A time exposure at Mauna Loa Observatory, lasting a little over an hour, produced star trails as Earth rotated. (Jonathan Kingston/Aurora Select for the New York Times)

What are Star Trails?

Simply stated, star trails are the streaks in a photograph left by stars during a time exposure as the Earth rotates.  Objects in the foreground of the photograph remain sharp as they are not moving relative to the camera, while the stars, depending on which cardinal direction the camera is facing, will form concentric circles or streaks in the sky.

Why does Photographing Star Trails require a different technique with a Digital SLR?

In order to capture star trails, one must leave the shutter open on the camera for a long amount of time, often for more than an hour.  This allows time for the earth to rotate and form the streaks associated with star trails.  With film one simply set the camera to the “bulb” setting, locked open the shutter with a cable release, and returned a few hours later to close the shutter.  This method unfortunately does not work well digitally as one of the drawbacks of digital SLR photography circa 2011 is the longer the shutter remains open, the more the image degrades with visual grain, otherwise known as digital noise.

The solution is to slice the single long exposure one would have used for a film capture, into many small separate time slice exposures and then combine them back together into a single frame with a technique known as “Stacking”.  Dan Newton of Liquid In Plastic has written an excellent post HERE on how to photograph star trails using the stacking technique.  I suggest you read it before proceeding.

How to stack your star trail time slices into a single image in Photoshop:

I recommend using Dan Newton’s ”Method 2″ to stack your star trail image slices into a single image in Photoshop.  For a large part of this year the page that Mr. Newton linked to in Method 2 (Chris and Dawn Schur’s photoshop action) was broken, so I have taken the liberty to re-record their action verbatim and make it available for download on this website.

Downloading and loading the star trail stack action into Photoshop

1) Click here to download the Star Trails Photoshop Action.
2) UnZip the file.
3) Copy the action into the Photoshop>Presets>Actions folder.  On the Macintosh the pathway is as follows:  Applications>Adobe Photoshop CS5>Presets>Actions.
4) Open Photoshop and click on the Actions Palette (Window Menu>Actions).
5) In the upper right hand corner of the Actions Palette click the icon with four small horizontal lines.  A drop down menu will appear.  Scroll down and click “Load Actions…”.
6) A finder (explorer) window will open.  Navigate back to your Photoshop>Presets>Actions folder, highlight the “Kingston Startrails Stacker.atn” and click “Open”.
7) In your actions palette you should now see a action called “Kingston Startrails Stacker”.

Running the Startrails Stacker Action in Photoshop

1) Open the dark frame that Mr. Newton talks about in his blog post in Photoshop.
2) Go to the File>Automate>Batch menu in Photoshop:

• Under the “Play” box select the Set: “Kingston-Startrails-Stacker” and the Action: “Star Trails Stacker”
• Under the “Source” set the drop down menu to “Folder” then click the “Choose…” button and select the folder that contains all of your star trail image slices.
• Be sure that “Suppress File Open Options Dialogs” is checked
• Be sure that “Suppress Color Profile Warnings” is checked

3) Click “OK”.
4) Wait while Photoshop assembles the stack.  You will see a bunch of images opening and closing in rapid succession.
5) When Photoshop is finished go to File>Save and give the resulting file a relevant title.

Dan Newton has some excellent suggestions on modifying the  action under his “Method 2″ section here that I suggest you experiment with.

That’s it – Star Trails 101!

Watch how to create startrails using the stacking method the in the following video:

In the following video I demonstrate the above steps as well as show you how to use a Star Trails Photoshop Droplet that you can download HERE to further automate the stacking process.

How to create a Star Trail image using a Photoshop Action from Jonathan Kingston on Vimeo.

How to load and use a star trails Photoshop action to stack multiple, consecutive exposures into a single frame to create a star trails image.

This is the first time in a number of years I have not participated in the pre-release beta testing for Photoshop, and I must say it is strange to be out of the loop and unconstrained by an NDA.  One thing I know for certain is that Adobe works on 18-24 month product cycles, and CS4 is just about to hit the 18 month mark.  In my humble opinion its lunacy to have such a short product cycle as the consumer barely has time to adopt and learn the updated program before it is depreciated by the next product cycle, but, that being said – everybody gear up for the next life change from Adobe!