As in any kind of photography, one of the most important things is building a decant composition. In star trail photography it is important to show as many stars as possible but without a decent anchor / foreground, to "hold" the sky and make the picture unique, it will lose interest.
Since star trail photography is done at night and in an area as dark as possible, it is crucial to know the scene in advance or at least to arrive during daytime in order to plan your composition properly and in an optimum way.
One more important thing to consider is that, since we are shooting at night, our foreground will also be dark and if silhouette is not our goal, we will need to have at least one frame where the foreground is lid to show its details.
When building our composition it is important to consider earth's rotation and how it will affect the final result. In short, earth the north star are moving in the same direction and at the same speed. This is why we get the feeling that the north star is fixed in its place. All other stars are rotating around the north star which means that if it will be in our frame, we will get an effect of circles. If our camera will face to the south-east, we will get lines, almost horizontals that concave in one corner and convex in the opposite corner.
The stars are moving 15 degrees per hour around the north star. We see more stars in the skies in darker areas than in light polluted areas. Combining these two factors will determine the length and density of star trails we will get in the final image, considering also the total time of shooting.
The idea is to shoot many frames in a row using a shutter release controller with a lock option / intervals option or, (if available) programming the camera to do so. if shooting towards the north, it is advised to shoot for at least 2 hours straight.
So, how it is done in practice ?
Place your camera on a steady tripod with the chosen composition in frame. remove or set anything that may cause her shiver from a passing wind.
Create your first frame where you can see properly the foreground of your frame. You can do so by using a high ISO, longer exposure or with the help of a flashlight / flash.
Do a test frame of the night sky. The goal here is to see that we are getting good looking sky without too much light pollution that will interrupt the frame but, that we see as many as possible stars in the sky and that they are in focus. We will work in M mode (or bulb for intervals) and make sure that our WB is not in auto mode (preferably in Tungsten mode). Aperture should be as open as possible to get the best results and exposure time should be around 30 sec. We can play with the ISO to lighten or darken the skies as needed. Once we achieve a good frame we keep the settings and erase this frame (but make sure to keep the "foreground frame").
Close the lens lid and shoot 1-2 frames like that (with the same settings). These are called "dark frames" and used in the post processing. Remove the lid !!
Lock your shutter release controller and wait to see that the camera works properly – ends one exposure and immediately stars another one. Leave the camera to work for as long as you decided.
Once finished, close the lens lid and shoot 1-2 more dark frames (important to do it immediately, when the sensor is still hot).
That is it for the shooting, now it is time to post process.
Process the foreground frame with emphasis on the visibility of the foreground. Ignore the appearance of the sky and if possible, darken them.
Process the first frame of the night sky. Your emphasis here is the visibility of the stars.
Synchronize all the night sky frames to the first one.
If you were shooting in RAW format, this is the time to export all the photos, including the foreground and dark frames, to JPEG or TIFF.
Go to your dedicated software for combining all photos (I personally work with StarStax). Upload your frames into it (dark frames have a dedicated location, not with all the others).
Considering the foreground frame, there are 2 options here - (a) Add it together with all other frames or (b) Leave it out and combine it with the final result later, in Photoshop.
In StarStax I mark the "gap filling" option as it fills the gaps in the trails created between frames.
If we chose to upload to the software the foreground frame, the image received is our final result and the only thing left to do is some light cleaning in Photosop. If you look closely, you will notice a dot before each trail. These are the stars captured in the foreground frame. I, personally, do not like them and erase them one by one.
If you left the foreground frame out, now is the time to combine it. Upload the trails frame and the foreground frame into Photoshop as layers and merge them together using masks.
That is it for now. Do not forget to visit my site for updates http://www.amirehrlich.com/