Milky Way Photography
Milky Way photography is a blend of art and science, and image processing is difficult as cameras filter out the red light produced by hydrogen emissions. So we only capture a very limited amount of colour in the nebulae, and images need a lot of restoration to bring out faint colours and details hidden in the shadows.
My previous attempts at wide-field astrophotography had always been a bit underwhelming, as I never really understood what the finished image should look like. So I thought I’d try to process my Milky Way photos a bit more scientifically…
The sensors in modern digital cameras are so good these days that most cameras with a long exposure setting can take Milky Way photos. Many even have built-in noise reduction for long exposures. For my setup I’ve used a Panasonic G6 micro four thirds camera that’s smaller, lighter and more portable than a Digital SLR camera yet produces similar quality results. Cameras with smaller sensors are often criticised for being less capable in low light, but when combined with a fast lens with low focal ratios they can be quite useful.
Although some cameras are better at low light photography than others, the most important factor is to use a fast wide lens that enables more light to hit the sensor. Any wide-angle lens with a focal ratio of f/2.8 or below should produce fair results. But the lower the focal ratio and the wider the aperture, the less noise will be visible in the image.
You’ll also need a sturdy tripod to keep the camera still, especially if there’s a breeze. Camera vibrations will also blur long exposures, so try to avoid manually activating the shutter. If you don’t have a timer in your camera you’ll need a cable or wireless shutter release.
Clear Dark Skies
Ideally a dark sky site away from city lights is the best place to capture the Milky Way. You can actually take Milky Way photographs in light polluted areas too, but you’ll need to use a light pollution filter either on the lens or in front of the camera sensor.
It’s worth checking where the Milky Way is in the sky using planetarium software before hand. In the northern hemisphere summer our galaxy’s core will be towards the southern horizon, and the spiral arms will stretch north straight up overhead. It’s also a good idea to try to include a foreground object if possible, to give a frame of reference and add context to the image.
Unfortunately the one thing we can’t plan for is the weather! A clear dark sky with a new or crescent Moon is best to see the faint glow from the Milky Way. So don’t forget to use Scope Nights to find your nearest dark sky site & stargazing forecast ?
If we record images in RAW format we can adjust the white balance during image processing. Many astrophotographers recommend to use the daylight or sunlight white balance in your camera which is usually around 5000 Kelvin. During photo editing I found a white balance between 3800K to 4500K produced the most balanced and realistic colours with my camera setup.
We need to use a high ISO to capture as much light as possible, but it should also be balanced with the amount of sensor noise which increases with ISO. With my camera I found ISO 1600 was the highest I could go before noise became too intrusive, but you may well be able to use ISO 3200 and above if you have a more modern camera, so you’ll need to experiment.
The exposure time will vary depending on your camera ISO and lens aperture. Again you’ll need to experiment with this, so I’d recommend taking lots of different exposures to start with. Exposures over 30 seconds will create star trails, so it depends what type of image you’d like to create. I found that 10 to 20 second exposures, produced the best balance between exposure and star trailing.
Unless your camera has an exact infinity focus, you’ll need to manually focus on a bright star as the Milky Way will be too faint. Many manual focus lenses will travel past infinity, so be careful when focusing.
When the number of light photons captured by the sensor is so small, background noise starts to cause problems during image processing. Noise is a particular problem when using conventional cameras for astrophotography as they’re not designed for such low light conditions. Many manufacturers have included long exposure noise reduction within the camera firmware, and while this makes Milky Way exposures possible there are many other steps we can take to reduce noise.
In this case I wanted to see what was possible with a simple camera setup, so the images I’ve used here are all single exposures and just use in-camera noise reduction. But you could use more advanced noise reductions techniques by taking dark, flat and bias exposures around the same time as capturing the Milky Way. This data can then be subtracted during image processing to cut noise even further. It’s also worth taking multiple exposures and stacking your image frames in dedicated astrophotography software. This will also improve the signal to noise ratio and improve your images even further.
When taking long exposure photographs don’t forget to use the in-built timer or a remote/cable release to avoid camera shake.
Processing Milky Way Images
To achieve colour balanced and natural looking Milky Way photos, it’s advisable to calibrate our screens before image processing. As I’m not stacking image frames, I used Lightroom as it’s the software I had to hand. But there’s plenty of other photography editing apps that offer similar editing functions. There are also many websites offering plugins and presets that do all the work for you. But in my experience you’ll still end up having to do most of the work yourself, because no image is the same and they all need different amount of processing.
Before starting to process our images it’s a good idea to have a goal in mind otherwise we can end up going round in circles. If we want to create a detailed yet realistic photo, we’ll need to bring out structure in the dust clouds, and colours in the nebulae. Whilst also keeping the colours of the stars as much as possible. Many Milky Way photos have blown out highlights with white stars and a false coloured sky. Which is great if you want to create a piece of art, but not if we want to create something a bit more realistic.
Milky Way Colours
Although the initial colour balance of our Milky Way photo will be wrong, we can use science to decide how the finished photo should look. Our Galaxy’s core consists of older red and yellow stars, while the spiral arms are populated by younger blue and white stars. Interstellar dust also scatters blue light, which causes colours to shift towards the red part of the spectrum as we see during sunsets on Earth. The dark dust lanes in the Milky Way should therefore contain shades of red and yellow.
It’s also worth taking time to identify the constellations and astronomical features and where they’re located in the photo. We can use this information to understand different parts of the image.
Stars have different colours because they’re made up of different types of gases and burn at different temperatures. Just like the hottest part of a flame is blue and the coolest part is red. Stars also obey the same physical rules. The hotter the star burns the bluer it appears, and the cooler a star burns the redder it appears.
