Over the Christmas holidays the thought occurred to me that it might be fun to try shooting some wide star trail panoramas – wide enough perhaps that the stars could be seen rising on one horizon and setting over the other. A minute with google was enough to demonstrate that this was not a new idea – Vincent Brady has been shooting what he calls “planetary panoramas” with four cameras bolted to a custom rig. Nonetheless I haven’t seen that many star trail panoramas, no doubt because it requires multiple cameras and lenses, and it seemed that there might be some interesting images to be made with this technique in Colorado.
Having now tried a few it’s time to collect some tips and lessons learned. There are two parts to creating a stitched star trail image, the image capture and the post processing, and although both are pretty straightforward as always there are things to keep in mind.
You don’t need identical cameras and lenses. I’m sure it would be better if you had duplicate equipment, but I’ve gotten by fine with a combo of a Canon 5D3 / Zeiss 18mm and Canon 7D / Canon 10-22mm (shot at the wide end, i.e. at 16mm equivalent focal length). Modern stitching software can handle it.
The cameras need to be mounted as close together as possible, especially if you have any foreground. I’m using a Novoflex QPL 350 rail to mount both cameras to an Arca-Swiss mount on my ballhead. It’s a reasonably solid arrangement, though in high winds I’m seeing a little bit of flex visible as slightly wiggly trails viewed at 100%.
Having two cameras bolted to a single rail is optimal if the image you’re after (like the one above) has the horizon midway through the frame. Often you’ll instead be canting the cameras skyward to capture more sky and less foreground. In this case I think the rail is not optimal – it would be better to have each camera independently leveled against the horizon. This would need a rail and two heads, which I haven’t tried yet.
The simplest approach is to start shooting continuously as soon as it gets dark enough to see stars, but while there’s still enough light to capture the foreground. At Colorado’s latitude, with exposures of 30s, f/3.5 at ISO 1600, that works out to be about an hour after sunset. It’s pretty gloomy by then, so you’ll want to set up well in advance.
Check how good the edge resolution of your lenses is when shooting stars. My Zeiss 18mm is very nicely sharp at the edges wide open, the Canon 10-22mm (also a good lens) less so. It may be advantageous to stop down one stop or so for optimum quality of trails.
The basic concept is that the stacking software chooses the brightest pixel across the stack of images, so if you start as the light is fading the first frame will provide all of the image apart from the stars and any moving elements such as bright clouds. It’s fine then to change exposure parameters between the first frame and subsequent ones. For example, if you need plenty of depth of field, you could start with a single f/16 frame, then wait till it gets dark enough that a wide open image is darker and continue from there.
Long exposure noise reduction (dark frame subtraction) needs to be off.
I use lockable wired shutter releases and my fastest cards to keep the cameras shooting with as little gap as possible between frames. Over an hour or two, you’ll find that one camera is a little slower and a noticeable lag develops. This is fine, but to make the stitching as accurate as possible try to end the total exposure at the same moment (I put a lens cap on the “slow” camera just as the quicker one finishes its last frame).
The basic workflow here is (1) process individual frames for white balance and noise reduction, (2) stack left and right images independently, (3) stitch together, (4) adjust final image. This is going to involve quite a bit of destructive editing and reprojection of the image, and since you’re starting with long exposures that haven’t had the benefit of dark frame subtraction a bit of care is needed. The good news is that you’ll be ending up with a lot of pixels in the final panorama, so even if it’s not perfect at 100% a large print (never mind a version to stand tall on Facebook) can still look good.
I start by applying a consistent white balance to all the images, along with a first pass at global adjustments (shadows, highlights, saturation etc). It may be helpful to brighten the foreground an extra stop, this makes stitching later easier and you can take that stop back in a final process of the image. Don’t overdo sharpening at this stage (I export from Lightroom with sharpening set to “low”) – there will be time for that later. Correct for vignetting but leave the distortion as is… the stitching software can handle it and there’s no point interpolating the pixels more times than necessary. Be wary of using gradient filters at this stage as they may not line up perfectly between left and right images.
Camera lenses are marked in f-stops rather than T-stops (transmission). This means that even if both cameras shot at the identical exposure, one image may be brighter than the other if you’re using two different lenses. You need to measure this difference and correct it at some point, and it may as well be done now.
Different cameras / sensors will need different levels of noise reduction. My 7D is quite a bit noisier than the 5D3 at ISO 1600, so in Lightroom I’m using something like 25 luminance noise reduction for the 5D3 but 40 for the 7D.
Hot pixels can be a problem. In the sky these need to be dealt with manually, but in the foreground you can take advantage of the fact that only the first frame is going to count toward the stack. I mask out the foreground in all but the first frame and push it 8 stops toward black to suppress hot pixels.
I use StarStax for stacking. It’s free, fast and works well. This step is straightforward.
Once you have the “left” and “right” stacks, it’s time to stitch them. I use Autopano Pro, which is not free but which works unbelievably well for almost all panoramas. Except these. I don’t find I’m able to stitch these super-wide star trail panoramas automatically. Instead I use a two-step approach. First I add back the EXIF data on focal length (which won’t be output in StarStax .tif files) and have a go at an automatic stitch. If you’re lucky the mid-ground (e.g. the mountains in the image above) may stitch fine, but the sky won’t. You’ll need to add maybe 20 manual control points to the sky, distributed horizontally and vertically across the region of image overlap, to get a good stitch. Finding them is pretty easy – planes with flashing lights (which make trails of dots across the image) are probably best, but the start and end of the trails work too as long as you remembered to end the exposures at the same time. Once the sky and mid-ground are set, the foreground normally falls into place automatically.
With the image successfully stitched, you can import back into Lightroom for final adjustments and sharpening!
All of the above is pretty obvious, but there are enough steps that I’ve screwed up a fair few times. Hopefully I now have the mechanical part sorted out, so the next step is to find locations where the technique makes for a good image…