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| Star Trails Tutorial |
Processing and Stacking Multiple Exposures:
So, now you have a set of 20-30 exposures, but you need to assemble them into one image. This is where things get complicated, and attention to detail is important if you’re interested in the highest quality. The simplest method is to open all your files in photoshop and stack them in ‘lighten’ blend mode. There’s an easy way to do this if you have CS3 or higher:
1. Process your raw files (or jpegs), and save them to a folder
2. Open Photoshop
3. Navigate to File-]Scripts-]Load files into stack…
4. Select the files you wish to merge, and select ‘Create smart object after loading layers’
5. Then go to layers-]smart objects-]stack mode-]maximum
6. (to get rid of the smart object and get a regular layer you can flatten the final image)
Essentially, for each pixel in the image, photoshop will choose the brightest pixel of all the layers in the stack. Since the stars are the only things changing (usually), this will simple draw out your star trails. But now, if you zoom in to 100% you will see gaps, like this:
32x 5 minute exposures taken at iso 400, f/5.6, with the Canon 5D and Sigma 15mm f/2.8 fisheye lens, stacked using lighten blend mode.
After you resize to a normal web size (800 or so pixels in the longest dimension) you won’t really notice them, but in a nice print you will. You could sit there and use the clone tool to fix every gap, but if you have 25 exposures and 100 bright stars, that’s already 2,500 clone fixes – not something you want to do. There is one other possibility that sometimes works (only if you shot the circular pattern of the stars rotating around Polaris):
1. Duplicate your flattened image
2. On the duplicate image, select the entire image with the crop tool
3. Move the center of the crop box (there is a little cross hairs) to the center of rotation of your star trails. Choose this very carefully
4. Move your mouse to one of the corners, and rotate the image just a little.
5. Select this image, and paste it on top of the original
6. Set the blend mode to ‘lighten’
7. Crop a little, or use the clone tool or layer masks to get rid of the white edges in the corners
This only works if there is hardly any distortion in the picture, which doesn’t really happen with wide angles. It’s also really hard to get the center of rotation just right.
Naively you might think that those gaps are there because of the slight break in time between exposures. For a long lens you might be right, but for an ultra wide angle, 1 second is insignificant. The gaps are there because of the blend mode, by using the ‘screen’ blend mode the gaps disappear! But, then the sky becomes much too bright, so the key is to use a clever combination of the two.
Lighten Screen Blending (LSB) – Advanced Multi-exposure blending:
This method overcomes many of the shortcomings of just using ‘lighten’ layers. It works best if you shoot raw. First, lower the exposure by 1-stop (alternatively you can do this in photoshop after you’re done). When doing your raw conversion use a ‘linear’ conversion. That means you will do no curves, levels, or contrast adjustments in your RAW conversion (for ACR set all the values except white balance and saturation/vibrance to zero, make sure to set the tone curve to linear as well). Otherwise you will be affecting each end of the star trail, making the gaps more pronounced. Save the files as Tiff or psd in a new folder.
Load your files into a stack (using file-]scripts-]load files into stack), but do NOT convert them to a smart object. Now you’re going to perform a little algorithm, alternating the use of screen and lighten layers strategically to get rid of those little gaps.
Now suppose you have the following layers:
layer-1, layer-2, layer-3, layer-4
You want to duplicate each layer, except for the first and last:
layer-1, layer-2-copy, layer-2, layer-3-copy, layer-3, layer-4
Now set layer-1 to blend mode ‘screen’, select it and layer-2-copy, and merge them (select both and hit command e, or go to layers->merge layers). This is why we reduced the exposure earlier, because now you’re essentially adding exposures. Set this new layer to ‘lighten’ blend mode.
Now do the same for each successive pair, so set layer 2 to blend mode screen, and merge it with layer 3-copy, then set the new layer to lighten.
After doing this for all your layers you will end up with a stack of layers all set to ‘lighten’ blend mode, and by strategically using the screen blend mode we have made sure each gap is filled in.
Now flatten the image and go ahead and do your normal processing. The result? Here you go..
Same files as before, processed at -1 exposure in raw, and blended using the LSB method. No further adjustments. I don't really understand the other gaps and weird star lines, they're long than a single exposure, some I'm not sure what's happening.. The two gaps per trail you do see are when I had to replace my battery.. I need to go in and fix those all, oh boy!
If you could have a low noise 2-3 hour exposure (ie. on a very cold night, or some time in the future with better cameras), you’d be better off doing that, why waste your time processing?
A note on dark frames:
If you want to get into astrophotography, I recommend you do some more research, and read a book. There are many techniques that are much better for getting detailed images of stars (not star trails), including the use of dark frames. I have not found dark frames to be useful with my cameras and techniques, but you might find them useful – I recommend you do a search on google.