Star Stacking for Pinpoint Stars and Low Noise
How to get clean Milky Way photos
The Milky Way rises over a fairly intimate canyon view, Canyonlands National Park, Utah.
Nikon D810A & NIKKOR 14-24mm f/2.8 lens @ 14mm for all shots.
Sky: Star stack of 10 shots for low noise, all at ISO 10,000 (way higher than needed I later learned).
Foreground: 2 separate exposures: I used one foreground shot at ISO 1600, f/2.8, for 15 minutes focused for the stars to get most of the landscape in focus (with f/2.8 at 14mm a lot is in focus), and I took another foreground shot at ISO 3200, f/5.6, for 30 minutes to get more of the close foreground on the left in focus. I then blended the resulting star stacked sky and the foreground exposures in Photoshop to create an image that has low noise and good focus from the foreground to the stars.
Introduction
If you’ve done any night photography then you’re likely very familiar with the noise of exposures in low light with a high ISO. But most of the noise is actually caused by lack of light hitting the sensor, not the ISO, and can be improved with a longer shutter speed to capture more light, boosting the signal to noise ratio.
However, when shooting the Milky Way you’re usually going for sharp non-trailing stars which means you’re limited in how long you can expose before the stars appear to move (trail) too much in the frame.
If you try shooting with a short enough shutter speed to capture completely motionless stars, the noise level can be so high that the exposure is either unusable or needs a ridiculous amount of noise reduction, resulting in large size prints that can look very grainy or blocky due noise or noise reduction. Noise reduction tools these days are great, but they can’t fix everything all the time.
Fortunately there are ways to capture both pinpoint stars and achieve low noise. You can either use a star tracker with very long shutter speeds to capture a lot of light and thus a high signal to noise ratio, or you can use star stacking, a method of capturing multiple photos with short shutter speeds that are then aligned and averaged in software to reduce noise. Or you can combine both methods.
Noise & Star Examples
There are 100% crops of shots from a Nikon Z6 with the NIKKOR Z 14-24mm f/2.8 S lens. All shots are at 14mm, f/2.8, and ISO 6400. They have been adjusted to similar brightness levels in Lightroom to show detail.
14mm @ f/2.8, 30 seconds, ISO 6400 — Reasonably low noise but streaky stars.
14mm @ f/2.8, 10 seconds, ISO 6400 — Pinpoint stars but lots of noise.
14mm @ f/2.8, 10 seconds, ISO 6400, stacked result of 20 exposures — Pinpoint stars and low noise. The best of both worlds!
Notice how the top image at 30 seconds has reasonably low noise but the stars are streaking. Lowering the shutter speed to 10 seconds (second image) gives pinpoint stars but the lack of light results in a lot more visible noise. But if you take 20 of these 10 second exposures in a row, and then stack them in software, you get the result in the third image — pinpoint stars and low noise. This is the benefit of star stacking!
What About a Star Tracker?
Before we get into star stacking, you might be wondering if you could just us a star tracker, a device that sits on top of your tripod and turns with the rotation of the earth so that your camera can follow the stars, capturing long exposures with no star movement.
This is fine if you’re just doing shots of the sky without a foreground, but if you’re capturing the foreground then it will blur in the tracked exposures, so if you want a sharp foreground you’ll need a separate exposure (or more than one) of the foreground with the tracker turned off. I do this anyways even without a tracker so that I can get detail and low noise in my foreground. Blending the static foreground shot with the star tracker shot would require dealing with blending the blurred foreground of the tracked shot with the sharp foreground of the static shot. So, the blending gets more complex.
Also, there are limited benefits to using a star tracker for wide angle astro shots. Stacking 20 shots of 10 seconds each will result in roughly the same image as if you had taken a single shot for 200 seconds with a tracker (20 shots x 10 seconds = 200 seconds). The stacking shots combine in software to produce an image as if you had the shutter open for the entire accumulated shutter speed (amount of light hitting the sensor). That said, a tracker is required when you are using long lenses and shooting deep space objects that require the camera to move in order for the object to stay within the field of view.
