Astrophotography is a fascinating subject and with care, you can produce some stunning images of the heavens with just a digital camera, a tripod, and perhaps a useful app on your phone. Your options include twilight images, and photos of the sky long after the sun has set. Not only that, you also have some options to capture the stars in creative ways too. Have you ever wondered how you can capture the movement of the stars and turn the night sky into something that looks like the stargate sequence from the end of 2001: A Space Odyssey? Well, we have information that can help you on your way to capturing the majesty of the cosmos in work that looks really cool as well.
With some basic equipment, and some easy to understand techniques, you too can explore the wonders of the universe and capture the moon, stars, and our own Milky Way galaxy this very night, if you wish. Let’s get started and talk about cameras first.
The gear you need is relatively simple. Your camera can be anything from a DSLR or bridge camera, down to a compact camera. You may find you need to be able to use manual settings, purely for the sake of image consistency, and it also helps to be able to shoot in Raw format although it’s not a deal breaker if it can’t.
We mention Raw with perhaps annoying regularity but it is with good reason. Raw images, as the name suggests, is the data captured from the sensor with no compression or processing of any kind applied to it. It is literally raw and untouched sensor data. This gives you the absolute best original image quality to be working with as you get into the post-process stage of editing your photos, whether you are eking out as much detail as you can from a landscape photo or, in this case, making the night sky look its absolute best.
If your camera also supports mirror lockup, then use it. Mirror lockup moves the mirror out of the path of the sensor before the exposure starts to reduce the chance of vibration causing a loss of sharpness. People do argue that it really has little to no impact on the resulting sharpness and if you are using a big, heavy tripod, that is more than likely. If however your tripod is not so sturdy, then it may still be worth using this function.
The night sky is a big place. It encompasses from one horizon to the other and to do it justice, a wide-angle is normally the lens of choice. Wide-angle lenses in the 14mm – 28mm range are considered wide angle enough for the task. If you have a camera that has a lens with a focal length in this area, you should be good to go. Don’t discount your choices if you have a compact camera with a built-in lens that has a longer focal length than this.
You could still get some great shots with an equivalent 28mm lens or longer, but your field of view would be much narrower than with its wide-angle brethren. Sky shots generally need that wide field of view so you can capture a large amount of sky, as well as some foreground interest such as mountains or trees.
Keeping it steady
If you want great, steady, shots of the sky, you will need a tripod. You are going to be dealing with exposure times possibly in the tens of seconds. Hand-held is simply not an option. You could rest your camera on a rock, or other sturdy base, but if you want proper control over your composition, it’s going to have to be a tripod or nothing.
Let’s face it, if you are serious about your photography, you will have a tripod anyway. When shooting long exposures, the sturdier the tripod the better. A bigger, heavier tripod may be a pain to cart around with you, but it will keep your camera steady as a rock through those all-important exposures.
Another useful item is a cable release. Back in the day, a cable release was a very simple affair. It was a bulb connected to a tube with a plunger at the end that you screwed into the shutter release button of older cameras. Squeezing the bulb pushed the plunger into the shutter button, firing the camera. That is where the term ‘Bulb Mode’ comes from when talking about using long exposures that range from 30 seconds to minutes, or even hours.
These days, cable releases are much more complex beasts. They are programmable in various ways that allow you to literally ‘fire and forget’. You program what length of exposure you want, how many shots you want to be taken, and the amount of time the camera pauses in between each shot. This is much better than standing there in the dark with your finger on the shutter button, holding it down while a 2 minute exposure counts down and possibly shaking the camera in the process.
Since we are talking about long exposures, a lot or current cameras offer what is called Long Exposure Noise Reduction (LENR). Exposures that last many minutes can cause the camera sensor to heat up. This heat can generate additional noise that can degrade the quality of the image. When you take a shot with LENR active, the first normal exposure is recorded, and then it shoots a so-called ‘dark frame’ that is the same exposure duration as the first. It records the amount of noise caused by the heat of the sensor and it uses this frame to cancel out noise in the first normal exposure.
