Ever wonder how images like the above were made? There are two main ways: 1)one VERY long exposure, or 2) stacking several long exposures. In this tutorial post, I'll go through the planing, strategy, and technique of the second option - stacking.
First up is planning (see my post on that here). After you determine the location, the right night, ascertain whether or not you have do-able weather, and select where to place your camera, then comes the fun part - guessing the exposure length, aperture, and ISO, and subsequently figuring out how many you need to take. Looking at your sensor size + lens combo should give you a good idea of how many frames of a particular length you need to make if your goal is a star circle (facing N to Polaris). I have a whole separate post on this here. In this post, I want to focus on the post-processing portion of making an image like the above.
So you have your frames, and (hopefully) your "plate" exposure that you shot for the foreground. Now what?
Step 1: get organized by placing all frames in your stack in a unique Lightroom Collection.
Step2: Synchronize ALL develop settings across EACH star trail frame so they are exactly the same.
Step3: Separately develop your "plate" frame to taste.
Step 4: Select all frames in your stack in Lightroom's filmstrip along the bottom of the screen. Right-click on one of the photos, and choose "Edit In / Open as Layers in Photoshop"
Step 5: In Photoshop, drag the plate layer to the bottom of your layers, if it's not there already.
Step 6: In the layers panel, select ALL star trails layers, and change the Layer Blending Mode to either "Lighten" or "Screen." Choose one based on which looks better - if shot on a moonless night with dark sky and bring star trails, I find "Lighten" to work better. This should allow all the trails to add up and create your finished blend OF THE SKY ONLY.
Step 7: Merge all the star trail layers together via the "Merge Layers" command
Step 8: Add a layer mask to the merged star rails layer. Use regular mask-painting techniques with different brushes, hardnesses, opacity, etc. to cover over the foreground of the star trails layer with black paint (remember: white reveals, black conceals). The black paint on the mask will "conceal" or cover up the part of that layer, allowing the part of the next-lowest layer (in this case your foreground plate) to come through to the top in your photograph. Take care to make the blend look realistic.
If you find gaps in your trails, they will need to be dealt with. Stephen Christensen of the Star Cricle Academy has an excellent post on how to manage this scenario - here.
That's it in a nutshell! Any questions, feel free to leave a comment or shoot me an email.