Capturing the Milky Way can be a rewarding experience for any photographer. It can also be challenging and occasionally frustrating. Bringing out the detail in that big, bright band of celestial objects isn’t always easy. This article will explore one way to enhance your images of our galaxy, using a process known as image stacking.
Opening Image: 11 sky exposures, 9s each @ f/2.4, ISO10000 Stacked in Sequator. (Clouds blended back in from a single 14s exposure @ f2.4, ISO6400) + 13 foreground exposures, 13s each @ f/2.4, ISO6400 stacked in Adobe Photoshop. Captured with the Irix 15mm. ©Chris Maust Photograhy. All rights reserved. Published with express permission.
Let me say right at the start that not everyone is going to be a fan of this technique. You can create beautiful images of the night sky and the Milky Way with a single, long exposure capture. In fact, the Irix 15mm f/2.4 lens has become a favorite of many photographers for doing just that.
If you’re the kind of photographer that likes to try new things, however, I recommend giving image stacking a try.
What You Already Know About Shooting the Milky Way (Probably)
Let’s talk for a moment about the problems ‘most everyone knows about in relation to shooting the night sky. It may be redundant, but it’s going to help explain some things a bit later.
Light pollution: The farther away from lights on Earth, the more clearly you and your camera will see the Milky Way. Those lights can also cause color shifts in your photos. You can purchase filters designed to reduce these effects, but the simplest solution is to get as far away from the lights as possible.
Motion: Another major consideration in astrophotography is actually the reason that we can capture star trails. Because you’re standing on a rotating planet, everything you point your camera at in the night sky will appear to constantly move. There’s a limit to how long you can leave your shutter open before that movement across the sensor starts to show.
There are two ways to deal with this motion:
Limiting exposure times is a straightforward way to avoid trailing. To calculate the maximum practical exposure time for a lens, we use the 500 rule:
500 / effective focal length = maximum exposure time in seconds
Note the word “effective”. It’s important to consider the size of your camera’s sensor when using this rule. If you’re using a full-frame camera, you can simply use the focal length of the lens. A camera with a smaller sensor will have a narrower field of view with the same lens, so the crop factor comes into play:
500/(focal length x crop factor) = maximum exposure time in seconds
So, if I’m shooting with a Canon T6 and a 50mm focal length:
500/(50 x 1.6) = 500/80 = 6.25 seconds (round down to 6 seconds)
The same lens on a Canon 5D:
500/50 = 10 seconds
Now note the change if I switch to the Irix 15mm lens:
T6: 500/(15 x 1.6) = 500/24 = 20.8 seconds
5D: 500/15 = 33.3 seconds
With the Irix 11mm lens:
T6: 500/(11 x 1.6) = 500/17.6 = 28.4 seconds
5D: 500/11 = 45.4 seconds
As you can see, the shorter the effective focal length, the more time you have before motion becomes a problem. That’s just one of the reasons the Irix 15mm and 11mm lenses are ideal for capturing Milky Way and star photos – you can get nice, long exposures.
This rule is the most consistent, simple way to calculate your exposure length limit. It’s best to be a little conservative with your setting. It’s also important to remember that the longer the exposure, the more noise will be generated.
Tracking and Guiding are other ways to avoid motion in your astro shots. Tracking refers to aligning your mount with Earth’s polar axis and constantly or periodically adjusting the alignment to compensate for rotation. Guiding requires more sophisticated equipment and involves adjusting the alignment based on the position of an object within the frame.
Tracking and guiding are typically used for Deep Space Object (DSO) imaging.Fortunately, we don’t necessarily need either for wide-angle shots of the night sky. Instead, we can use one of the software applications designed to automatically or manually align our images during the stacking process.
Noise: This is one of the worst enemies of the night photographer. There are several kinds of noise that can affect your digital images. If we generalize them a bit, the two most significant types when shooting at night are random noise amplified by shooting at high ISO settings and pattern noise, a.k.a. “hot pixels” caused by camera circuitry and components during long exposures. Let’s quickly examine each of those:
- Random Noise: This type of noise is present in every image and is a result of the random distribution of photons across photosites on the sensor. The disadvantage to shooting at high ISO settings is that both the signal (the image being captured) and the random noise are amplified when you increase the settings. This noise is difficult to remove, because it is truly random.