What is Star Stacking?
Star stacking is a method of overlaying multiple night sky exposures of the same composition, aligning the images so the stars all line up between each exposure, then averaging the exposures, producing a result with far less noise than a single exposure. The noise differs between each exposure, and even from pixel to pixel in the same exposure the noise differences can be high, so the averaging process greatly reduces that variation, resulting in a much smoother sky.
You can extend this to get pinpoint stars by using a short enough shutter speed for your given focal length so that the stars do not appear to move in the frame. So you get the best of both worlds, pinpoint stars and low noise.
Low Noise Foreground
The star stacking method reduces noise in the entire image, not just the sky. Even though the sky is aligned separately from the foreground in the stacking process (when using Starry Landscape Stacker or Sequator, details below), the foreground area can still be stacked and averaged to reduce noise. Depending on the ambient light of the scene, this might mean that you don’t need to do any separate foreground exposures to get a foreground that has detail and is in focus with low noise.
But, where I’m often shooting, this isn’t normally the case. I still will take separate foreground exposures at a lower ISO (normally ISO 1600), for longer shutter speeds (usually multiple minutes per exposure), and when needed I will change focus and take multiple foreground shots to get everything in focus.
In the image above you can see how stacking greatly reduces the noise in the foreground part of the image. The star stacked result is significantly cleaner and has more detail. However, the foreground is still too dark and too noisy because there simply just isn’t enough light collected from the foreground in 10 seconds, and while stacking cleans it up, it can’t bring out more detail that wasn’t captured in the first place. So I would take another exposure at ISO 1600 for several minutes, and blend that cleaner, brighter result with the star stacked result of the sky in Photoshop.
Milky Way Over the Coast of Maine
Nikon Z6 with NIKKOR Z 14-24mm f/2.8 lens @ 14mm and f/2.8 for all shots.
Sky: Star stack of 20 exposures at ISO 6400 and 10 seconds each.
Foreground: Single shot at ISO 1600 and 10 minutes.
Exposure Settings for Star Stacking
Shutter Speed
Pick a shutter speed that will capture pinpoint stars with the focal length of the lens you are using. There is a complex formula, known as the NPF Rule, that can be used to calculate this with respect to your location on earth, the direction your camera is pointing, your focal length, and the size of the photosites on your camera’s sensor.
This is easily done using a tool that does the math for you, such as the Spot Stars tool in the PhotoPills app. You can then test out the shutter speed it gives you and take some test shots and see what works for you. I find that the shutter speed given by the NPF rule with the “Accurate” setting in the Spot Stars tool of PhotoPills can often be pushed a little longer (increase the shutter speed a bit) and still have sharp stars. For example, with a 14mm lens on a 45MP 35mm camera, the NPF rule gives 7.8 seconds, but I find that 10 seconds works perfectly fine for sharp stars.
For 50mm on a 45MP 35mm camera the NPF rule gives 2.2 seconds, which I push to 3 - 5 seconds. That may sound extremely short, but isn’t so bad if you have an f/1.4 or f/1.8 lens. Remember how f-stops work, doubling or halving the amount of light for every full stop. So, a 3 second exposure at f/1.4 captures the same amount of light as a 6 second exposure at f/2, or a 12 second exposure at f/2.8.
ISO
Generally speaking, you’ll need an ISO of 3200 or 6400 for star shots. The actual ISO you can use will depend on the amount of ambient light, your shutter speed, and your camera’s high ISO performance.
In some cameras, if you’re star stacking with shutter speeds of around 10 seconds or less and your lens has strong vignetting at the aperture you’re using, you could end up with very strong magenta color noise issues along the edges of the frame when you apply vignette correction in your raw editor, which brightens the dark edges, revealing the color noise. This happens because the amount of light hitting the edges, in particular the corners, of the frame is much less than the amount of light hitting the center of the frame due to the vignetting of the lens, and if the light level is dark enough then you’re not overcoming the amplification/circuitry noise of the sensor and camera, so boosting the dark areas in software then shows that noise. The solution here is to use a low enough ISO to prevent or at least minimize the amplification noise. Try ISO 3200 or less if you are experiencing this. If you can’t get rid of it, you can leave in the vignetting, or you can color correct the magenta fringing.
