I don’t live in one of the best locations for astrophotography, but when we have some clear skies in my part of the UK, I like to head out and capture star trail images—a series of long exposures that gradually capture the earth’s rotation. The premise is that you capture anywhere from 20 to 150 long exposures, each one recording a small amount of star movement. In post-production, you then stack these images and expose the brightest pixels, producing a unique circular effect.
Watch the video above for a comprehensive, 10 minute practical guide on setting up your camera to capture these ‘star trails’, featuring live footage of the images being taken and a guide on how to find the Polaris star using smartphone apps.
Practical: planning and shooting
You generally frame your shot to include the Polaris Star (otherwise known as the North or Pole Star). It lies almost in line with the axis of the Earth’s rotation, so whilst it does move slightly between shots, all the other stars appear to ‘rotate’ around it, producing the swirling effect that has proven so eye-catching.
While my astrophotography knowledge isn’t vast, I’d like to share with you some tips and pointers if you decide to head out on a clear night and capture some star trails.
Without a doubt, you should definitely have some preparation beforehand if you’re going to tackle this type of shooting. This includes:
- Wrapping up warm. Even on nights where the air doesn’t seem too chilly, you’ll be doing a lot of standing around, which means you can get incredibly cold. Put on multiple layers, invest in some gloves—give yourself the best chance at keeping warm. You can also carry disposable hand warmers, which double up as a tool to stop your lens from fogging up (more on this later).
- Charge your batteries. Don’t venture out with a couple of batteries at 50% charge—you’ll be lucky if they last. Long exposures mean your camera is constantly capturing images for hours—sometimes longer. There’s nothing worse than a battery going empty halfway through your 120 image capture session.
- Scout your location beforehand (or use a mapping system like Google Maps). If you’re going for the classic swirling star trail look, you need to point your camera towards the Polaris Star. Being restricted to this direction will influence the composition if you’re hoping to capture foreground detail too. There might be a fantastic building or structure you want to include, but if it’s not in the same direction as the Polaris Star then it won’t work. Bear this in mind when picking your location.
In terms of equipment, you’ll need the following:
- A camera with support for long shutter speeds up to 60 seconds (the majority of cameras with a Manual shooting mode should have this).
- A sturdy tripod to eliminate wind shake and ensure consistency between shots.
- An intervalometer (either external or built into the camera) or a remote shutter with a ‘locking’ function.
- A wide angle lens. Ultra wide angle lenses with large apertures (e.g. f/2 or f/2.8) are preferred but can be expensive. If you don’t have a wide angle lens, go out and get some shots regardless! (You can get some creative results with a narrower field of view, seen later in this article).
Once you’re on location, you need to locate the Polaris. To help with this, you could use one of the many available smartphone apps which can use your phone’s GPS and give you a 3D representation of the solar system. SkyView, for example, allows you to search for the star by name and will give you an on-screen marker to help you along.
Shooting the star trails
With the Polaris Star located, you can then set up your camera and tripod. Here are some pointers for camera settings:
- Shooting mode: shoot in Manual (M) mode so you can control the aperture and shutter speed individually. The aperture generally wants to be as wide as it will go (for ultra wide lenses this will be f/2.8, f/2 or in some cases f/1.8), and the shutter speed will be a value suitable for long exposure shooting—usually 60 or 30 seconds.
- Focus: Manual focus is a must for this type of shooting. Focusing in the dark is tricky, however; even more so if you’ve deliberately picked an area with minimal light pollution from nearby cities, towns or villages. If you’re using a camera that has live view, it will typically have some kind of magnification or focus peaking that can help you nail focus. In most cases, however, you can generally set the focus distance to infinity on your lens and this will provide a sharp result. Be wary if using a zoom lens: my Olympus 12-40mm is tack sharp at 12mm with focus set to infinity, but once you start to zoom in the focus needs finer tweaking to maintain sharpness.
- Composition: how you frame your shot is up to you. For the example shown in this article, the shot has been composed so that the Polaris is almost dead centre. You can, however, frame it to the side or even corners of your image. In one example which you can see at the end of the article, the lens has been zoomed in and excluded the Polaris entirely to capture some abstract star trail motion.
- Shutter speed: this is important, as it will influence how many shots you need to take. 30 seconds seems to be quite a common figure, but you could also try 60 seconds. There are no hard and fast rules about the shutter speed you use, but you’ll want some motion blur appearing on the stars, so a minimum of 30 seconds is recommended.
- ISO: the most common ISO seems to be around 800, which is sensitive enough to capture the stars and some background detail. In all honesty, however, you could try sticking to 200. The exposure looks worryingly low in-camera, but don’t forget that you’ll be stacking around 60 (if not more) images together. This, combined with working in 16-bit precision, means you can get clean and workable results.
Once your camera settings are all correct, you’ll then want to trigger continuous shooting whereby the camera captures exposures continuously one after the other. My camera has a built-in intervalometer, where you can set the interval between shots (as short as possible) and the number of shots to take, then set it going and leave it.
If you don’t have a built-in intervalometer, you can also use a separate intervalometer device which will provide the same functionality. A more low-fi (and cheaper) approach would be to use a remote shutter with a lock switch. For this approach, you would simply put your camera in manual mode, ensure you have the correct shutter speed (e.g. 60 seconds), press the remote shutter’s button in and then flip the lock. This keeps the button depressed which sends a continuous signal to the camera, so it will keep shooting exposures.
