Star trail photography tutorial using multiple exposures – Part 2

Stacked star trails - Part 2

In Part 1 I explained how to capture a set of photographs for creating a stacked star trail photograph.  In this article I will explain the editing of those images.

What you will need:

  • A number of images captured over a period of at least an hour
  • Lightroom (for this turorial)
  • Photoshop or any other image editing software that lets you work with layers and masks
  • A pretty decent computer with at least 8GB RAM, or a lot of patience

The first step is to import all the image into Lightroom, where we will do basic exposure adjustments in preparation for stacking them with Photoshop. Here is a screenshot of what it looked like right out of the camera:

As you can see the images are pretty dark, so we’ll want to adjust the exposure to bring out the details.  We do that by editing one photo, and then synchronising the settings to all the others in the set. So, let’s select the first image, and go to Develop mode.

Something I always do before anything else is to Enable Profile Corrections and Remove Chromatic Aberration. The first option uses a database of lens properties to determine the amount of distortion present in your photo, and correcting it, while the second removes purple or red fringes on the edges of light to dark transitions, usually more noticeable with less expensive lenses. More about this in an upcoming article.

These options are under Lens Corrections in the develop settings section at the right:

Now adjust your exposure, contrast, highlights, shadows etc. until the image has the exposure you want. Here are the settings I chose to use:

Now go back to Library mode, and select all images (Ctrl-A on Windows, Cmd-A on a Mac). Make sure all images are highlighted, then click the Sync Settings button on th bottom right of the screen. You will see a window where you can specify which settings to synchronize. In this case we want everything, so click Check All and Synchronize:

Lightroom will now apply the same settings to all your photographs, and once it has updated all the previews you will see the entire selection with the same exposure and other settings as the first image.

Before continuing, scan through all the photos and pick one that you want to use as the main or top image. Make a note of the file name, we will come back to it.

It is now time move on to Photoshop. Make sure all photos are still selected, then right-click on any one, and in the popup menu select Edit In -> Open as Layers in Photoshop. Do not expect things to happen quickly though, it can easily take an hour or more for Lightroom to export all images and open them in Photoshop.

Here is what it should look like in Photshop, note the layers on the right:

We now need to change the blending mode for all layers. Click on the top layer to select it, then scroll down to the last layer, hold the Shift key, and click. All layers should now ne selected.

Click on the Blending Mode selector, and change the mode to Lighten.

You will now see your startrails, but the problem with the moving windmill is also clearly visible:

Remember the image you selected as the master above? It is now time to bring that into play. To view each layer separately select one layer, then right-click and select Show/Hide all other layers. You can now click the small selection button to the left of each layer to scan through and find the master layer, then drag that layer all the way down to the bottom. Hide all other layers.

Now comes the extremely tedious stuff. For each layer you need to follow these steps:

  • Add a vector mask
  • Zoom in until the area to be masked more or less fills the screen
  • Select the brush tool (or press B)
  • Ensure the background color is black (press X to switch)
  • Now paint over the areas that you need to hide. The more precise you are with this the better the final image will be, but it doesn’t have to be pixel-perfect.

Do this on each layer, turning layers on and off as you go so you can see the effect of your masks. This can take a long time, and may require several sessions to complete.

When you have done this for all layers (except the bottom one of course), select all layers except the bottom one, and flatten them.  Now add a black vector mask on the top layer (the one you just flattened) by holding Alt and clicking the mask icon.

You want to paint with a white brush on the mask to reveal the master layer below, but only the static parts, not the star trails.

You can now flatten the image, save it as a master file, and continue to edit it as usual for landscape photos.