Star trail photography tutorial using multiple exposures

Star trail photography tutorial

This is the first of a two part tutorial on shooting and editing stacked star trails. In this part I’ll describe the process of capturing the photos that we’ll use in the second part to create the final image. Part two, editing of these images, is here:

Star trail photography using stacked images differ from single, long-exposure shots in that the former is a (possibly large) collection of images stacked on top of each other to form the final image.

Photography is often a matter of solving problems, and this shot is a very good example of that. Before we jump into the details I’d like to explain the mental process I followed to arrive at this particular solution.

Dewald Kirsten and I were on a scouting trip for an upcoming workshop in the Tankwa Karoo, on the Hammelhoek guest farm. We drove around during the day to see if we could find a good subject to use as foreground for a star trails shoot when we found this windmill. We knew immediately that we found our location, but there were some problems:

  • It was almost full moon, so I knew there would be plenty of light, perhaps too much to get pronounced trails
  • the wind was quite gusty, and was shifting direction by almost 180 degrees causing the windmill to move around quite a bit
  • I know that to get the trails to connect in a complete circle you need at least an hour’s capture, but I wanted to keep noise to a minimum
  • I wanted the windmill head to be fairly sharp and recognisable, but the changing direction of the wind would completely blur the blades in a single long exposure

We left the location, planning to return that night for the shoot. On the way back I thought about the problems posed by the scene, and what opportunities there were for creating a good image. Shooting a single long exposure is fairly simple – just calculate the exposure, adjust the camera, open the shutter and wait an hour. However, there are some drawbacks to this technique, most notably the problem all CCD sensor have, and that is heat. The longer the shutter remains open, the warmer the CCD chip gets, and this causes noise in the final photograph. Additionally, as explained above, everything that moves during the exposure will be very blurred.

The plan I came up with was to shoot multiple exposures, then blend them in Photoshop. This will result in beautiful star trails. Once I got that, I would pick one image where the head was aligned in a pleasing way, and blend that on top of the trails image to provide a sharp image throughout. For step by step instructions on how to do this see Part 2.

Here is a timelapse video to show what the scene looked like:

What you will need

To do a shoot like this, you will need the following:

  • DSLR camera (duh)
  • Some way to trigger multiple exposures, most cameras have a timelapse feature, but I use a Giga-T Pro II electronic trigger which allows me to program various scenarios
  • Tripod is a must of course
  • We’ll get to the editing in part 2, but you need Photoshop or some other tool that allows for editing and masking in layers

The shoot

When we arrived back at the location later that night, I chose my composition, keeping in mind the location of true south. This is where the centre of the star circle will be. In the northern hemisphere it is quite easy to find true north by simply locating the North Star (Polaris). Here in the southern hemisphere we don’t have a star to help us. I know there are ways to do that by drawing imaginary lines from the Southern Cross, but I find it much easier to just use a compass app on my phone. If you use something like that, make sure it indicates true north, not magnetic north. I won’t go into details about that here.

Next came the decision on what camera settings to use. To keep things simple with the electronic trigger I had to keep my shutter speed below 30 seconds, so I started there with a test shot. I then adjusted the settings until I was fairly happy with the exposure. The settings I ended up with was:

  • Exposure: 25 seconds x 120
  • Aperture: f/6.3
  • ISO: 400

So why didn’t I use ISO 100, and something like f/4? Good question. I have no idea, must have been the beer at dinnertime. When you get to do this, always try to keep the ISO as low as possible to reduce noise. It was certainly bright enough that night.

Using a Hahnel Giga-T Pro II electronic cable release

If you have a different electronic cable release chances are that it will work very similar to the Giga-T. If you do not, and will use your camera’s timelapse feature, now is the time to dig out the manual. My experience with Nikon’s intervalometer function have always been very hit and miss, so I prefer an electronic cable release.

You can find more details on Hahnel’s website. I got mine from Orms in Cape Town.

The Giga-T’s timelapse function uses two sequences in combination. Each one has an interval function (INTVL1 and INTVL2) in seconds, and a counter (N1 and N2). The second sequence acts as a multiplier of the first (N1 x N2). The number of intervals on both are limited to 99, so to arrive at the number of shots I needed (120) I configured the first sequence (INTVL1) with an interval of 30 seconds (just to make for an even number and to give the CCD a few seconds to cool down a bit), and 60 exposures. To get to 120 shots I set up the multiplier (N2) to 02, with an inteval of 30 seconds (same as INTVL1). This gave me 60 x 2 = 120 exposures, 30 seconds apart. Remember the shutter speed is set to 25 seconds. Do not set the delay shorter than the shutter speed, that will result in the camera firing on every second trigger. See below for pictures of the settings.

Once this was done, it was simply a matter of triggering the Giga-T, and hanging around for an hour while the camera got on with capturing the 120 images.

Instructions on editing these images is here:

INTVL1 set to 30 seconds

INTVL1 set to 30 seconds

INTVL2 set to 30 seconds

INTVL2 set to 30 seconds

N1 set to 60

N1 set to 60

N2 set to 02

N2 set to 02