With the introduction of new technology, products and processing techniques, the way we capture the night sky has changed significantly in the past few years. The most basic method of capturing the Milkyway is a single exposure, for this you need high ISO, a short shutter speed to avoid trails and a lens with a good maximum aperture. If you are lucky enough to get great conditions whilst shooting (Cool temperatures, dry air and minimal light pollution) and use a lens which has a large maximum aperture (F1.4 would be ideal) you can get a decent shot with a single RAW file.
A single sky shot from the Dolomites. 24mm at F1.4, ISO 6400, 10 Seconds.
However in most cases, we find less than ideal conditions, and to help get more detail in the Milkyway, we can do one of these two things:
Stacking multiple images to get an average exposure
Tracking the stars in order for us to shoot a longer shutter speed, thus allowing us to reduce our ISO and not requiring a lens as wide as F1.4.
Let’s look at these two in more detail and look at some examples:
Stacking
This involves shooting multiple exposures with the same settings, then using software like starry sky stacker or Photoshop to align them and get an average exposure from all the shots, this effectively removes most of the noise (Noisy pixels are usually randomly spread across the frame in each shot, and taking an average of all the shots helps remove this). There is no fixed rule on how many frames to stack, generally speaking, the more frames you use, the better results, however the rule of diminishing returns does apply here, so once you have stacked for example 20 or 30 frames, you won’t get much better results if you stack more frames. Some people also like to include ‘Dark frames’ when stacking, I have never seen a difference in the results, so I don’t bother with this.
100% crop of a single Exposure from the stacked sequence at 24mm, F1.4, ISO 6400, 10 Seconds.
The number of frames I chose generally depends on conditions such as how much time I have and how many compositions I have planned for that locations, if I have many other compositions in mind, I will generally shoot less frames to save time.
A sequence of 10 frames shot at 24mm, F1.4, ISO 6400, 10 seconds.
As an example, below is the result of a sequence of 10 frames shot at 24mm on a Full Frame, an aperture of F1.4, an ISO of 6400 and a shutter speed of 10 seconds. Shooting these 10 frames took about 130 seconds (10 x 10 second frames with a 3 second gap between each frame).
10 Stacked Images as above (F1.4, ISO 6400, 10 Seconds).
Tracking
This involves using a device known as a star tracker. Once aligned to Polaris (Northern Hemisphere) the tracker rotates at the same speed as the Earth, but in the opposite direction, effectively cancelling out the Earth’s rotation and therefore keeping the stars from trailing. Using a tracker provides a much cleaner single file because your settings are completely different, lower ISO and you don’t require a particularly fast lens.
Tracked Image: 100% crop of an image shot at 24mm, F2.8, an ISO of 640, 363 seconds.
As an example, below is a single tracked RAW file at 24mm on a Full Frame, an aperture of F2.8, an ISO of 400 and a shutter speed of 484 seconds (Over 8 minutes!). Shooting this single raw file took over 12 minutes though, as I had to polar align the tracker to ensure the axis of rotation of the tracker is the same as that of Earth.
Tracked Image: 100% crop of an image shot at 24mm, F2.8, an ISO of 400, 484 seconds.
Observations
The tracked shot is sharper across the frame and has less coma / aberrations / distortion due to being able to use a smaller aperture (Not shooting wide open).
Noise levels are significantly lower in the tracked image as the ISO is so much lower than in the stacked images.
The tracked shot has more rounded / pin-point stars that the stacked one
Advantages of Stacking
Quicker to capture the sequence of shots required due to spending less time setting up and also less time shooting
Ability to shoot in tight locations where Polaris is not visible (Required to polar align a tracker)
No need to carry a tracker with you, helping save weight in your camera bag
No need to shoot a separate foreground image (With a tracker the foreground will be blurred by the movement of the tracker, so you will need to shoot a separate frame for the foreground and blend them together in post processing).
Advantages of Tracking
Better image quality, making post-processing a much simpler process
More detail in the Milkyway dust lanes and better contrast throughout the frame
More glow from nebulae captures due to the longer exposure
Cheaper than buying a lens which would be used solely for Astro photography
Tracked Image: 24mm, F2.8, an ISO of 400, 484 seconds.
My Conclusion
There is a significant difference in the final image between a tracked image and a well-executed stacked image. I personally prefer tracking as I feel it gives a cleaner result overall. I would rather spend more time in the field shooting and being creative than spending more time post-processing an image. If I am in a position where I can’t polar align my tracker (Either due to Polaris not being visible, or I am in a sketchy location where I do not have space to comfortably work with a tracker, I would stack images instead without doubt, but where I have the choice, I would always choose to track.
You can read my review of the tracker I use here (Includes a discount code!)
Note: Images on this blog cannot be compared directly as they are post processed differently.