This is the third in a series of tutorials on the subject of photographing the Milky Way. Part 1 deals with the gear, planning and taking the photos. Part 2 is an overview of processing single exposure images of our galaxy. This article will cover a technique mentioned briefly in Part 1: image stacking.
“Stacking” is just what it sounds like, It’s the process of taking multiple photographs of a scene, then using software to align and blend those images. In Milky Way photography, there are two common purposes for using this technique:
- Reducing electronic noise caused by long exposures and high ISO settings
- Achieving maximum front-to back sharpness in the foreground
There are separate stacking methods for each of these purposes. We’ll discuss both in this post. Both involve taking the necessary steps in the photography stage and the processing stage.
Shooting the Foreground for a Milky Way Image
We’re going to start with the process of shooting the foreground, assuming you plan to include it in your Milky Way photos. Why? because it’s probably best to make this the first part of your shoot. You’ll have better light for focusing and you won’t need to bother with light painting the foreground after dark. It also gives you the opportunity to tweak your setup and framing before nightfall.
Fortunately, we’ve outlined the process of focus stacking the foreground in a previous tutorial. (By the way, it’s an extremely easy process, thanks to the convenient hyperfocal scale on Irix lenses!) You can learn about it in Focus Bracketing and Stacking Landscape Photos with Irix Lenses.
We have the perfect lenses for the job!
Now that we’ve mentioned one of our unique lens features, let’s take a minute to talk about the two super-wide-angle lenses that Irix shooters already know are ideal for landscape/Milky Way shots.
Our first lens, the 15mm f/2.4 quickly gained fame as an outstanding astrophotography lens. Not long after, our 11mm f/4 lens showed its night-sky prowess, especially when stacking images. Take a look at both to see why they’ve taken wide-field astrophotography by storm!
Noise in Milky Way Photos
The most common stacking method in astrophotography is intended to help cancel noise in your images. Noise is arguably the most prominent problem in night sky photos and it comes in two “flavors”:
Luminance Noise looks much like the grain that was often a problem with high-speed films, which was caused by the number and size of silver halide crystals in the film emulsion. The cause in digital photography, however, is entirely different.
This type of noise is caused by high ISO settings. Each time your sensor captures an image, the “signal” or the light striking the sensor is recorded, along with electrical noise generated by the sensor circuitry. Because the ISO setting in a digital camera amplifies the sensor output, it affects both the signal and the noise. This amplified noise is what creates the bright spots we call luminance noise.
Color Noise appears as a pattern of multicolored dots in the image and is particularly noticeable in shadow (dark) areas. This type of noise is caused by long exposures, which cause the sensor to heat up, which in turn causes color fluctuations in the pixels.
Color, or “chromatic” noise can start to appear at relatively short exposure times, depending on the camera and sensor. It can render an image file unsuitable for use, without a method to reduce or remove it.
Both types of noise usually exist together and at the very least, reduce the sharpness of an image. Both can be dealt with to some extent in post processing, even with single exposures.
In addition, most digital cameras have settings for in-camera reduction of both types. The problem is that both types of reduction extend the waiting time between photos and can reduce image sharpness, so that has to be considered when working in the field. Astrophotographers often turn these features off or leave them on low to medium settings.
How Image Stacking for Noise Reduction Works
It’s important to remember that when you shoot the Milky Way, you need to consider Earth’s rotation in order to avoid elongated stars or star trails. That means keeping your shutter speed relatively short, which, in turn, means increasing your ISO setting. (See Part 1 for details on calculating your exposure.)
Given the information above, you can see that a typical Milky Way photo with sharp stars will probably contain considerable luminance noise from the high ISO and hopefully less color noise. By aligning and stacking several exposures, we can blend those exposures with an algorithm that helps eliminate the noise.
To improve your results, there are several types of extra frames you can create with your camera, each with a specific purpose in the blending process. These images are known as calibration frames.
It’s worth noting here that there is a lot of debate over which, if any of these are necessary, particularly for wide field astrophotography. Photographers that image deep space objects with telescopes often use all of the calibration frames.
Many Milky Way photographers feel that only dark frames are necessary. Others believe that only flat frames are needed. Please also see the section on in-camera noise reduction in Part 1 of this series. We’re going to give you the information on each of them and recommend that you find what works best for you:
- Dark Frames are exposures taken with the same exposure settings you used when shooting the sky (those are your “light frames”). In this case, however, you take them with the lens cap on.
Ideally, these should be taken before you leave the shooting location, to ensure that they’re taken at the same ambient temperature. (Temperature affects the amount of noise generated.)
The dark frames will contain roughly the same amount and pattern of color noise as your light frames. When you stack the images, you can subtract the data in these from the light frames, helping reduce the chroma noise.
