Star Track Photography

Back Porch Milkyway,  Chesterfield New Hampshire

In the last several weeks I have been exploring star field and star trail photography. It have dabbled in these areas in the past including taking long star exposures on film years ago, but recently my interest has been renewed by a new project. Over the last year or so I have had the wonderful opportunity to work with Rabbit Ear Films, a small local group of dedicated, talented people, on a documentary film about Mount Monadnock. I am a rank amateur at video but have taken the opportunity to learn while contributing in small ways to this exciting project.  Our goal is to produce a feature length documentary, meeting the high broadcast standards of PBS. Leading our group is Steve Hooper a veteran editorial photographer for the Keene Sentinel and Dan White who works with Florentine Film, Ken Burn's production company. My role has largely been to collect high resolution "B Roll" video of views around the mountain. This works well for me since it is largely video versions of the still images that have been a major part of my portfolio. It is the kind of work that the 5D Mark II handles especially well.

Recently I began thinking about using time-lapse techniques to record star fields moving across the iconic profile of Mount Monadnock. I thought it would make a nice addition to the film.  I have experimented at home using an intervalometer and my old Canon 5D.  I decided to dedicate my 5D for time-lapse to spare the shutter on my newer Mark II. Recently Steve Hooper and I spent a clear cold evening recording the stars over Monadnock from the lovely home of friends in Marlborough New Hampshire. Over about 2 hours we record more than 200 images and after processing we were rewarded with about 13 seconds of a dramatic moving star field video. Unfortunately the lower resolution video here does not reflect the impact of the full resolution version.

Canon Intervalometer

A key component for this process is an Intervalometer which is essentially a remote trigger that permits precise control of exposures longer that the Canon's max of 30 seconds. Importantly for star photography the intervalometer also allows the capture of multiple exposures set at any interval desired. In this case the 200+ images were all 20 seconds long and taken every 30 seconds. You can set the parameters and then go get coffee or even better, take a nap. At the end the images can be assembled into a video using a number of different software solutions (I have been using QuickTime). As you might expect there are many important details that make for a spectacular video and I am still learning. I will discuss this in more detail in the future when I have more experience, but today I would like to discuss another use for these images.

Star Track Photography

Back Porch Star Tracks Chesterfield NH
Densely Packed

You have all seen pictures of the night sky taken over minutes to hours which show the stars tracing concentric circles around the north star, my Northern Hemisphere bias is showing. The pictures are a palpable demonstration of the rotating earth, but they also can be quite beautiful especially when the star trails are framed by interesting foreground elements. Years ago, when I experimented with this using film, I opened he shutter on "bulb" and timed long exposures with my watch. The results were great but noise was always a problem. The same long exposures can be captured on digital cameras with or without the use of an intervalometer, but there is also a noise problem with such long exposures on digital sensors.


Over prolonged exposures digital sensors heat up and the result can be increasing amounts of distracting noise. The severity of the problem varies with different cameras but with longer exposures it becomes a issue for them all. The digital solution is to take multiple images and combine them into a single picture using stacking, blending the layers to create a array of smooth continuous arcs. In my examples, I created the star track by using a selection of the images originally collected for the moving star fields. I experimented with the number of images, fewer creating a less densely packed sky as in the tracks over Monadnock. In the tracks over my back porch, more images created a much busier perspective.


The procedure starts with collecting multiple sky images. I hope I don't have to mention that a tripod is a necessity - don't be offended. You want a clear sky without a great deal of light pollution or intrusion of the moon, but some foreground elements can provide pleasant framing and context.

With the camera on manual you can experiment with exposure. Longer exposures will cut down the number of images required and increase the number of visible stars. The nice thing about the multiple-image approach is that, when you get home you can fine tune the final image by adjusting the brightness and the number of

Beaded Star Tracks with 10 Second Pauses
300x

images that you include in the stack. Shooting in RAW would allow the greatest leeway for post processing, but since QuickTime doesn't take RAW images, I use high resolution JPGs. It is generally best to take pictures immediately one after the other to avoid gaps that could result in a dotted line effect.  I was able to use my star field images, which had a 10 second pause between images, without any grossly apparent  gaps, but a close inspection shows that even this short pause leads to noticeable beading at high magnification.



Once home I pull up all of my images in Adobe Bridge;

Edit Single Image in Adobe bridge

I start by editing one representative image, adjusting brightness, contrast and color, and I try to find a balance between sharpness and noise reduction. When I'm happy with the results I save the settings and then load them to all the other images

Load Images Into a Stack of Layers

In Photoshop, from the "Files" drop-down menu, I select "scripts" and then the "Load Files into Stack" option to create a single file with each of the images as a separate layer. The result is a single image file with all the original images as layers in a stack. Because the camera was locked down on the tripod I haven't found it necessary to align the layers. 

Apply "Lighten" Blending Option to each layer

In order to get the stars to shine through all of the layers the blending mode must be changed to "lighten" on each image layer. This blending mode allows anything in a layer which is brighter than the layers above to shine through. This can be done manually, but I have created an action to make the change on each layer with a single click. There are a number of programs available which do this manipulation automatically, but I find it helpful and entertaining to reveal the layers one at a time, watching the arcs build until they reach the a level which seems best. 




Once I am pleased with the blended image, I flatten the file and can

Plane Trails from Boston

then fine tune the, much smaller, image. One inevitable subject for editing are the ubiquitous airplane trails. In the time-lapse videos these flash by like shooting stars, but on star track images they are random streaks across the image. It is of course a matter of taste, but I think it is worth the time to edit out these scars. Of course airplane trails can be reduce by shooting away from busy flight routes and by shooting later at night. Air traffic falls off significantly after midnight.

Star Tracks over Monadnock (Cleaned)
Less Densely Packed

That's about it. Not too complicated, but with impressive and unique results. Just a couple of other technical points. First if you plan to record many images over an extended period of time you should be sure to use a fresh battery. This is especially important when you are out in the cold. Once you have all the settings locked in, switching off the camera's LCD monitor can extend battery life. If you decide to get a intervalometer you will find that the official Manufacturer versions are quite expensive. The Canon intervalometer is $135, but you can get knock-offs for around $30 that look and work the same. My intervalometer came from China, it cost $25 and has worked without any problems.


So get out on the next clear dark evening and give star photography a try.  There is a lot to learn and I feel I am just beginning. As spring arrives it will be much less uncomfortable and we can come home with something truly special.