F-Stop
You want to use the brightest (lowest f-stop number) aperture possible on your lens while still getting sharp stars. If you see heavy coma distortion of your stars or general star flare, try stopping down a little bit at a time until it goes away or is minimized. You may have to live with some amount (or a lot) of coma distortion depending on your lens.
Number of Exposures & Intervalometer
Star stacking requires capturing multiple shots at the same shutter speed so that they can be aligned and averaged later. The more shots you have, the less noisy the star stacked result will be, but only up to a point. There are limited returns after a while.
In general, I’ve found 20 exposures to be plenty to give a result with very low noise. But even 5 or 10 stacked exposures will make a big difference in noise. Experiment and see what works for you.
Built-in intervalometer of many Nikon cameras.
You can take the shots manually by just hitting the shutter button multiple times, but a much easier way is to use an intervalometer that can be programmed to shoot X amount of shots at Y seconds each. Some cameras, like many of the modern Nikons and Canons, have a built in intervalometer that can be used, or you can get an external remote/intervalometer.
Example Settings for Star Stacking
Here are some example exposure settings for star stacking at various focal lengths. ISO 3200 or 6400 would normally be used for all of these. Remember that you need to take several exposures for star stacking, at least 5, but I normally take 20.
Focal length: 14mm
Shutter speed: 10 seconds
Focal length: 24mm
Shutter speed: 6 seconds
Focal length: 50mm
Shutter speed: 3 seconds
Take these shutter speeds with a grain of salt — test and see what works for you. You might increase the shutter speeds and still be happy with the stars. Some lenses will have extremely heavy vignetting wide open (widest aperture / lowest f-stop number) and will need more light (longer shutter speed) to get a reasonable signal to noise ratio in the corners of the frame.
Star Stacking Software
Star stacking isn’t anything new, pure astrophotographers (space images without a landscape) have been doing this for a long time, and use programs such as Deep Sky Stacker, among many others, to do the stacking. But landscape astrophotography requires the alignment of the stars in the exposures to happen without the static foreground objects messing up the alignment. This can be done manually in Photoshop, but there are a couple of software programs that are designed for star stacking with landscape astrophotographs, and they make the process much, much easier.
Mac: Starry Landscape Stacker
Starry Landscape Stacker is the program I use, it is incredibly good at automatically finding the stars to make a mask for the sky, so that the foreground is not included in the alignment process. You can help the program find the sky by adding red dots that indicate where the sky boundary is located, and you can fine tune the auto generated mask with a paintbrush.
Starry Landscape Stacker is available in the Mac App Store, and a free trial that has all the features but outputs images with a watermark.
You can read more about it here, and Ralph Hill, the author of Starry Landscape Stacker, has a great collection of video tutorials on YouTube, and it is also detailed in my Milky Way Master Class.
Windows: Sequator
Sequator is the star stacking program available for Windows that can deal with landscape foreground masking, like Starry Landscape Stacker above. It is available for free. I demonstrate how to use it in my Milky Way Master Class.
Photoshop
Star stacking with landscape foregrounds can be done in Photoshop, but it is a tedious pain, and in my past experience Photoshop sometimes fails to align all the stars, particularly towards the edges of the frames. I strongly suggest using one of the programs above instead.
Blending Sky and Foreground
After you complete the star stacking steps, you can take the resulting noise-reduced sky image and blend it in Photoshop with foreground exposures from the same scene to produce your final image that will have the entire scene well exposed, sharp, and with low noise from the foreground to the stars. I cover this in detail in my Milky Way Master Class, and in my book Night Sky Photography: From First Principles to Professional Results.
Practice!
Now that you know what star stacking is all about and how to do it, head out on the next clear moonless night and give it a try! Figure out the best shutter speeds for your favorite focal lengths, and practice with one of the software tools mentioned above.
You can also join me on a workshop where I go over all the details in person.
Happy Shooting!