In practice, people argue that it is not worth the effort of having to wait 10 minutes for your first exposure, for example, and then wait another ten for the LENR frame to be recorded. You could literally be there all night! You may be better off keeping LENR disabled and using more effective noise reduction options at the post process stage.
Long or short exposures?
As you prepare for your shoot, one thing to consider at the outset is what kind of pictures you are hoping to capture. If you are looking to shoot the sky and keep all the stars as well-defined points of light, then you will need to shoot relatively short exposure durations so the movement of the earth does not transform the stars into smeared light trails. Of course, if the reverse is true and you do want to capture long star trails, then you are not as constrained by your exposure time. Later in this article, we have two examples of night sky images. One is a typical star trail photo and the other is a capture of the Milky Way. We will cover each one in a bit more detail shortly.
This is once again down to personal preference. Portrait and landscape orientation works equally well. Obviously, portrait orientation means you can see more of the sky vertically up from the horizon but the broad sweep of a landscape shot is just as good. The only time portrait orientation is preferred, is when trying to capture the polar star Polaris. Also called the North Star, this is the star that doesn’t move through the night sky as the earth turns.
Shooting a star trail with the North Star in the frame means you will capture an image that looks like a giant pinwheel, with all the other stars rotating around the one point. The North Star can be found at the end of the handle of the ‘Little Dipper’ (Ursa Minor). You can also use the two stars on the farthest side of the bowl of the ‘Big Dipper’ (Ursa Major) as pointer stars. Draw an imaginary line that connects the bottom of the bowl to the top of the bowl, and then extend that line outwards about 5 times the height of the bowl until you see a much brighter star. That is Polaris.
There are two methods that can be used to capture star trails. The first is to shoot one long exposure. This can range from 10 minutes up to 30 minutes, and in some cases even longer. Prevailing weather, light pollution, and the phase of the moon will have an impact on the exposure time. If the moon is bright, you’ll need to be careful; it will put a limit on your exposure times as its ambient glow will start to overpower the stars. A black, moonless night will let you shoot very long exposures to capture the movement of the stars with enough starlight to illuminate your environment. Obviously, you don’t want to do an hour long test shot only to find you are using the wrong settings.
A great method to get a test shot, taken in a reasonable amount of time, is to bump the camera ISO up to its maximum and shoot wide open, with your lens’ maximum aperture. Once you have a balanced exposure, you can use those settings to calculate how the settings should alter to keep the same balanced exposure if you take the ISO down to 100. Luckily, there are plenty of apps available for your phone that can do those calculations for you. One tried and tested app is Expositor. It has been around for a while now but it does a great job.
The alternative to shooting one exposure over a long enough period of time to register movement, is to shoot much shorter exposures with settings that capture the brightness of the stars, without showing movement, and stack these images together. You can use Photoshop to do exposure stacking or you can use software such as Image Stacker, DeepSkyStacker, and StarStaX to automate the process for you. Alternatively, the short exposure approach is different, but for many, it is the preferred way of doing things.
This time you need to get some balanced settings that give you a much shorter exposure time, but shows the stars clear and bright with as little visible movement as possible. This usually means an exposure time of 30 seconds or less. This is where the large maximum aperture of f/2.8 mentioned before comes in handy. It means you can get shorter exposures at f/2.8, with lower ISO settings than if you had a lens that had a max aperture of f/4 or f/5.6. With the camera settings ready, you need to shoot a sequence of images over a period of time that replicates a single exposure of, say, 30 minutes to an hour.
One clear evening, with no moon in view, we set out to capture some star trails. We were armed with a Canon 5DMK3 and a good EF 16-35mm f/2.8 wide-angle lens. We had a robust Manfrotto tripod and an intervalometer to automate the shooting process for us. In addition, as we were going to be trudging around at night, we had stout shoes and warm clothing including hats and gloves. Clear nights can get cold in the UK and we knew we would be sitting around for many minutes at a time waiting for the exposures to finish. Last, but not least, was a powerful head torch so we could navigate the dark environment easily. We found a spot that we knew faced due north.
