Creating Star Trails
The night sky of South Africa is brilliant and free from light pollution. It is mesmerizing to see how many stars there are up there. I have always wanted to get an image of star trails above of my tree houses. This year I was going to do it.
With digital cameras it is not possible to leave the shutter open for hours like in the old days. A long exposure on a digital camera will cause the CMOS sensor will heat up. Instead your image will be a collage of many different exposures each capturing a star at a different position. You must use the timer and an interval that gives you an acceptable spacing between exposures that will still look like a complete streak after you do the Photoshop work.
Practice at Home Before the Trip
I started by practicing at home: this helped me refine and fine tune the number and timing of exposures to get the type of trials in my vision. Some of my settings did not give me long enough star trails; filling the image top to bottom when combined. Some of my settings gave 200 + images, but the intervals were either too small or too large. If the intervals between exposures was too large, the trail was a dotted line and if it was too small the trail was solid and bright, but short and could have been achieved with alot less images.
Under the Brilliant Southern Hemisphere
In Africa I waited for night with no moon then set up my tripod with my Canon 5D Mark II. It was still light out and I used this waning light to get several exposures of the tree house HDR style. Then I waited until it was fully dark.
From my experiments at home I had decided on the following settings:
| Lens: Canon EF16-35mm F/2.8 zoomed in at 20mm | Lens Mode: preset the focus then switch to manual |
| Camera Mode: manual | Aperture: f2.8 |
| ISO: 160 | Shutter speed: 30.0 sec |
| TC-80N3 timer/remote control was set to interval timer mode. Set for 1 minute 31 sec for a total of 3 hours. |
I was pretty nervous leaving my good camera out there alone so long. What if a giraffe or zebra came blundering upon it – they would at best knock it over and at worse trample it. It was getting very late and I was glad to pack up the camera and get some sleep.
Processing the Results
The next morning I am ready to see my results. First I create a folder for images and copy them onto my computer. From here there are a few different ways to prepare the files. If you have light pollution in your photos you may want to adjust in Lightroom to minimize the effect first. In Lightroom you can make changes to one image in Develop Module then copy the changes to all the photos in the series.
I had no light pollution and no other need to adjust the photos so I did not need Lightroom. I simply transferred the files onto the computer.
Load the Files into Photoshop as Layers
My computer could handle loading all of the raw files into a single PS document, but if you wish to work with a smaller files: use Lightroom's Export function to export as smaller tif or jpgs files.
To Load all the files into layers in Photoshop:
From Bridge: CMD (CTRL) click to select the multiple files. Then choose Tools>Photoshop>load Files in Photoshop Layers
A new PS document will be created with each image on a separate layer. You can also access this command from Tools menu in mini bridge if you are using the new CS5.
Getting the Layers to Combine to Show all the Stars
Once the images are loaded as layers you need to make all of the images combine. The need here is for the layers to work together such that if there is a light object (a star) on the layer it will show, otherwise use the background (dark). Changing the blend modes on each of the layers is the best way to do this.
Difference is the blend mode I used. Difference mode does a complex comparison of the overlying layer and the image underneath. Black results if there is no difference. Lighten and Exclusion blend modes may also give you good results.
To set every layer blend mode to Difference: Set the blend mode of one of the layers, then choose the layer, right click and select Copy Layer Style. Select all of the other layers, right click and choose Paste Layer Style
At this point I save my document with all of the stars layers then flatten the image and save under a new name. This way I have ease of use and preserve memory, but I can go back if I need to . I copied the stars layer and added a slight blur to get rid of the pixel look from the individual exposures. I also used a curves layer to bring out the dimmer stars. This made some of the most prominent stars too bright so I masked them out of the effects of this curves layer. If you prefer you can add a Hue Saturation layer to change the starts to all white. My camera interpreted some of the stars in different colors like blue and red. I desaturated about 60% and decided I liked a small touch of color
Adding in the Foreground Building
An image (or multiple bracketed images) of the building taken before dark is placed on a layer above the star trails. First you need the night sky to replace the overexposed dusk sky in the building shot. Since the sky is all a narrow range of highlight and mostly a white-blue sky which contrasts with the black night sky so this is a good application for the Blending command. Click on the Add a Layer Style button and choose Blending Options. I had good results using the blue channel only. You want the bright to vanish in favor of darks on layers below, so move the slider from right to left. When you see a good result, split the slider using the option (alt) key to refine the transition.
From here you will want to add a mask to this layer and mask out any edges that still show a fringe of light or things like tree branches that have become disembodied. Blurring your mask also helps soften any edges and makes good transition from light areas to dark areas revealed in the stars layer.
This project was heaps of fun and well worth staying up late to do some star gazing in the Southern Hemisphere.