- Flat Frames help equalize the brightness of pixels exposed to the same amount of light. This allows you to remove the effects of anomalies like vignetting and dust.
These frames should also be shot right after your sky shooting session, being careful not to change the focus or the ISO setting. There are several methods for shooting these frames. One of the simplest is to point your camera straight up and cover the lens with a white sheet of paper. Illuminate the paper directly with an even light source, such as a laptop screen or LED panel. For the best results, find a light source and exposure setting that puts your histogram close to the center of the x axis or slightly toward the right.
- Dark Flat Frames perform the same function as your flat frames, but they’re applied to the dark frames.
Use the exposure settings you used for your flat frames, and simply shoot these with the lens cap on.
- Bias Frames record the noise output of each sensor pixel when no light is striking the sensor. They can then be used to subtract this “bias” noise from the light frames during the stacking process.
These frames should also be taken at the same temperature as the light frames. Put the cap on your lens and make sure no stray light can enter through the viewfinder. Set the fastest shutter speed available and start shooting.
How Many of Each Type?
Calibration frames take time. Dark frames, especially, can keep your camera tied up for a long time after you’ve captured your light frames. Nevertheless, “darks” are the type that most astrophotographers consider the most important, so if you take the time to shoot only one set of calibration frames, shoot darks.
The general rule for calibration frames is, “More is more.” Theoretically, the more frames you shoot, the more you’ll boost the signal and cancel out noise. In practice, though, processing stacks takes time and is a drain on computer resources. That means it’s important to strike a balance that works for your hardware and software.
Darks: Try to shoot at least 10. 20 to 50 is better if your system will handle them.
Flats: 20 to 25 is usually sufficient.
Dark Flats: Take the same number as flats.
Bias: 50 or more isn’t too many. These don’t take long to shoot. Keep in mind that bias is often applied by the software in DSLR and mirrorless cameras. Also keep in mind the limitations of your processing computer.
Shooting for Stacking Milky Way Photos
Let’s look at a typical shooting session for Milky Way photos, when you intend to stack the images. In most respects, the shoot will be much the same as a “normal” night out shooting the galaxy. After all, you’re already taking multiple exposures, right?
If you answered “No” to that last question, please re-read Part 1. Not only will you increase the chances of capturing just the right shot, but if you shoot a series, you leave yourself the option of stacking, even if you don’t use calibration frames. In fact, there are a few methods you can use to “fake” calibration frames with your processing software, although the results will vary.
So, for the shoot, you can plan to follow the instructions in Part 1, at least until you’re done shooting the sky. Remember that you’ll want to shoot the foreground first, while you still have light. Here’s how a night shooting the Milky Way might go if you’ll be stacking images. This is AFTER you’ve taking the planning steps outlined in Part 1, of course:
- Arrive early and set up.
- Frame your shot based on where the Milky Way will be.
- Shoot the foreground before the light gets too low.
- If you plan to focus stack the landscape, allow yourself time.
- If you wand to use a single image for the landscape, take a few at different times so you can later choose the one you like.
- Focus on the stars and lock your focus. (Your Irix Lens is equipped for this.)
- Check PhotoPills and determine the best exposure time for sharp stars.
- Set the proper exposure settings and set your intervalometer.
- Set up your tracking mount, if you have one. (We won’t cover tracking in this tutorial.)
- When the Milk Way appears in your frame, start the intervalometer. (Don’t change your framing if you intend to use foreground shots taken earlier.)
- Enjoy your evening under the stars! Watch for appreciable changes in temperature that might cause condensation buildup on the front of your lens. If you expect a significant change, you should consider a dew heater for your lens.
- When you’re done shooting the sky, shoot the calibration frames you intend to use. (Remember that several of these should be taken at the same settings and ambient temperature as your light frames.
As you can see, the shooting session is very similar to any other Milky Way shoot. You’ll be able to determine which of the sky photos to stack when you’re ready to process them, so the only real additions are the foreground frames and the calibration frames.
Stacking Your Images
If you’ve created focus stacked images for the foreground, you can blend those images before you start, then use the resulting image as the foreground in your final images. You can refer back to Focus Bracketing and Stacking Landscape Photos with Irix Lenses to stack them easily in Photoshop.
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 quickly becoming one of the most popular. 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. If you spent a long time shooting, simply group your images into batches for stacking.
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.
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.
Putting it all together
This concludes our series on shooting the Milky Way. With the information in these articles and the help of our wide angle lenses, we believe you’ll see amazing results from your efforts to shoot the galaxy in no time.
We’d love to see your Milky Way photos captured with Irix lenses! Feel free to sharre them with us:
Tag @irixusa on Instagram or Facebook.
Post as a message to Irix USA on Facebook Messenger.
Post to the Irix Shooters group on Facebook.
We look forward to seeing and sharing them!