Star Trail Stacking Tutorial

Starry Night Star Trails At The Crystal River Mill
Near Aspen, Colorado

I get a lot of questions about how to make star trails in some of the images I've posted.  So in the spirit of sharing, I thought I'd answer some of the most common ones.  In this tutorial, I'll focus on a technique where I stack multiple frames of star trails.  And in the final part of the tutorial, I'll show how I do a very popular technique - applying Comet-Like Star Trail Processing.

What Does It Mean To Stack Star Trail Frames? 
A star trail image is simply a long-exposure photograph usually captured from a stationary position that includes stars in their apparent motion across the sky as the earth rotates on its axis.  You could shoot a single long exposure image that shows the motion of the stars as long lines in one frame…  or you could shoot multiple shorter exposures of the stars and combine them in post-processing.  This second option is called star trail stacking and is the focus of this tutorial.

Don't get me wrong…  I really like the effects of a super long exposure image with the colors and tones that can appear even through the starry lines.  We try so hard to crank as much as we can out of our sensors in a short amount of time to get the sharp points of light.  Sometimes, I really like doing the opposite and just opening it up to see what the sensor can capture.

I'll post another tutorial later on that focuses on a great technique I use for the super-long exposures without stacking.  But for now, lets get back to star trail stacking.  :-)

Why Bother Stacking?

Wonders Of The Night
Mayflower Gulch, Colorado

Noise Reduction
• Your camera's sensor warms up in a long-exposure which combines with other sources to create some noise.
• Combining multiple exposures can significantly reduce this noise - thanks to the random nature of the noise.

Creative Processing Option
• Shooting one big long-exposure prevents you from being creative in post-processing - where I can selectively apply effects to the different stages of the stars' trails.  More on this below…
• Opening the door from a single frame (in Lightroom) to multiple exposures (in Photoshop) means you can blend in a long-exposure, low ISO frame with the foreground looking as good as it can be.  Newer cameras are great but they all have their limits with a single exposure.  I can't say enough about th is technique and what it does for the quality of a nightscape image.

Shooting Efficiency
• Long exposure times (like 30-45 minutes) can make it challenging to test a shot, tweak your settings and shoot again.
• Shorter exposure times (more like 1-3 minutes) allows you to do this much more easily.

How Do You Capture Star Trail Frames?
I suggest first picking a lens, then an exposure time you're aiming for.  Then, open up the aperture as wide as it'll go to let in as much light as possible.  Then, I set the ISO wherever it needs to be to show a good signal in the histogram.  On my Nikon D800, that usually works out to ISO 1600 (on the low side), ISO 3200 (most often), and as high as 6400 (in some cases).

I like aiming for an exposure time that shows the stars as points to most viewers, but as short lines to viewer who are able to look at the stars up close at full resolution.  If the exposure time is too long, the stars will all look like lines and you'll lose some options on the creative processing side.

Suggested Exposure Times For Stacked Frames Of Star Trails

  • Wide Lens (24mm to 16mm Fisheye): 30 seconds - 3 minutes
  • Medium Lens (35mm - 50mm): 20 seconds - 2 minutes
  • Long Lens (85mm - 200mm): 5 - 20 seconds

Common Questions

Question: How Many Frames Do I Need?
There are a lot of factors including how long your shutter speed is and how thick the sky is with stars.  But that said, I typically find that 15 to 30 frames is plenty.  In the field, I usually shoot a few more but in the end, I find that using more will make the sky too busy with star trails.

Question: Are There Any Tricks For Doing This?
Yes!  Use a remote timer to automate the shooting of these multiple frames in sequence.  They have to be exactly the same duration so you'll want to take the guesswork out of it.  And since you will sometimes need a shutter speed that exceeds 30 seconds, you won't be able to use your camera's internal timer.  A remote timer from a non-brand-name provider can help keep the cost down but know that this is a critically important tool for a star trail photographer.

How Do You Process Star Trail Frames?
When it comes to stacking star trail frames, you have some choices on how to do it.  I've found 3 distinct approaches that feature using a dedicated, standalone program that handles the stacking, or you can stack them manually in Photoshop, or finally, you can use a script to automate stacking them in Photoshop.

Standalone Applications

Watch Your Step
Stacking In Star Trails Standalone Application

Many of my first star trail images were made with Star Trails (the first application listed below) which comes free from a man in Germany.

I don't have significant experience with either StarStax or Advanced Stacker Plus because I tend to do my stacking in Photoshop.  If you like the standalone application approach, they are good options to consider - so I included them in the list.

Star Trails


Advanced Stacker Plus

Manual Stacking In Photoshop With Comet-Like Star Trail Processing
The best part about manually stacking your star trails in Photoshop is that it opens the door to some creative processing like Comet-Like Star Trail Processing.  "What Is Comet-Like Star Trail Processing?", you ask...  well, if we stack our star trail frames by just opening in photoshop and arranging the frames as layers in the same image before changing the blending mode on all but the bottom frame to lighten, we'll see what I would call straight line star trails.  But if we process the head of the trail so it's bigger and the tail so it fades out, we'd have trails that look like a comet.