We found Ursa Major and used it to pinpoint the North Star. Our composition was set up to include the North Star in the shot. A few test shots revealed the best settings of aperture f/2.8, bulb duration of 30 seconds, and ISO 1000. The intervalometer was set to fire the camera and take 30 shots using those settings. The lens was manually focussed at infinity, and the button was pressed. 15 minutes later, the sequence was done. We then moved location to the side of a small lake, and set up a composition that would show the stars reflected in the lake waters. The settings were almost the same as before, apart from boosting the ISO to 1600, as it was nearly pitch black. Another 30-shot sequence was captured.
The Milky Way
Having captured some stunning star trails, it was time to try something that we had not attempted before, but had always wanted to. Getting shots of the Milky Way is not difficult, but it does require a bit of planning and forethought to get some nice examples. You need to know where the Milky Way is going to be in the sky, what elevations you are dealing with, and also what times it will be visible. Luckily, there are plenty of apps that can tell you.
A personal favourite of ours is PhotoPills. It is a very feature rich app that’s capable of everything from calculating hyperfocal distances and planning sunrise and sunset shoots, to exposure guides and, in this case, Night AR. This feature allows you to view a scene on your mobile device in real time, and overlay the positions of stars, the sun and moon and most usefully for us; the Milky Way. This meant we could see in advance, where the Milky Way would be during the best hours of darkness, later that night. Therefore, we knew where and when we needed to be ready. Midnight saw us arrive at Haytor Rock in Devon with our camera kit.
This time we were armed with a Nikon D810 and a Tamron 15-30mm f/2.8 ultra-wide angle lens, as well as some torches, in case we wanted to try some lighting painting too. We set ourselves up with the granite tor as our prominent foreground item and, since the sky was clear and there was no light pollution, we could easily make out the Milky Way splashed across the sky. This allowed us to set up our compositions accordingly to combine the rocky tor and the starry sky to best effect. Once the composition was set up, we needed to figure out settings to capture the stars without motion blurring them. There are a couple of equations that photographers can call upon.
The 500 and NPF rule
The 500 rule was originally developed for a full frame film camera and requires you to set your camera to ISO 3200, Aperture to f/2.8, and shutter speed to 500 divided by the focal length of your lens. For example, if you were shooting with a 50mm lens, your shutter speed would be 10 seconds (500 / 50 = 10). Many would advocate the use of the newer NPF rule which is a much more complex equation, but takes into account the better quality imaging technology on offer today. Apps are available that calculate the maths for you based on the camera you are using.
We decided to go with the easier 500 rule to begin with, and since we were using a 15mm lens, this would mean a shutter speed of roughly 30 seconds with the aperture at f/2.8 and the ISO set at 2000 for a decent exposure. Your focus for images of this nature require you to set focus manually at infinity, or to lock focus on a distant object and then set it to manual so the focus, once set up, didn’t change. A few test shots showed that this was working well, and on the Nikon D810, even the ISO value of 2000 still gave good, clean, results. What was not working so well was our foreground interest. The rock looked dark and uninteresting. Since we had torches, we decided to try some of the exposures with torchlight being shone on the rock face from camera right. Suddenly the images transformed, and the granite tor looked far more interesting when lit from one side. We were able to come away that evening with some great images of the Milky Way, ready for processing back on the computer.
So you have your sequence of images for your star trails and Milky Way shots, the next stage is to process them in your favourite Raw editing program. Adobe Camera Raw (ACR) and Lightroom (LR) are perfect for the job, but you may also have software provided by your camera manufacturer. The main thing to be aware of is to process your entire sequence all at the same time, so that you have consistency. Try not to be too heavy handed but make sure your stars are bright and clear. You can then save the sequence as a series of good quality jpegs or tiffs, ready to be stacked. The stacking sequence in Photoshop is relatively easy. You just need to load all the images as a series of layers into one document. Then make sure they are all selected and set the blend mode to Lighten. All the brightest detail will be merged and you will be able to see the path taken by the stars in one beautiful star trail.
The single Milky Way images can be processed to reveal maximum detail and then brought into Photoshop for any final finessing that you may require. With that, you now have two very different approaches to capturing the night sky.