- Pattern Noise: This noise generally appears during long exposures and is recognizable as very bright points of color within the image. This effect is usually more pronounced at longer exposures, due to sensor heat, among other factors.
Although this type of noise may seem random, it will actually repeat a recognizable pattern in exposures taken with the same camera at the same settings and temperatures. Because of that, it can be managed easily by creating one or more “dark” frames at the same settings with no light striking the sensor. Those frames are then blended with the original in a way that subtracts the noise.
This dark frame subtraction is one of the methods used by astrophotographers to remove noise from extremely long exposures of DSO’s like planets, nebulae and neighboring galaxies. It also happens to be the way that Long Exposure Noise Reduction built into your digital camera works.
How and Why Stacking Works
Okay, so now that we’ve covered the most common problems, let’s get technical just one more time, to better understand why image stacking can help your Milky Way images.
Did you notice that random noise is the only problem in the list above that didn’t have a definite solution? While it can’t be eliminated, we can help reduce its effects by blending multiple images taken successively. Here’s why:
When you take a properly-exposed photo, the signal (the light you want to record) is normally much stronger than the noise. That means that you can blend multiple images of the same scene using an averaging algorithm and the signal will increase more than the noise.
The signal in each of your images will also vary somewhat, due to atmospheric interference, sensor variations and several other factors. By averaging the signal data gathered during multiple exposures, we can “fill in the gaps” in the data collected and end up with a more accurate representation of the signal.
Those two principles are the basis of how this process works.
I won’t go into a lot of detail about preparing for the outing. Common sense will tell you what you need in order to spend a few hours out where you can get away from the light pollution. Just think in terms of a camping trip – in fact this is a great excuse for one!
As far as gear is concerned, you’ll need your camera, your Irix lens (You do have an Irix lens, right?) and a sturdy tripod. An intervalometer will be a big help, although you can use a remote or cable release. Just be sure you can keep your hands off of the camera for the light frames to avoid vibrations.
So, get out there, away from the light pollution and start with your basic camera settings. The following settings are only my recommendations, so feel free to research and experiment:
- Save your images as RAW
- Manual Exposure Mode
- White Balance: Daylight or Auto
- ISO: 1600 or higher
- Aperture: maximum (smallest f/number) or stopped down by one
- High ISO Noise Reduction: Normal
- Long Exposure Noise Reduction: You’ll need to make a choice here. This camera function handles pattern noise fairly well on most cameras. On the other hand, you’ll have to wait while the camera creates a dark exposure with the same duration as the light frame, so you’ll have to allow for that delay. You can opt to switch this function OFF and shoot dark frames for pattern noise reduction during processing. I’ll explain how a bit later.
A note about the foreground: If you want to include the foreground in your final image, shoot it first, right after you frame your shot. You might want to start a bit before the Milky Way is where you want it to be in your shots. See this article for some tips on getting the sharpest possible foreground images.
Focusing: You’ll want to set your focus to infinity. If you’re using an Irix lens, that’s easy – just set it at the click stop and lock it. If you’re using a lens with a conventional infinity mark, verify the setting by focusing on a bright star, zoomed in on live view. Get your focus as sharp as possible and lock or tape it there before you compose the shot.
Exposure time: Opinions differ on the best method for determining exposure times for stacking. Frankly, the math for determining Signal to Noise Ratio (SNR) is complex and there are several variables, including the light pollution near your shooting location. Since we’ve already touched on SNR and shooting locations, let’s shoot for something basic here.
Using the 500 Rule, we determine how long the shutter can be open before we start to observe trails. Let’s say your focal length allows a 30-second exposure. The lack of trails doesn’t necessarily mean your stars won’t be a little bit oblong-shaped. Using a little shorter time might keep the stars sharper, so let’s go with 20 seconds.
If we want to shorten up the exposure times, we can take ten 2-second exposures for an equivalent total exposure, right? We’re going to blend and average the exposures, though, so we could also shoot ten 20-second exposures and collect more signal data along with some increased noise.
Most astrophotographers will opt for somewhat shorter exposure times, because color (pattern) noise will begin to interfere with the color data in the final image as it increases. Most also agree that anything too far below 10 seconds doesn’t provide a high enough SNR ratio.
As I’m sure you’re beginning to see, this becomes a matter of personal preference. Make a decision based on your situation and equipment. Once you’ve determined the exposure time you’ll use, then you can adjust your ISO setting based on the shutter and aperture settings.