Look closer at the straight lines in the image titled "Watch Your Step" above and compare it with "Wonders Of The Night" to get an example that shows the difference in how star trails look with and without the Comet-Like Star Trail Processing.  In general, I like the effect the processing creates but I'm careful not to overuse it.

1. First we need to get our star trail exposures opened in Photoshop and sorted in the right order so they're ready for processing.  You could just open your files individually and build a single image with all the layers before sorting them or you could make use of some automation.  One option is to open them as layers from Lightroom by selecting your images, right-clicking, choose Edit-In, choose Open as Layers in Photoshop.  Another option is to export your images as tif or jpg and then open Photoshop and click File, Scripts, and then Load Files into Stack and just choose your files to open.

Wonders Of The Night
Setting Layer Opacity On The Tail

2. Then, we'll need to determine how much to reduce the opacity on each layer to make the tails fade out smoothly.  To do this, divide 100 by the number of frames you have.  As an example, 20 frames would need a 5% decrement for each layer.  In this example, you'd set the bottom layer at 100% opacity, then the one above that would be 95%, then 90%, then 85...  continuing all the way until you reach 5% being the last needed frame at the top.  This sequence of steps creates the fading tails of the comet.  For artistic purposes, you may decide to try this again with the order of the layers reversed - resulting in the comet facing in the opposite direction.

Wonders Of The Night
Adding Star Spikes To The Heads

3. Our next priority is to create the comet heads.  We do this with one of a couple of options for enlarging the size of select stars on our comet head frame (that we just set at 100% opacity).  First, let's make a copy of this layer.  I typically like using Star Spikes Pro for the comet heads but you can use Topaz Star Effects, a fog filter (in Photoshop with a mask, or other more creative approaches.

In Star Spikes Pro 2, I just set the number of spikes as high as it will go (32) and the length of the spikes down to a relatively short level.  Check the image to the right to see the other settings I used in the example. 

Photoshop Scripts
I like having the fine control of stacking directly in Photoshop but as you can see, it can be quite time consuming.  And if you choose to shoot a lot of exposures (like 50 or more), the slower nature of this manual approach can become a pretty big deal.  This issue pointed me toward making the most of Photoshop's scripting capabilities - which are pretty impressive.

It's especially nice to see that some people have already looked to make the most of this scripting capability by creating some that directly address the problems.  To be more specific, I was looking to stack what could potentially be a lot of star trail exposures in a way that doesn't take me a month or bring my computer to it's knees.  Along the way, I was looking to gain speed but I did not want to give up the flexibility I had in applying some Comet-Like Star Trail Processing.  In doing my research, I found that Salim Waguila had created a Star Trail Stacker Script that seemed meet my needs and he'd made it available on his website for free (with a donation). 

Salim Waguila's Star Trail Stacker Script:

I went ahead and followed his instructions to install the script and to give it a try and was very impressed.  I went right back to his site and gave a donation.  Not only is it really fast but it's made for people who want flexibility in their stacking.  For the purpose of stacking star trails, I thought this script rocked!  If you use his script, please make sure to support his work by giving a donation.

Salim Waguila's Star Trail Stacker Script - Start

1. I started by reviewing all the information Salim has on his Star Trail Stacker page (shown above).  This will explain all about what the script does and how to install it.

Once the script has been installed, you can start it from Photoshop by clicking File, Scripts, Waguila_StarTrail_Stacker_V1.1.  You'll then be prompted to select a folder that contains the images you're looking to stack, so it helps to have your images in a folder ready to grab.

You can process as many images as you like thanks to the approach Salim used in designing his script.  Rather than building a giant file with one layer for each image, he merges before the next image is added.  As a result, the file size doesn't grow, the time to process grows instead - giving you an unlimited number of star trail images you could potentially process.

Salim Waguila's Star Trail Stacker Script - Options

2. I like the speed of automation but I really like having flexibility along the way.  And this area is where Salim's script really shines with a plethora of options to choose from.

I usually like the Lighten blending mode, the comet style left to right, and add a final layer for star spike pro effect checked.  With this approach, the script fades out the tail of the comets and allows me to add processing for the comet heads.

Of course, there are lots more options that are described and shown in detail on Salim's Star Trail Stacker page linked above and I encourage you to check them out.

Salim Waguila's Star Trail Stacker Script - Complete

3. After selecting your images and your options, the script then does the dirty work of stacking your images.  For me after having done it manually, it feels like a breeze of fresh air as I watch the script fly through my images, stacking them one at a time.  This speed of processing encourages me to try some different options so I can make sure I'm getting the best possible star trails for my needs.

In the end, I found Salim Wagula's Star Trail Stacker script to be outstanding.  It has the speed I was looking for and offers tremendous flexibility and support of Comet-Like Star Trail Processing at a cost that simply can't be beat!  If you have any interest in this subject, you won't go wrong by trying Salim's Star Trail Stacker script.

I think Salim's work deserves my best recommendation for Star Trail Stacking.  I very much appreciate what he did and that he offers it as his contribution to the community.  Thank you Salim!  If you like his work as well, please show your support by giving a donation.  :-)