Back in film days, I tried doing star trails. I didn't have a good view of the sky from my yard in Massachusetts, so one night in 2004 I set up my my medium format camera in a wildlife refuge some distance away. It is hard to experiment with film because it is one long exposure, and it takes at least a day to develop the film. For medium format slide film, I used to mail that in to Adorama and turnaround took at least a week. If a plane flies across the sky during your exposure, that's in your image. With digital, the usual technique is to take many shorter exposures and combine them. Plane lights can be edited out of individual images.
With the arrival of the 14mm lens a few days ago and with a clear view to the north from my Montana yard, I decided to give star trails a try last night. Using the built-in intervalometer in the Canon 6D Mark II #12, I took 271 30-second images between 9:39pm and 12:08am. (It's actually 32 seconds, from what I have read.) As the first image was being captured, I set off my flash unit several times, which is why the trees and part of the lawn are lit up.
In Photoshop, I stacked the images, set all of the layers to "Lighten," flattened the image, ran it through the RAW filter to bring out more color and detail, and cropped it. My neighbor's house lights were on for only two images right around midnight, so I edited those two files to take the lights out. The process is ridiculously easy. But it takes a few minutes for the computer to do the stacking and lightening. I tried to save the file before I flattened it, and after an hour Photoshop told me the file exceeded 2Gb and could not be saved as a PSD. I was supposed to save it as PSB, which is large document format. So, bottom line is it is an easy but time consuming process. Not bad for a first attempt, IMO, but now I have to find a more interesting foreground.
If one doesn't have the full version of Photoshop, there is a free program called StarStaX which will accomplish essentially the same thing. One downside is StarStaX does not read RAW files, so they have to be converted (JPG or TIF) before starting the process. I didn't explore all of the features, but it works and it is interesting to watch the process as it builds the image. I tried "Save after each step" to assemble the images into a video showing the trails growing. I tried both Premiere Elements and Photoshop to make the video but there were some gaps in the trails that showed up during playback. The individual frames look OK, so maybe it needs to be encoded differently. Or maybe I can be happy with the single still image that I got. From exploring this over the past few days, you can make this as simple or as complex as you want to.
As mentioned, the 6D has a built-in intervalometer, but there are some limitations. The number of exposures can be set as high as 99, or unlimited. I set it on unlimited and had to go out there at 12:08 to stop it. And the maximum exposure that can be set on the 6D is 30 (or 32) seconds. What I would like to do is use the 5D Mark III (which does not have an intervalometer), set the exposure to one minute and let it run for six hours, which would be 360 exposures. I found a fairly cheap ($20) device on Amazon that will allow me to do this.
And one more thing: If you have the lawn sprinklers set to run overnight, remember to turn them off. The back of the camera (fortunately not the lens) got a few drops on it before I stopped them.
Update: I was checking my camera settings and realized I actually shot the images in Aperture Priority mode rather than Manual. As a result, the exposure time was 25 seconds, not 32. With the intervalometer set at 33 seconds, there is an eight-second gap between images. If I zoom way in, there is a noticeable gap between the star images. I'm going to try again tonight with the Manual setting and get that gap down to one second, which hopefully is not detectable.
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