For most shoots, you’ll want to capture 60 exposures if not more—sometimes nearer 80. Do bear in mind, however, that this is a recommendation if you’re using 60 second exposures. For 30 second exposures, expect to double that number (so you would shoot a minimum of 120 exposures). Of course, don’t take these numbers as gospel; experiment and see what number works for you. Perhaps you’ll find you can get away with fewer exposures, or you might even prefer to shoot more.
An issue that plagues astrophotographers is the dreaded lens mist. After a while, the temperature of the lens will drop below the ambient temperature and dew or mist will begin to form. This often tends to creep up on you halfway through shooting the images, and whilst you can still get usable results, it’s a shame to invest the time and effort into setting up a shoot only for half the images to be compromised.
Thankfully, there are ways to combat this. One is simply to leave your camera outside in the cold air (preferably in a bag) for longer, meaning it will acclimatise to the outdoor temperature. Another is to use some form of heating around the lens—a cheap and cheerful solution is disposable hand warmers! Simply activate them, then fix them to the underbarrel of the lens using rubber bands. There are products available that hold the hand warmers in pouches which wrap around the lens in a bid to improve the heat efficiency.
The editing stage is just as important as the shooting of the images. Having shot RAW, we can pre-process these images into 16-bit TIFFs before dropping them into a stack for editing. For this I’ve used Olympus Image Viewer (the camera manufacturer’s own software) to get the best results, and have added some very light noise reduction. 16-bit precision is important because we’ll likely be doing some tonal stretching and filtering like adding a radial blur—this kind of work falls apart very quickly in 8-bit with posterisation and banding becoming problematic.
Stacking, merging and retouching
Once the TIFFs are ready, we can go to File>New Stack in Affinity Photo, add the images to the file list, and uncheck Automatically Align Images (since aligning all the stars to each other would defeat the purpose of our objective). Clicking OK will then create a new document with all the images placed into what’s called a Live Stack Group. From here, all the work is basically done for us—we just need to change the stacking operator to Maximum, which will expose the brightest pixels and give us our initial result:
This is a respectable starting point, but we need to put in some work to really make it pop. Before doing anything else, go to Layer>Merge Visible, which creates a single pixel layer composite of the stack. You can then uncheck (hide) the initial stack in the Layers panel. This is for performance reasons—things are much smoother working with just one composite layer, as Photo doesn’t have to constantly redraw the whole stack of images.
We also have some retouching to do—the Polaris Star isn’t actually at the centre of the trails, nor is it sharp. This is because it also moves with the Earth’s rotation, just by a lesser degree compared to the other stars. To fix this, what we can do is duplicate one of the images from the stack, drag it above the merged pixel layer and mask just the Polaris from it. We can then position this dead centre and have a strong Polaris Star.
Photo’s Inpainting Brush is useful for getting rid of the original Polaris trails. You can do this non-destructively by creating a new empty pixel layer, then setting the Inpainting Brush to sample from Current Layer & Below.
After that, we can do another Merge Visible operation to create another flattened pixel layer. On this new layer, let’s apply a Radial Blur filter using a small value, and set the Polaris as the centre point. This helps to smooth out the jagged appearance of the star trails and makes them look more pleasing to the eye.
Once all the retouching is out of the way, we can move onto the tonal adjustments. I tend to work a lot with Brightness/Contrast, HSL and Selective Colour adjustments in my work, and I’ll often use multiple variants of each as I build up the layer stack, tweaking tones as I go. A simple Brightness/Contrast adjustment usually comes first, to boost the image’s tones, followed by an HSL adjustment; this is typically to single out certain colour ranges and increase or decrease their intensity, sometimes tweaking their colour hue as well. In this image, for example, I’ve reduced the Yellow saturation entirely and shifted the hue of the Cyans to lend the image a deep blue tone.
Further to that, we can use a Selective Colour adjustment followed by another HSL adjustment to exaggerate the blue tone even further. After that, let’s take advantage of Photo’s unique Live Filter Layers by adding a live Unsharp Mask filter as a top level layer. By using a large kernel value of 100px, this enhances local contrast within the image, which is perfect for making the star trails ‘pop’. If the effect is a bit strong, you can always tame it by reducing the layer’s opacity—I’ve reduced it to 50% here.
We’ve saved one of the most dramatic enhancements for last! In order to break up the patterning of the star trails slightly and focus the viewer’s eye on the Polaris Star, we can add a gradient fill with a blend mode (tweaking the opacity to taste too). Photo is great for this approach because you can do it non-destructively via the use of a Fill layer (basically a vector layer). Applying the Gradient Tool on a Fill layer allows you to jump back in and edit the gradient at any point in the future.
Here, I’ve used a Radial type fill for a blueish centre that radiates out to a murkier green. Combined with a Screen blend mode, this produces a kind of misty overlay. To combat this, we can usually add a final Brightness/Contrast adjustment to really bring the image to life. The final step is to crop using a Diagonal guide, which helps ensure the Polaris is the centre focal point.
That concludes the typical editing for a star trail image.
Related tutorial videos
These videos are produced by James as tutorials for Affinity Photo and are hosted on Vimeo and YouTube—they focus on post-production techniques for astrophotography. Don’t forget to check out the other tutorials (there are over 200!) as well.