When I process a Star Trail, I often do it in Photoshop. There are programs available that will process a star trails for you and have useful functionality, but they can have limitations too. This tutorial will take you through the steps to create a simple Star Trail image in Photoshop and review how I set up in the field.
In the Field
There is more than one way to obtain a star trail image. You could do this with a single (very) long exposure. If something goes wrong during the exposure, you have lost the entire star trail. The method I usually like to use is to take a series of shorter interval exposures and then blend them together. When shooting star trails or even time-lapses, I have often taken a single frame (or more) from the sequence to edit and use as a stand alone image. In order to do this, I have to set up my exposure times to minimize star trails in a single shot.
The sequence I am using for this tutorial was actually shot to be a time-lapse. I was hoping the aurora would do something more exciting, but it didn’t explode until much later. However, since the aurora didn’t change very much, this was a good but short sequence for a star trail.
ISO: 6400, f2.8, 14mm, 10s + 1s interval, 90 images total
Nikon d810 + Nikkor 14-24mm f2.8
I always start in Lightroom. I have loaded all my images into a folder and have applied some basic colour correction, contrast, adjusted the tone curve and vibrance. Chromatic Aberration correction has been applied but not the lens correction as this can create banding when the images are blended together.
Depending on your processing power and how many images you have, you may have to do the next steps in batches.
The first step is to select some (or all) of the images. Right click and select “Open as Layers in Photoshop”
It might take a while for the layers load. Once they are in Photoshop you will see them in the Layers section on the right.
The next step is to select all the Layers (click the top layer, hold shift, click the bottom layer). Then change the blend mode to lighten.
Unfortunately, all the planes, satellites and other unwanted bright objects will come through as well.
My preferred method of getting rid of these is to go back and find the image numbers in Lightroom, then edit those individual layers in Photoshop. You can view an individual layer by holding alt/option and clicking on that layer.
After you are done editing out all the unwanted objects you can merge the layers together. Make sure all the layers are visible again by holding alt/option and click on the layer that is currently visible.
If you are blending the layers in batches, save this image (I usually a save it as a tiff file) and go through the same process for the next batch of images. Once you have combined all the files in your star trail sequence, bring in the individual tiff files as layers and blend them in the same way. You can do more editing at this point if you like.
This is a simple example to get you started with your star trails. There are more complicated cases and I have actually done some creative editing for some of my star trail images. Notably the one below which is a south facing composition.