Astrophotography II: Milky Way Images
by Forest Chaput de Saintonge
Let’s turn our attention to the Milky Way and how to capture it.
First, make sure you’ve read the first article, because much of that information is a prerequisite for understanding the following things.
When capturing the Milky Way, we need to primarily understand that we are trying to get pin-point stars instead of trailed ones. This puts an upper limit on our shutter speed. If we use a speed too long, the stars will move a measurable amount in our image and we will have tiny star trails instead of nice, round, crisp stars.
The rule we like to use here is something called the “400 Rule," which states that the longest shutter speed you should use is equal to 400 divided by the focal length of your lens in millimeters (assuming a full-frame sensor). (Some folks use the 500 rule, but I find that dividing by 400 is much more accurate.) As an example, if you are using a full-frame camera with a 16mm lens, the equation would be 400 / 16 = 25 seconds. This means that a shutter speed longer than 25 seconds would result in trailed stars. I usually like to round down to the nearest full-stop shutter speed; in this instance that means I would stick to 15 seconds.
Now, let’s look at a crop-sensor example. Let’s say you are using a Nikon camera with a crop sensor, which would have a crop factor of 1.5x. So, if you are using a lens that has 35mm printed on it (or a zoom that’s zoomed to 35mm) we need to take the 35mm and multiply it by 1.5x to get 52.5mm. So, the 400 rule in this case would be 400 / 52.5 = 7.6 seconds. Round down to the nearest full-stop and we get 4 seconds. So, with this setup, any longer than 4 seconds and we can expect our stars to trail instead of looking nice and crispy.
Obviously at this point it’s worth mentioning that the longer your lens is, the shorter your shutter speed can be. This is why doing deep sky astrophotography (capturing nebulas or galaxies) with a long telescope or lens requires a tracker in most instances -- because we can't achieve a slow enough shutter speed to "see" these objects without an additional device. That is something we will address in Part III of this series of Astrophotography articles.
Now. Let’s complete the exposure triangle. To make it easy, you always want your aperture wide open. For a kit lens that means f/4 or 5.6; for a faster lens, that means 1.4 or 2.8. Either way, stick to the lowest number your lens is capable of, which will let in the most amount of light.
For ISO you'll want to use the same trick we talked about in the previous article. Essentially, you want to "guess and check" your ISO until the histogram looks like the image below. We are looking for the darkest things in the image (background sky) to not be clipped to pure black (touching the left edge).
Now that we have exposure figured out, let’s look at some other factors that can influence the success of a good Milky Way image.
One of the first things is the location of the galactic core. The core is the most interesting part of the Milky Way and is only visible during certain parts of the year in different locations. I would strongly recommend using an app such as Photo Pills to determine if/when/where the galactic core will be when you are shooting. This will allow you to pre-plan your composition and ensure that the core is where you want it in the frame.
I STRONGLY recommend making your first attempt at capturing the Milky Way when the core is visible. It will be way more interesting and motivating to see the core in your frame rather than trying to chase the faint "wispies" that are elsewhere in the band of the galaxy.
Nerdy side-note: The “Milky Way” is the galaxy that we are a part of (a spiral galaxy). The reason it looks like a band across the sky is because the Milky Way is shaped like a pizza. We are essentially getting a view of that pizza from the perspective of a piece of pepperoni sitting on the pizza. If we look toward the center of the pizza (the core) we see more pizza, if we look the opposite way (towards the edge) we don’t see as much because we are close to the edge of the pizza. If we look up from the pizza we don’t see it at all.
Anyway. Get the core in the frame. It will help a lot!
Next is light pollution.
This one is huge. In order to get good contrast between the Milky Way and the background sky, we want as dark of a sky as possible. Contrast is the name of the game in astrophotography and the darker the sky, the more contrast we get.
Use a light pollution map like the one at lightpollutionmap.com to find a nice dark sky area near where you live. The darker, the better. Ideally, you can find one where the nearest cities are in the opposite direction of where the galactic core will be. If there are no dark skies in your area, it might be time for a road trip!
On the topic of light pollution, one of the biggest things that creates light pollution is the moon. Ideally, a night with a new moon or as unilluminated of a moon as possible is ideal. If the moon is full and you have a night to go shooting, stick to star trails, as any attempt at the Milky Way will most likely end in disappointment.
- Is the moon new or close to new?
- Are you going to photograph in an area with little-to-no light pollution?
- Have you checked to see if the core of the Milky Way is visible when/where you want to photograph?
- Have you pre-scouted the location and thought of some good foreground elements?
Now that we have that out of the way, let’s move on to the process of actually capturing an image.
First, I would plan to arrive early enough to make composition and focus as easy as possible. Usually, this means around sunset. I use Photo Pills to plan out my composition and get my tripod and camera locked down and in position.
Next is focus. If it’s light enough, just use manual focus and live view to focus on something far enough away from the camera that that lens considers it to be at “infinity.” I recommend a distant mountain peak or something at least a few hundred feet from the camera. This will ensure that the stars will be sharp in your image as well. Once focus and composition are figured out, set the aperture and shutter speed as we discussed above and start with something around ISO 1600. This will change as we start shooting (and review our histogram) but it’s a good starting place.
Now wait for darkness (or for the core). Take test images and adjust the ISO. Once you nail it, keep shooting until you get something you are happy with.
Once you have an image you’re happy with, that’s awesome! Now let’s look at some editing ideas.
First, editing Milky Way images is simple if you follow the steps above. Some simple adjustments in Lightroom or Adobe Camera Raw will yield great results. I find most people struggle with the editing when trying to make the Milky Way look good, but that's often because the images were shot on a full-moon night or near a large city. If you stick to good shooting conditions, it should be straightforward.
I do strongly recommend processing the foreground and background separately using local adjustments in Lightroom, ACR, or Photoshop. Adding contrast to the sky while separately bringing up the shadows in the foreground will produce some pretty awesome results.
Here are a few ways to make your images even better!
- Stacking. This is an advanced technique that we will talk about more in the Astrophotography III article. Essentially, because of the random nature of noise, the more images we shoot and stack together, the cleaner the result will be. For this reason, just cracking off 20-30 images of the same composition without anything changing between frames can help make your images almost noise free. You can then use a program like Starry Landscape Stacker to put them together into a final single image, subtracting the noise. Again, more on this in the next article.
- Compositing. Take one image exposed for the foreground and one for the background. Composite them together in Photoshop and boom! A perfect image right from the get-go!
- A tracker. Again, this is something for the next article. But here’s a hint: One of the things causing our images to have more noise is the high ISO. We need a high ISO because our shutter speed needs to be short to ensure round stars. Add a tracker in the mix and all of a sudden we can handle a much longer shutter speed, enabling a lower ISO and a much cleaner final image!
That’s all for now! In Part III, we will discuss how to start getting images of deep sky objects like galaxies and nebulas!
Here's my article Astrophotography I: Star Trail Images, in case you missed it.