There is plenty of information on the web about night photography and star trail photography. As I decided to immerse myself in the subject most information I found was contradictory and sometimes unclear, hence this post on the techniques I use.
Before we start, this is just how I do it; they're the methods I have tested and tried and applied to my workflow and equipment. Yours might differ, but I think you'll glean some useful information even if you have a different setup.
The first lesson learned
I embarked on my first star-shoot armed with a digital camera, a lockable shutter release cord and a tripod, thinking I'd just put my camera on the tripod in bulb mode, open the shutter for an hour and hope for star trails. The result was more than disheartening. Sure, I had star trails, but they were hardly apparent, and the image was full of digital noise and coloured blotches.
My first lesson: You can't take high-quality digital photos with excessive shutter speeds of 30 mins, an hour or more.
The imperfections and severe quality reduction in the images caused by something called Thermal Noise are created by the electronic camera sensor heating up during the exposure, making the noise and colours (called Hot Pixels) in the image.
There are a few ways of reducing digital noise in long exposure photography. The first I came across was the in-camera Long Exposure Noise Reduction (Long Exposure NR). This camera function effectively takes two photos, the first is your desired photograph and once complete, the camera takes a second but with the shutter closed. The second image is effectively a black photograph but will still show the same thermal noise build-up of the first image. Finally, the camera evaluates both images and subtracts the noise of the second image from the desired, first photo. Clever, indeed and it works to a degree. The downfall of this method is after, for example, a one-hour exposure the camera needs time to take another exposure which can also take up to another hour. Long star trails may require up to 2, 3 or 4 hours exposure time. Do you see the problem with this method? No, Long Exposure NR isn't going to work for star trails.
The way forward
After this noise fiasco, I decided there is only one way to get quality long exposures, and that is not to take long exposures. The only way to capture digital, quality long star trails is to shoot lots of shorter exposures and merge them all in Photoshop to form one image.
Unfortunately, that knowledge opens up more problems because we need a way of taking maybe 200 or 400 images with exact similar exposures in precise intervals without moving the camera in any way. Now I was looking into buying a gadget specifically designed for this, called the Pclix XT, an Intervalometer which lets you do time-lapse photography. Around the same time, I was buying a new digital camera and ended up buying a Nikon D800. Imagine my surprise when I found out the D800 has a built-in Intervalometer...Christmas and Birthday at the same time.
The first hundred
It took a while to figure out the D800s Interval Timer Shooting function. I'm not going to describe here because it's likely you have a different camera and also because it's a pain in the backside to get your head around, so try it at home in the warm before trying it out in the field. Chances are you may have to resort to the Pclix solution. (You sure don't want to be sitting next to the camera pushing the shutter release 400 times.)
The first result. Trails still a little shaky due to settings testing but otherwise a clean image.
Best camera settings for Star Trail Photography
Much information I found was a little contradictory, so here is a rundown of what I found works for me and why.
I use the lowest f-number I can (largest aperture), usually f/2.8. You can get well-exposed images at smaller apertures, but I found the stars are not as clear and that's not what you want.
I use my lowest native ISO, which is 100 on the D800 (yeah, it goes to 50 but it's only simulated). You'll get the least amount of noise at the lowest ISO so going high isn't necessary. One issue with night photography is residual ambient light which can be exaggerated with higher ISOs and will overexpose your image quickly (more on that later).
You'll need to be shooting in full manual mode so select your shutter speed. The in-camera exposure meter is relatively useless at night, so you'll need to play around and find what's best for your camera and lens combination. I generally shoot either 25 or 30-second exposures with a one-second interval.
I always shoot raw...except for long exposures where I opt for jpg. The reason is the file size. You'll later be dealing with a lot of single images which will burn computer resources. Now my Mac is fast, but I like to keep the post-processing faster than the shoot.
I've played with different settings, but by far, my best pictures are in automatic white balance. I tried Tungsten to make the skies bluer but found it contaminated the star colour too much, and any foreground subjects were difficult to colour correct in post-processing.
Out in the field
It would help if you found something you want to photograph in the day time. Either scope out your subject several days before or go before sunset. Setting up in the evening light makes it easy to compose your image. The alternative of setting up in the dark is quite hard. Getting the right focus being the most significant hurdle as you don't want to spend two hours shooting off hundreds of images to find they're all not sharp.
So daylight set up is most accessible, but you'll have to wait a lot longer before you can shoot once the sun goes down. The reason is ambient light from the sun is still around up to and over an hour after sunset which will overexpose your first images.
I've found the best times to start shooting are around midnight where it's usually dark enough. I do like to take a couple of single shots earlier to give me some foreground detail which I edit in later in Photoshop.
We don't do dew
Probably one of the most significant issues with long night exposures is the build-up of dew on the lens, ruining the clarity of your images. Caused by the lens being colder than the ambient air temperature, dew is one of the biggest headaches you'll have to solve. Depending on the weather and time of year, be prepared to find a suitable solution as you will encounter this problem.
There are solutions out there for astronomers, but they're expensive. You can also find instructions for homemade electrical solutions on YouTube, but I don't want to carry more equipment and battery packs with me.
In my opinion, by far the most convenient and serviceable solution is the gel hand warmers from camping stores which, when secured around the lens, stay warm for up to an hour. They're held in place with a double-layered neoprene band and wrapped with thick bubble wrap for extra insulation. Not pretty, but it works. I also take my Giottos Rocket-air blower with me to blow the lens clear from time to time.
Ambient light - from towns and villages (or even the sun) will give your photos a heavy orange tinge. It's best to find subjects as far away as possible from any form of street lighting, but it's going to be very hard to find areas with no ambient light unless you are lucky enough to live near a massive desert.
Boredom - If you can't set your camera up and go off to watch TV or sleep due to camera security, you'll need something to do to keep busy. I usually take my iPad with plenty to read or listen to for hours I'll be out.
Light - You'll need adequate lighting if you're off into the absolute darkness. A good lantern and a headlamp will do the trick nicely.
So you're back in the warmth and sitting in front of the computer. I load all the images into Adobe Lightroom where I'll pick one of the lighter images and edit that to how I like it. Adding clarity and maybe tweaking colour temperature, high lights and shadows etc. From there, I'll copy and paste the develop settings to all the other images.
Starting from the end of the series of images I'll highlight around 20 to 30 and select "open as layers in Photoshop". In Photoshop, I change each layers blend mode to Lighten then merge those layers. Then select the next 20 to 30and repeat the process. The process of changing the blend mode to Lighten makes the star trails show through the image stack giving you the trails. Merge everything, and you have your photo.
You now have a comprehensive guide to getting started. The rest you'll have to play with and adapt to your workflow and equipment.
I have explained the confusing Intervalometer issue in my blog post Nikon D800 Intervalometer explained
About Robert Rhead
Robert Rhead is a English landscape photographer, UX/UI designer, web developer and graphic designer. He currently holds the LRPS certificate from the Royal Photographic Society, has won English and international photography awards and has featured in various high-profile photography and lifestyle magazines.