The Making of Star Trails over Old Faithful
March 22, 2016
I began planning my image Star Trails over Old Faithful about five months in advance. First, I booked four nights at the Old Faithful Snow Lodge during the period surrounding the February 2016 full moon. Next, I began solving the shoot’s technical challenges. The first was exposure. I wanted to capture the geyser at its full height, lit only by moonlight. Too short an exposure, and the geyser would be under-exposed. Too long, and the geyser would only be at full height for a fraction of the exposure time. The National Park Service Yellowstone website said that a typical Old Faithful eruption lasted from 1 ½ to 5 minutes – a rather broad range. With eruptions occurring only every hour and a half, an exposure strategy of guess-and-check sounded very inefficient. Then I remembered that I had snapped a few frames of an Old Faithful eruption during a brief family vacation the previous summer. By looking at the time stamps on the photos, I was able to determine that Old Faithful’s peak height lasted only 10 seconds or so.
That gave me an exposure time; what about the aperture? I wanted to keep the ISO to a minimum to keep noise under control. That dictated an aperture of f/2.8, the widest aperture on my Canon 16-35mm lens. The final parameter was ISO. I knew that the geyser would be white, as would the snow surrounding the geyser. That meant that I could shoot test frames before the eruption to determine the correct ISO to properly expose snow at 10 seconds, f/2.8, and use the same settings for the eruption. Testing once I was on location showed that an ISO of 1250 would give me correctly exposed snow under a full moon. I confirmed that exposure for the geyser with test shots taken during a partly cloudy night when it was impossible to shoot satisfying star trails.
My normal approach to shooting star trails is to use an exposure of four minutes, f/4, ISO 200. Using that approach for Star Trails over Old Faithful posed a problem, however. An exposure of four minutes would probably turn the geyser into an amorphous blob of steam, which meant I would have to shoot the geyser and star trails separately, then combine them in Photoshop in a believable way – a difficult task. I decided that a better approach would be to shoot back-to-back frames at 10 seconds, f/2.8, ISO 1250. I’d start the sequence some time after an eruption, continue through the next eruption, then end the sequence after about two hours, which would be long enough to create graceful star streaks after I combined all the images.
I was fortunate to have one completely clear night in Yellowstone, which allowed me to execute my shoot just as I’d planned it. When I returned home, I began experimenting with ways to combine the images. The easiest approach to revealing all the star trails would have been to stack them all in Photoshop and change the blend mode to Lighten. The Lighten blend mode looks straight down through the layer stack from each pixel and reveals only the lightest pixel in the stack. In this case, the lightest pixel would have come from either steam or a star streak. (The land was essentially the same in every frame, so it didn’t matter which pixel was revealed.) With 615 images in the sequence, however, that approach was impractical. The layered file would probably have been about 90 gigabytes. So I used Floris van Bruegel’s free Photoshop star-stacking script, which creates an effect similar to the Lighten blend mode while eliminating the tiny gaps between individual star streaks that Lighten creates.
I performed an initial test and immediately spotted several problems. The first was that Polaris was obscured by the steam column in the frame with the biggest plume; the second was that stacking 615 images of steam meant that the plume of vapor was rendered as a featureless white; the third was that the star trails were showing through portions of the steam plume, which looked rather odd.
I picked out one favorite image showing the geyser and steam plume in the ideal position, selected the steam using the Quick Selection tool and Refine Edge, then created a Photoshop action that loaded the selection onto an image and filled the selection with black. I then ran that action as a batch on the folder of images (excluding the frame with the ideal geyser plume). The action effectively eliminated nearly all of the unwanted steam plumes from the final image. I then located all the images where the steam plume extended beyond the selection boundary and used the Brush tool to paint with black over the remaining bits of unwanted steam. I then re-ran van Breugel’s star-stacking script and created an image that effectively combined a single geyser image with star trails from all 615 images. This complex procedure had one drawback: it created a subtle halo effect around the plume. I eliminated the halo by using the Clone Stamp tool to retouch the edge of the plume and finally declared Star Trails over Old Faithful complete.
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