Star Trails Over London
By Leigh Cousins 12/10/2015
Canon 5D Mark II 16-35 USM 16mm 3.2secsx350 f/4 ISO1600

My first star trials photo. Taken from Richmond Park overlooking the heavily light polluted London City! Richmond Park itself is the darkest place in London and gunning for a Dark Sky Reserve listing. admittedly I chose the worst location to emphasis this, but walking into the centre of the park truely is dark and the stars are seen in their hundreds, but I had the idea of London glowing below the stars with this particular bench in the foreground. With all the research I did, light pollution seemed to crop up quite often, I wanted to see it for myself.

I thought I'd take this chance to go through my workflow and pinpoint some problems I had to deal with, if your interested in trying star trail photography or even night skies then read on...

The Workflow

The Idea
Very simply, a background silhouette, lots of clear sky and a foreground object. Richmond Park has little to choose from in terms of foreground, a tree, bench, road or maybe a pond for some nice reflections. I chose this location as I have sat there many times looking out over London, the picture to the left shows roughly what the location looks like... But much darker!

The Research
Research the weather, clear skies with very little cloud cover if none, humidity high but not essential.
The phases of the moon are very important as the moon sheds a lot of light into your scene and most of the stars will not be visable.
North. Face the North star if you want that spiral effect.
If you can, scout the locations you have in mind during daylight, make sure everything works. One big problem I had was planes.. 5-6 of them constantly flying around!

The Timing
Timing your shot is what will make it. What you have in your mind, be it dark landscape star trails or in the city, taking your shot 6 hours after sunset or before sunrise would yeald the darkest skies and more prominent stars, but anything 2-3 hours each way will still be good but have a faint orange glow of the sun.

The Shot
What you read online regarding star trails photography is just a basic guide. Your settings are completely independent on your shot and your equipment. I used an ISO of 1600 at f/4 with a 3.2sec exposure time. In manual mode I set the camera to multiple shooting, did a few test shots and then using a shutter remote I could let the camera do its work as I wondered around trying to pass a couple of hours. Problem was that after 45minutes my card was full with 500 shots so it was probably better to increase the aperture to compensate for the exposure time.
Focusing on the stars at night is never easy, lucky enough I had the London skyline in the far distance to use as my focus point which was close enough to infinity. If you don't have any distant objects to use then setting your lens to almost infinity should be perfect. Always take a test shot to be sure!

The planes, something I never thought about until I started the shot and just saw them all flying around. The final blend was littered with bright lines across the image which I decided to edit in every shot before combining.

The car headlights. Even though the only car that pasted that night light up the scene and the bench, it also produced a bright hue in the sky. You wouldn't think it would affect the final merge so much but it did. It produced a tight banding across the entire image as you can see from an early render I did.

The Post
Post production stage. My finished shot is a combination of 350 shots blended together, I had to remove the first 100 or so shots and also 4 shots in the middle where the car passed by. At first I used Lightroom to edit and sync all the images before importing to Photoshop but due to the crazy planes I decided to >Load files into Stack with Photoshop and remove the plane lines from each photo before editing in Camera Raw and blending together.

Then it was just a case of overlaying the shot where the cars headlights illuminated the bench and painting that over the final image. I did take some shots where I light painted the bench myself with a torch but I found the unexpected headlights shot to work very well.

Conclusion
Realistically for nice long star trails a minimum of 2 hours shooting should be good, this would mean increasing the exposure time to around 20 seconds, thus producing 360 shots over a 2 hour period. In light polluted areas this is easier said then done but again, play around with the settings to best suit your shot. Adding a 2-5 second delay between shots is not advised as you will have breaks in the trails.

Study the scene for unwanted light like planes or cars, even a little light leaked into one shot will have an effect on your final shot.
Clouds will also ruin your shot, you can see in my shot the clouds starting to roll in from the right, if I had stayed longer they would have stretched across half the image.

Planning is essential. The last thing you want to do is take 2 hours of shots, then spend hours waiting for Photoshop to process and merge them together, only to find errors that render the image as unusable!