What is a tracked and un-tracked image? I can't find anything about it.

Question

I've been trying to figure out from seeing an Instagram photo where the guy made a long exposure shot combining the stars and the crashing waves into one and I can't understand how he did it. He said he had a 4 minute tracked image for the sky and then a 4 minute non-tracked image for the foreground to help reduce noise. For some reason finding the definition of what a tracked image is proving to be much more difficult than it should.

Answer 1

Since the stars move in the frame during long exposures, you need a way to follow, or track their motion in the sky, so their light can be collected in points instead of being spread in short segments.

Actually it's the Earth rotating, but since we are rotating with it, this is what we see.

To compensate for Earth rotation you use an astronomical mount or a star tracker, a mount for your camera rotating around an axis that's parallel to our planet's, and that do a full rotation in the same time (1 day).

Depending on the camera and lens you plan to mount on and how much time you need it to track the stars, this kind of mounts can be as simple as a cooking timer or really big and complex. In the past DIY savvy people actually build this kind of equipment for themselves. They call it "barn-door mount".

Sometimes they are called equatorial mounts, and actually this is astronomical jargon, so I'm not surprised you couldn't find a clue.

When properly used, these mounts let you take long exposures of the stars, with pinpoint stars.

This way you can also take several shots of the same star field, and then stack them together to get a final image with less noise and more dynamic colours and light (it's more complex than this, but I hope you get the point).

If there's also some landscape object in the frame, you may want to do the same, stacking several untracked images to get a less noisy image of the landscape. Finally, you can merge cut away of mask the sky from the stack of the landscape, and put it over the image of the sky to get an overall nice, bright and crisp final image.

In the past few years telescope manufacturers made several small, light and convenient astronomical mounts like the SkyWatcher Star Adventurer Mini, often called just "sky trackers", aimed at photographers that do this kind of "landscape astrophotography".