Star Trails & Timing

Let me first say, "RTFM!" When you're finished come on back.

Okay, now that you read your camera's manual you know how to use it. Or so you think. And that is why this star trail picture is not a picture of star trails, but star dots. Though I screwed up, it did make a pretty cool image. So how did I mess it up? Easy, I did what any photographer can do. I confused minutes with seconds. 

Before I get to that, a little back story. When this image was created we were camping in the Eastern Sierra at Twin Lakes just west of Bridgeport, California. The weather was tremendous with a New Moon, so the sky was good and dark. It was a perfect night for capturing star trails. 

To create a nice circle, if you aren't sure how to do it, point your camera in the direction of Polaris, the north star. The earth's axis points, more or less, right to it. If you are in the northern hemisphere, that is.

For this image, I set my tripod in an alpine meadow adjacent to the campground. On it was my old Nikon D7100 with a 17-50mm f/2.8 Sigma lens. I shot at 17mm with the lens wide open for 20 seconds/interval at ISO 1600.

The way star trails were made in the film days you shot with your lens wide open and the shutter would stay open for a length of time long enough to get trails. The longer the shutter was open the longer the trails. It was best to have a fully mechanical camera so the shutter could stay open as long as needed.

With an electronic shutter it would only stay open as long as your camera battery would last. The downside of star trails on film is that you would record skyglow along with your trails. So you would get both star trails and an orange glow from distant cities. Not ideal.

With a digital camera you instead take a series of shots at a specific interval then compile all the images in stacking software. If you want longer trails you take more photos over a longer period of time. Ideally, you want a couple of hours of images taken every 5 seconds for about 20 seconds each with the widest lens you have set to the widest aperture focussed at infinity. Doing so will get you nice trails on a black sky and no skyglow. 

Here is where the "500 Rule" should be mentioned. This isn't as important for star trails because, well, you want trails, but if you are photographing the night sky and want to avoid star trails you have to apply the 500 rule. That is, 500 divided by the focal length of your lens, which equals the longest exposure (in Seconds) before stars start to “trail” in your picture.

Why are there dots?

As I stated above, RTFM to mitigate mistakes. Or at least know the difference between 5 seconds and 5 minutes. And that is how this image has dots and not trails. When I set up the shutter interval in camera I set it to take a 20 second photo every 5 minutes when in fact I thought I was setting it to 5 seconds. 

To my surprise, when I woke up the following morning the camera was still working. As I calculated my intervals, it should have finished sometime around 4am. Yet it was still going at 6am. It was early, which further lead to my confusion until I realized my interval timing was wrong. Had I done 5 seconds I would have had trails.

After I got home and compiled the photos into a single image I was pleasantly surprised. This image is unique such that I have never seen anyone post a photo like it. So a mistake actually made for something unique and interesting, and gave me something to write about.

The takeaway is that you can salvage a mistake and make something interesting. The other takeaway is that you need to check and double check your settings making sure you will get what you think you will get. Lastly, pay attention to what the camera is doing after you start the series of intervals, and don't just walk away like I did. Let it take a few test shots.

I use StarStax to compile all of the images to create a star trail photograph. It's free to download and use on Mac, Windows, and Linux.