This helps colour balance the highlights in our image by using planetarium software to show the colours of different stars. For example our Sun has a temperature around 5800 Kelvin which means it would look white to someone in space. Although it looks yellow through our atmosphere due to shorter bluer wavelengths of light being scattered.
Photo Editing Settings
As this type of image requires so much processing and restoration, it’s worth taking some time to understand the histogram and your editing software beforehand. Although I’ve used Lightroom in this case, many image editing apps offer similar functions. I found the following settings particularly useful…
White balance and exposure will vary from image to image depending on your camera ISO and exposure settings and light pollution levels, so you’ll probably have to create different software editing presets for different camera settings.
Camera & Lens Correction
It’s important to add camera and lens corrections before you start editing. Some cameras have this built into the RAW file, but you may need to manually remove chromatic aberration and vignette. I found that manually adjusting the purple hue by selecting the purple fringes around a very bright star helped to keep the star colours intact.
Applying colour noise reduction makes a big difference, but be careful you don’t lose too much colour detail. Luminance noise reduction is also useful, but be careful not to make the image to blurred.
It’s very easy to make your photos look false and over processed, so you’ll need to experiment depending on how much noise you have in the original image. I didn’t use sharpening as it created too many artifacts. But if you do use it then I’d recommend masking the stars otherwise you’ll also sharpen the background noise.
I preferred to balance colours in the image using the RAW white balance and tint adjustments before making any other colour adjustments. It’s easier to do this while temporarily increasing the exposure and saturation. Once you have found the correct colour balance, it will make the rest of your editing much easier.
I found this one of the hardest things to do, as it’s tempting to just take the white balance right down to cut light pollution. But you end up with overly blue stars and nebulae. Try reducing light pollution using colour specific luminance and saturation adjustments instead. I found that during post processing I used temperatures around 4000 Kelvin to balance the colours. Each image required a slightly different white balance too, which was very time-consuming.
Milky Way photos are usually underexposed with most of the detail in the shadows. We therefore have to increase the exposure quite dramatically. But we also need to avoid boosting exposure too much as background noise will become too visible. If we wish to keep the star colours, we also have to be careful not to boost the highlights or white level too much either.
Digital image sensors have a limited dynamic-range and so highlights and shadows tend to lack detail. Darkening the highlights and lightening the shadows will bring out more detail, but be careful not to overdo it. This will lead to either too much noise in the shadows or clipped highlights.
I recommend making colour adjustments using a targeted adjustment tool or eye-dropper tool if available. This will really help target the detail and colours in the dust clouds and nebulae.
Tiny changes to colours can have knock on effects to other colours in the image, so keep an eye on your RGB values. Referring to an additive RGB colour chart helps to see what the consequences will be.
Vibrance and saturation sliders are commonly used as a quick fix, but it’s easy to go overboard with colours. The only time I made major colour adjustments was to reduce light pollution. I found that getting the white balance and tint balance correct in the beginning solved most of the colour problems. Reducing the overall saturation slightly can also make things look a bit more natural.
Milky Way images are initially very dark and flat with little contrast. They therefore need a lot of contrast adjustment. When adding contrast via a slider or tone curve, try de-saturating the image first. This will make you concentrate on the tonal differences and not the colours.
I found that adding a steep contrast gradient to the shadows using the tone curve brought out the most detail. Reducing the black level will also add contrast, but be careful as you can easily lose detail in the shadows.
Clarity & Details
Many tutorials recommend boosting the clarity to improve mid-tone contrast, but it didn’t give the result I was looking for. I found it burnt out star colours and stars took over the image and distracted from the Milky Way. Using the tone curves will show far more detail in the shadows where it’s needed. Reducing the clarity actually gave the effect I was looking for, as it reduced the stars and smoothed the background noise a little.
Referring to a small navigator or thumbnail view while editing gives an overview that shows how your changes are effecting the overall balance of the photo.
While most photo editing apps are good for basic adjustments, I found Luminar the best at providing the finishing touches. Photoshop is the obvious choice for many, but it can be a steep learning curve. I found Luminar much more intuitive and faster. Many programs also offer automatic merging of different exposures into a panorama, which is useful if your lens doesn’t cover enough of the sky.
As we’re pushing the camera sensor to the limit with Milky Way photography, noise will always be a problem. Using basic equipment and techniques as I’ve done here, can mitigate noise using noise reduction algorithms. But using astrophotography software to subtract dark, flat and bias frames and stack multiple exposures should produce better results. It’s also worth noting that simply reducing the size of the image when exporting will reduce image noise to a certain extent.
The other important factor that affects image quality is the rotation of the earth causing star trails and blurring. The best way to remedy this is to attach the camera to either a telescope tracking mount or a camera sky tracking mount. Tracking mounts are aligned to the celestial pole so they follow the stars as they move.
Tracking the sky should allow wide-angle exposures of several minutes, so up to 10 times more light can hit the camera sensor. This in turn means much less noise and far more detail and colours. Remember that foreground objects will blur when tracking stars, so you’ll need to take a separate shorter exposure and overlay any foreground objects.
Better results could also be achieved using a faster wider lens to let in more light, or the latest low noise camera sensor. A modified Digital SLR camera with infrared filtering removed will also show a lot more colour and detail in the nebulae.
A cheaper way of improving Milky Way photographs is to use a broadband light pollution filter. This type of filter will block out unwanted light and let through the important light from the Milky Way.
Hopefully I’ve shown with a little understanding of the science behind the picture, a single untracked exposure can produce fairly detailed and realistic images of the Milky Way. It’s definitely given me the motivation to improve my astrophotography when the clouds eventually disappear! ?✨