Greetings astrophotographers! Have you seen beautiful star trail photographs and asked yourself “how did they do that?” In this article, I’ll show you how to create your own star trail photographs to wow your friends and family. We’ll discuss the equipment, planning, and basic processing you can use as a starting point for this part of your astrophotography adventures.

What is a star trail photograph?

A star trail is a type of photograph that uses long exposure times to capture the apparent motion of stars in the night sky due to Earth’s rotation. A star-trail photograph shows individual stars as streaks across the image, with longer exposures yielding longer arcs.

Wikipedia

This is different from other forms of astrophotography, where you want to capture the celestial objects as sharply as possible. See the following two examples for sharp stars and Milky Way photography.

Where to begin – A little bit of physics

Let’s start with a couple of questions: Why do the stars travel through the night sky? Why are some star trails circular and others not?

Why do the stars travel through the night sky?

The stars travel through the night sky because of the Earth’s rotation. This is no different than the sun or moon moving through the sky overhead. 

***Note: If the Earth completes one rotation in 24 hours, then the speed of rotation is 0.25 degrees per minute. Thus, this is the rate the stars appear to move across the sky.

Why are some star trails circular and others not?

Now that we have established that the rotation of the Earth is responsible for the sun, moon, and stars traveling through the sky it’s time to talk about how we see the paths of the stars. 

Northern Hemisphere

Looking North: The spinning stars

If you aim your gaze at the north star, Polaris, you will see the stars rotating around it. 

Looking South: The semi-circle

Because of the curvature of the Earth, we in the northern hemisphere can’t see the southern center of rotation. That’s why star trail pictures taken to the south only result in a semi-circular pattern.

Looking east and West:  Something completely different – hyperbolas

The stars traveling exactly due east and west will trace a straight, diagonal line. The stars north and south of the diagonal line will trace arcs of smaller and smaller radii the closer to north and south you look.

Southern Hemisphere:

Swap the results of the Northern Hemisphere (You can see full circles looking to the south and semi-circles looking to the north).

Now that we got the physics out of the way let’s get into the rest of the astrophotography stuff.

Equipment for capturing star trails

Minimum equipment list – tripod, camera, a lens with a maximum aperture of at least 2.8, and an intervalometer of some type.

• Tripod – you need a steady foundation that will not move from one picture to the next.
• Camera – ideally, you want to use a camera that can capture images at ISO 1600 or better
• Lens – because you are working at night, using a lens that has a large aperture allows the camera to capture more light. This will provide you options for balancing your exposure.
• Intervalometer – This is a device that allows you to take photos at a set interval for your desired duration. If you have a shutter release cable and a timer you can do the job yourself. However, the intervalometer should allow you to program settings and let the camera work while you enjoy stargazing.
  • ***Note: some cameras have intervalometers built into their OS. Check your manual to see if your camera has this feature.

Planning and setup

Planning and setting up your camera equipment are key components to any of your astrophotography endeavors. If you have questions about planning, camera settings, and setup, check out “How to Capture Your First Astrophotography Image”. This article provides a more in-depth discussion of preparation, planning, and setting up for a successful night photographing the night sky.

Methods of capturing star trails

1 – Really long shutter speeds

When I say really long, I mean REALLY LONG. Four hours or more is not unreasonable for some star trail photographs.

Advantage

One exposure and you are done.

Downsides

1. Only one exposure means your shutter is open a long time. This could be minutes or hours, depending of your choice. The longer the shutter is open the more it is prone to sensor noise (pixels that don’t capture data due to the sensor heating up while being used to expose your image). You would need to experiment with your own equipment to see what the limits are.
2. You can’t make modifications to the exposure to reflect changing conditions for the duration of your photograph.

2 – Capture many shorter exposures of the stars that you merge afterward

Most of the time I use a series of 30-second exposures taken over several hours to capture the desired star trails. Each of the exposures will capture a small amount of movement for the stars. You will combine them afterward in your software of choice.

Advantages 

1. You can change your settings to adjust for any changes in your environment. 
2. Taking shorter exposures reduces the occurrence of sensor noise in your images. (This happens when the sensor heats up over time. This results in “hot pixels” or pixels that show up as red, green, or blue when you review the images on your computer.)

Downside

1. Capturing many hundreds, of images means you have to import, manage, and stack them.

3 – Anything else you want to experiment with.

There is no one right solution. You are creating art, have fun and enjoy the process.

Post-processing Methods and Tools

I had created a list of steps and screen captures, but decided that a video explanation would be more straight forward. See the following video for what to do after you have captured the photos you will use to create your star trail photograph.

**TIP – To speed up loading files into Photoshop I recommend you first export the files you plan to use as jpegs. Make sure they are exported as a series (imgxxx-1.jpg, imgxxx-2.jpg, etc.)

You will note that the instructor goes from Lightroom directly to Photoshop without exporting jpegs. This is fine, but will create a larger working file in Photoshop that may slow down your computer.

Next steps

Adding a foreground into your composition. This is really what sets your astrophotography apart.

Learn more

If you are interested in learning more about astrophotography check out the following resources:

• How to Capture Your First Astrophotography Image
• How to do Milky Way Photography

Congratulations on creating your first star trail photograph!

I hope you found this article interesting and helpful for your astrophotography journey. As always, if you have any questions drop me a comment.

Thank you for your time. Until next time, I’ll see you under the stars.