Star Trails over Cochrane
Our home city, Cochrane Alberta, creates it's own share of light pollution in the prairies of the Canadian south west, but a good fast lens, tripod and a little patience will still capture some nice star trails in the skies overhead. The adjacent image was only the second star trail shot I ever made but as you can see, good results are possible with very little experience of the topic.
Fortunately our house on the ridge overlooking the city aligns nicely with 'Polaris' (the north star) which means I didn't have to go far to get the composition, in fact I only went as far as the garden fence. You can see the effects of light pollution in the shot as the number of visible stars diminishes closer to the city but the brighter ones remain. There was also an issue with aeroplanes low on the horizon which had to be removed from each shot individually but as you'll see from the description below it's not as arduous a process as you might imagine. |
All in all the making of a star trail shot is a straightforward process which does deliver images beyond the 'norm' and in my opinion is worth the short post processing time required to deliver the finished product. There are online programs that offer to do the stacking for you such as 'DeepskyStacker' and 'StarStaX' but I didn't use these as I have Adobe Bridge and Photoshop and these are quite capable of doing the same thing without having to mess around uploading hundreds of images online.
Photographic Gear Used:
Canon 5DIII DSLR Camera Canon EF16-35mm f/2.8L II USM Lens Tripod Remote Trigger |
Camera Settings Used:
Focal Length: 16mm Aperture: F/2.8 ISO: 100 Shutter Speed: 30 seconds (136, 30 second images) |
In terms of gear it really doesn't matter too much, any modern camera with manual controls will be capable of making a shot like this with a good fast lens. The tripod is just a tripod, again make doesn't matter, if it holds the camera securely then it's good. Even sitting the camera on the floor or a wall would work too, as long as it remains in the same place for all of the shots it really doesn't matter. The remote trigger makes things easier but you could tape down the shutter button and get the same results (though the first shot as you stick the tape on and the last as you take it off would be spoiled).
Start by ensuring you have clear skies for the duration of your shot (or at least few clouds) If you live in North America there is a very good clear sky chart available online for free though it may seem a little daunting at first it's easy to learn and incredibly useful. Learn how to identify Polaris in the night sky if you want to centre the shot to make full circles. Using Ursa Major (the big dipper) as a pointer is an easy way to find Polaris, again there are many websites which will help you do this.
Start by ensuring you have clear skies for the duration of your shot (or at least few clouds) If you live in North America there is a very good clear sky chart available online for free though it may seem a little daunting at first it's easy to learn and incredibly useful. Learn how to identify Polaris in the night sky if you want to centre the shot to make full circles. Using Ursa Major (the big dipper) as a pointer is an easy way to find Polaris, again there are many websites which will help you do this.
Focussing the camera is often the most difficult task during darkness, with an illuminated foreground such as the city in my shot there's no such trouble but if it is dark and you have no lights to focus on, use the infinity setting on the lens, make an image and then zoom in on the image playback and ensure you have the focus correct, if it's not right just dial back from infinity a tiny amount and try again, a couple of shots should be enough to find the right focus.
Make sure your tripod is firmly held and will remain so for the duration (from an hour to several hours), make sure your battery has sufficient life left in it, set your camera to manual mode so the exposure remains the same throughout and then set up the best exposure you can achieve (depending on lens) and make a couple of test shots. Obviously the faster lens you can get the better, so a lens capable of f/1.4 can get away with a much lower ISO than a lens only capable of f/4 though modern cameras will produce clean images at huge ISO's, lower is always better. I maintained my ISO at 100 in order to get the 'cleanest' (low noise) shot possible and with f/2.8 available it was easily enough to capture the stars. The drive mode on the camera needs to be set to continuous so you can take consecutive shots then, with the remote shutter fired up (or duct tape on the button) simply let the camera do it's thing until you have a hundred shots or more, the image above was made from 136 images, at 30 seconds per image, that is over an hour of shooting so remember to check the battery life before you start.
Post Processing
Once all of the images were imported into Adobe Bridge I opened just one single image and edited it to ensure I achieved the look I was seeking. Once I was happy with that one image I closed it and selected all the others in one go (without opening them) I right clicked on the images and selected 'develop settings' and then 'previous version' from the list, this will change the remaining 135 images to match the one just edited.
With all images then edited and still selected I went into the 'Tools' menu and down to 'Photoshop' on the drop down list, from here select 'Load files into Photoshop layers'. This step can take some time depending on your system but once done you can now go into Photoshop to work on that huge file.
If you have no airplanes crossing the image then you can select all layers and set the blend mode to 'lighten', this will blend the images keeping only the light areas which are your stars, as they blend the movement becomes apparent. You can then finished off any other editing you want to do before saving your wonderful star trail.
If you do have aeroplanes crossing the shot it is a bit more of a pain, you have to find which images have the aeroplane light streaks and paint over them using black as the paint colour, when you blend using 'lighten' these areas will be eliminated. If you are struggling to find the plane trails you can turn off each layer until they become apparent, it's a bit of a pain but worth the effort for the finished product.
Once merged you may find that your foreground is too light, if this is the case simply open any one of your single images and align it on top of the stack, set the blend mode on it to 'normal' and then either mask off the sky (if you are familiar with photoshop masks) or simply erase the sky section of the single shot leaving your foreground exposed as you wish and your star trails visible above. That's it, flatten the image and save as the file type you want.
Aeroplane trails can be a real pain in the butt, this image shows my initial findings on merging 136 shots (click for larger version). I then had to chase down the offending frames and paint out the unwanted lights. |
This image was my first ever attempt at a star trail shot, as you can see there is no foreground but it is still nicely centered on Polaris. What is noticeable is just how many more stars are visible when light pollution is lower. I hope to try this technique again, perhaps in the mountains, and make more of these interesting images. As always I will continue to experiment a little with it and see what I can produce. |