- Less hot pixel noise. Unless you’re shooting on a very cold night, you’ll see more and more hot pixel problems with longer exposures.
- Minimize potential problems with moving trees in the wind.
- Keep the option to process a single sharp image (such a beautiful meteor passing through a frame) or to create a time-lapse video.
- You to choose the exact arc of rotation you wish to see in your final image without complex planning. Just use as many images as you need until you have the rotation you want in the image.
- It can potentially make it easier to clone out complications (meteors, satellites, planes, hot pixels, etc) from a short exposure frame instead of a full star trail image.
- Avoid the complication of trying to calculate a safe exposure to use for an hour. It would be very tricky to determine an exposure which shows the stars, keeps the right balance of blue sky, and does not blow out the foreground in the bright (and constantly changing) moonlight.
- Capture a foreground image. This might be taken at the blue hour, using light painting, or just a very long exposure with ambient light (potentially moonlight). The goal is to have something which is perfectly aligned with the star images (use a tripod) and ultimately has low noise (use ISO <1600).
- Capture a series of star images. The resulting length of star trails depends on how long you shoot the sequence, how wide your lens is, and where you point it in the sky (there is no movement around Polaris or the Southern Cross and increasing movement away from them).
- If you want flexibility to create a time-lapse or to process a single frame (perhaps if a great meteor runs through one image), you should shoot the night sky as you normally would. If you don’t need that, then you have the option to reduce noise by using a longer exposure and lower ISO, such as ISO 800 for 30 seconds each frame. Any noise you capture in such a stack may ultimately make the dark parts of the sky brighter in the final image (as we’ll be combining the brightest images), so reducing noise can help the final result.
- If you have an exposure delay setting, turn it off. Adding a delay between frames will not improve sharpness for such a long exposure, but it will likely cause minor gaps between the frames that make the star trails less smooth.
- Turn off long exposure noise reduction, this will similarly add delay between frames and cause gaps in the star trail. If you need to capture dark frames, do so before or after shoot the stars.
- If shooting in very cold conditions where you may get condensation on the lens, use a rubber band to hold a hand warmer on the bottom of the lens.
Lightroom / RAW workflow:
- Select the first image to process.
- Apply somewhat strong noise reduction. This will give us two benefits. First , it helps keep noise from brightening of the background that would result by allowing typical noise patterns when combining the brightest version of each pixel. Second, having star trails from minor stars makes the final result messy / complicated.
- Turn off sharpening, as this will reduce star edges and therefore make star trail gaps worse in the stacked image.
- Consider adding some clarity to help enlarge stars to minimize star trail gaps in the blended image.
- If you need to darken the ambient sky, a little bit of “dehaze” may help.
- Do not apply profile corrections before stacking, as this may cause artifacts in the trails.
- Select all the layers, sync your edits to apply the exact same processing to all images, then right-click and choose Edit / Open as Layers in Photoshop.
- Open all the sky images as a stack of layers in PS. Set all but the bottom layer to “lighten” blend mode.
- Put stars into groups of about 10, so that you can quickly toggle them to narrow your search when trying to find which layer may contain a meteor, plane, or other problem you need to clone out. You may then put these groups into a another group to collapse them all.
- If you need to show all layers again, <cmd/ctrl>-click one of the groups to expand/collapse all. Then click on the visibility icon of the top layer and then (without releasing the mouse button) drag down so that the mouse goes over the visibility icon of all the layers. When you reach the end and release the mouse button, all layers should show (or hide if you were hiding the first icon you clicked on). Alternatively, select all layers and then go to Layers / Hide All Layers.
- For dramatically smaller files and faster performance: select all your layers, right-click and merge to a single layer. You may choose to save your layers so you can edit further later, but it’s a significant tradeoff. If you don’t merge, put them all in a Smart Object or create a stamp visible layer to use the next steps.
- If you need to fill gaps in the star trails, try adding a Gaussian blur with radius 0.5-1.0.
- Now put your sky layer(s) and foreground layer into the document and add a layer mask to combine them.
The YouTube tutorial above was getting a bit long, so I didn’t show all the final post-processing I did. After I finished the video, I did the following to create the final image:
- Used ACR adjustments, Nik, and curves with dark luminosity masks from Lumenzia to help extract shadow detail.
- Used a darkening brightness/contrast layer in the top left to reduce some flare on the rocks from moonlight.
- Added a vignette with a “Not D1” BlendIf to protect the shadows.
Instead of using Photoshop to combine the stars, you may consider using StarStax. This software is free and designed specifically for stacking stars. A key benefit of this software for star trails is that it tries to help fill “gaps” between frames, which may give the edges of trails a jagged look.
The workflow is pretty simple:
- Select your images in Finder / File Explorer and drag and drop them into the app.
- Click the gear icon at top-right of StarStax and set the blending mode to “gap filling“.
- Click the 4th icon towards top left for “start processing“.
- When the processing is done, check “show threshold overlay” and increase the threshold to the highest value that keeps green on the stars (goal is to minimize green on the non-star pixels which you don’t want filled), then set the amount to a middle value where the results look best. When in doubt, just use a lower threshold to get the stars (since we’ll blend the foreground from another image, there won’t be issues there).
- Click File / Save As. The file format will default to JPG and is based on the file name, not what you’d normally expect to choose file format. Be sure to change the “jpg” extension to “tif” in order to save as a TIF file.
- Use Photoshop to combine this sky layer with your foreground layer.
I personally tend to use Photoshop over StarStax as neither truly eliminates the gaps and I can manage everything in one place. It’s also 8-bit for my use case, as far as I understand currently (see the list of tips below). I’m no expert on it, so please comment below if you know a solution or if a future update of StarStax addresses these concerns. That said, I think it’s a great option to consider, as results will vary for different source images.
Additional tips for working with StarStax:
- If you get an error “Cannot display image / Processing stopped!”, try using a somewhat lower resolution or 8-bit image exports. It seems like this error shows up after hitting a limit for the total amount of pixel data you send it. I’ve found that exporting with LR set to use a long dimension of 7090 pixels works great, but you might need something smaller or get away with something larger. 8-bit exports may be the best solution as StarStax seems to only export 8-bit anyhow, so you might as well keep the original resolution. But try before you do this, as I wouldn’t be surprised if a future update addresses this.
- If you exported in lower resolution to avoid a “cannot display image” warning, you can resize your sky layer to the exact size of your foreground for perfect alignment and great results. Alternatively, you can simply export your foreground using the same resolution as the sky.
- When adjusting the sliders, click and drag them. If you simply click, the overlay and gap filling amounts do not update the preview.