You’re going to want to shoot for a neutral exposure but with sufficient dynamic range and data to overcome noise. Experienced astrophotographers recommend starting with an EV of about -8 in a dark location and adjusting from there. Since that probably doesn’t make much sense to most of us, try this handy exposure calculator to find a starting point.
Take a few test exposures to verify your results. Your images may appear brighter and “flatter” than you expect, but don’t judge them by appearance. Use the histogram on your camera and try to balance the exposure so that the peaks are mostly centered or slightly to the right of center.
Once you’ve determined the right exposure settings, lock them in and start shooting. If you’re using an intervalometer and Long Exposure Noise Reduction, remember to program the delay for the process. Experience will tell you how many images you’ll want, but 10 is a good round number for your first experiment.
Dark Frames: If you’re planning to do your own pattern noise reduction, you’ll need to take a few dark frames. Simply put your lens cap on and cover your viewfinder to prevent light entering from there. Take a few shots like that and you’re good to go. Remember, these need to be taken at the same temperature as your “normal” frames, so do this before you leave the location.
If you’re wondering how many dark frames you should shoot, the fact is that more is usually better than less, but you shouldn’t need more than the number of light frames. Don’t forget that each of these takes just as much time as a light frame.
Stacking the Results
There are several applications that can be used to stack your Milky Way shots, including Adobe Photoshop. For this type of stacking, however, working with more than four or five images may be frustrating. Fortunately, there are several applications created specifically for star stacking. Here are a few of the most popular:
Sequator: This app is fairly new but gaining popularity quickly. It’s free, but only available for Windows operating systems.
Starry Landscape Stacker: This application is only available for the Mac platform. It’s very powerful and very popular, but it also costs $29.99.
Deep Sky Stacker: Another free application for Windows machines, this one is a favorite for DSO imaging, where hundreds of photos can be stacked along with darks and other control images. It can be used for your Milky Way photos as well. It does seem to run better on machines with lots of RAM and swap disk space available.
Nebulosity: This sophisticated application controls both the capturing and stacking of your images. It’s available for both Windows and Mac platforms and it supports a surprisingly wide selection of cameras. If you don’t mind taking your laptop out in the field, check this one out. The license fee is $20.
The image stacking procedure is basically the same in most applications: You’ll feed the program your image stack, and add in the dark frames if you shot any. Select a few options, then let the program register (align) and blend the photos.
Some applications will also let you specify an image to use to create a mask for the foreground. (Remember, your overall foreground will be remaining stationary while the sky appears to move.) You can use this mask in the application or in Photoshop to create a composite with a single, correctly-exposed foreground photo or the result of your stacked foreground images.
You’ll also find other ways to manipulate the final image. For instance, you can eliminate clouds that moved across your frame and use the clouds from a single image. There are so many things you can do with these applications and so many options that I’m going to simply provide links to a few tutorials at the end of the article.
Whatever you use to combine your images, the end result should be a Milky Way image with better detail and sharpness than a single, long-exposure image.
Image stacking can be used to enhance and manipulate photos in many ways. As we’ve seen in this article, one way is to improve the signal-to-noise ratio in astrophotography images. We can use a very simple stacking technique to reduce random noise and increase sharpness in images of the Milky Way.
The process involves shooting multiple exposures instead of a single long exposure. The images are then blended using software that averages the images. This averaging boosts the signal and effectively reduces the noise in the final images, usually with less deterioration of image quality than that of standard noise reduction algorithms.
Image stacking also provides opportunities to correct other issues with Milky way images, particularly when you want to include the foreground in your image or when objects such as moving clouds or planes interfere. Although this technique won’t appeal to all photographers, it’s an interesting exercise in post-processing and may result in surprisingly sharp, low-noise images.
Last, but not least, Irix ultra-wide, rectilinear lenses are ideal for capturing night sky images for stacking. With the click stop at infinity, a wide maximum aperture and the focus lock, they’re perfect for night photos of the galaxy and the landscape. What’s more, their optical performance has been proven to match or exceed competitive lenses, often at far less cost.
What’s Your Method?
The settings and methods in this article are based on my own research and experience. Irix USA and I happily acknowledge that opinions differ and there’s always more than one way to accomplish something. With that in mind, we welcome constructive criticism and comments. Leave yours in the comment box below!