Why Use a Star Tracker

Milky Way at Red Rock Coulee in Alberta by night photorapher Monika Deviat

Milky Way shot using a star tracker. Exposure time was 135s.

A Star Tracker can be a handy tool for a night photographer. For photography, a Star Tracker (sometimes just referred to as a tracker) is a motorized tracking mount that will move your camera at the same speed as the stars movings across the sky. There are also a few other options for speeds and what the tracker can do, but this post will focus on nightscape and in particular, milky way imaging.

When you are taking photos at night, one of the things you have to consider when choosing camera settings is that the stars move across the sky. As you increase the focal length, you increase the distance the stars travel across the surface area of your sensor in a certain time interval. A wider lens will allow you to have a longer exposure before you see an actual trail from the star instead of a pinpoint dot. The length of the exposure depends on a few parameters, including your personal preferences on how sharp your stars are.

Example of star trails appearing as exposure time increases in night photography.

Three slices from images with exposure times changing while all other settings were held constant. On your phone you probably can not see the difference between them. On a monitor it is probably a minimal difference.

Zoom in on stars trailing at different exposure times

Zooming in further makes the star trail more apparent between 10s and 20s.

The rule of 500 is often the standard practice to calculate an acceptable exposure time for stars. To apply this rule, you divide 500 by your focal length to get the shutter speed (if you have a crop sensor there is another factor to consider). However, this can still give you some trailing, so a rule of 400 can be used instead.What you are going to do with the images may also affect your decision. If you don’t mind a bit of a trail, you can get away with pushing your exposure time. If the photos only going on Instagram for example, slight star trails won’t be noticeable. I have printed one of my night images up to 30×40 (from my 12mp D3s) and even though there was some star trailing happening, it was not noticeable when you stood back to look at the print.

With newer high-resolution cameras, there are some other considerations, like sensor size. A very useful app, Photopills, has a calculator for this. You input your camera body, focal length, if you want super accurate pinpoints or will allow barely noticeable star trails, and it will give you an exposure time.

High ISOs are required to capture as much light as possible for night photos. Sensor capabilities constrain ISO choice, and again your personal preference or potential post-processing techniques. The larger the ISO value, the more noise you will see in your image.

I bought the Sky Watcher Star Adventurer (mine is unfortunately not the sexy red colour they currently have). I used the tracker during the 2018 Milky Way season. Here are a few benefits to using a star tracker:

  • Longer exposure times without star trails.
  • Can use much lower ISOs and therefore getting more dynamic range in your photos.
  • All this can lead to less noise in your images.

There are drawbacks to using the device though. Here are a few examples:

  • Polar Alignment (correctly positioning and pointing the device at Polaris) – this can take some time and every time you move your tripod you have to realign.
  • If you want to use longer focal lengths, the alignment needs to be precise.
  • The device, while reasonably small considering, is heavy.
  • You will have to take a shot for the foreground separately and then blend the two shots in photoshop (longer processing times).

The first time I tried to use the tracker, I had no idea if I would get the alignment right. It is a bit tricky. Luckily my friend Matt Quinn was available to answer a barrage of questions before I went out for my shoot.

Foreground image shot without the tracker and at 20s. I used Low Level Lighting techniques to light my foreground.

Star tracker used to photograph the Milky Way by Calgary Photographer Monika Deviat

One of my first tracked images taken this spring. The sky was already beginning to lighten as dawn approached.
The exposure time was 2.5min at ISO800.

Milky Way at Dinosaur Provincial Park, Night Photography by Monika Deviat

Blended and edited final image.

On one of my shoots where I used the star tracker, I found out I need to consider how water would affect the long exposures. As mentioned perviously, you have to shoot the sky with the tracker on and then take another shot for the foreground with the tracker off. I did this with a milky way reflection shot I composed. As you can see in the two images below, the reflections of the stars look strange. In both cases, the stars have “trailed” in the water.

Using a star tracker to photograph the Milky way

Tracked image – the stars are sharp and the foreground is blurred. The reflected stars are showing a circular rotation due to the movement of the tracker.

Foreground image taken with the tracker off. The stars trail in the sky, the foreground is sharp and the reflection has trailed too.

It’s possible to bring your ISO way down and thus have relatively long exposure times. In the example below, I shot for 5 minutes at ISO 200. I wanted my foreground to be a silhouette so I didn’t have to shoot another 5 minute exposure. But if I needed a second foreground shot at the same exposure, that would have meant taking 10 minutes for one image along with however long it took to align and set up the tracker and camera.

Milky Way in alberta badlands night photographer Monika Deviat Calgary

5 minute exposure to capture the Milky Way.

Since I bought the tracker, I’ve shot the Milky Way without the device more often than with it. In some cases, it was that I didn’t have room in my backpack on certain trips to take the tracker, weight and space are always a consideration when hiking or backpacking. When I go to a location, I like to get as many compositions as possible. Having to realign the device in between every composition could eat up a lot of the short window you have to shoot the galactic core. There were also occasions where I would have liked to use the tracker, but I was set up in a spot that did not have a line of sight to Polaris.

There are some situations I like having the tracker and others where the benefits don’t outweigh the negatives. I also have plans to shoot panoramas with tracked shots to create big detailed Milky Way images. I will be committing to getting less unique images and spending most of the time getting tracked images. Deep sky imaging is also something I would like to try out. Overall I’m happy with the tracker and the images I was able to produce with it. It is not something you need to have for nightscapes, but it can be a useful tool.

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