Astrophotography I - Star Trail Images
by Forest Chaput de Saintonge
So, you’re interested in Astrophotography and you’re wondering how to get started. You’re in the right place. This will be the first of a many-part series on the RMSP Photo Tips blog discussing astrophotography. We will start with star trails, move on to the Milky Way, and finally delve into actual deep sky astrophotography.
We need to start by defining astrophotography. Simply put, astrophotography is the process of photographing objects in the night sky. I would consider star trails the simplest form of astrophotography, running all the way up in complexity to long exposure, deep sky images of nebulae or galaxies.
Let’s start with how to photograph a star trail. If you’ve never shot one, that’s a great place to start. Learning to put together a good star trail image will teach you many of the skills you need to progress with your astrophotography.
Shooting Star Trail Images
First, the gear. All you need is a camera, wide lens (somewhere between 16mm and 24mm works well), plenty of memory card space, a full battery, a tripod, and most importantly, a way to lock down the shutter on your camera. I recommend one of these.
Plan to get to your shooting location right around civil twilight. This will give you enough time to set up your gear, frame your image, and more importantly, get accurate focus before the light gets too dim. Once you’re there you want to take a few minutes to plan your shot and do a bit of visualization when it comes to composition. For those of us in the northern hemisphere, stars all appear to rotate around Polaris (commonly called the north star). If you want a fixed star in your image, be sure to include that; if not, work with the eastern or western sky. A great way to previsualize this is to use the PhotoPills app to overlay a future view of the stars over the landscape in your scene. You can do this at civil twilight to give yourself the perfect composition.
Once you have the correct composition, lock the camera down. I recommend adding a bit of weight to the bottom of the center post on your tripod. This can really help with stabilit, especially if there is any wind.
The next hurdle is focus. For ALL astrophotography I recommend using manual focus with live view (mirrorless cameras are always on live view). Once in live view use the digital magnification feature to magnify a contrasty edge in your composition and perfect your focus. A great edge to use could be where the mountains meet the sky or simply a large bright star against the background. If you are lucky enough to be to your shooting location early enough, just use live view and focus on some distant trees. If the trees are far away, they should be at your lens’s infinity point. So, if we focus on them, we also know our stars will be sharp. If it’s already too dark to see much with live view, try to find a bright star and use that as your target.
Now we have focus and framing complete, the next step is to figure out our exposure and start a sequence.
To figure out exposure we need to eliminate variables. Astrophotography actually makes exposure pretty simple because we have quite a few “knowns” that we can use to eliminate variables. We know that we want to let as much light into the lens as possible. This, of course, leads us to use wide-open apertures like f/2.8, or f/1.4. I recommend using the maximum aperture of your lens. For example, I shoot with a f/1.4 lens and I deliberately set it to f/1.4 every night. That brings me to another point. Use manual exposure ALL THE TIME. It’s simply too dark for the meter on your camera to be anything but a hindrance. Don’t trust it; just use manual.
Now that we have the aperture set, let’s move on to shutter speed. We want to get a good arc of the stars moving overhead. Because of this we want a very long shutter speed (to see a lot of movement). If we go too long, however, we will saturate our image and be left with a lot of noise. To combat this I recommend taking a series of 30-second exposures one after the next (with as short of a gap between them as possible) and stitching them together in Photoshop or StarStax. For that reason, shutter speed is easy too; just set it to 30 seconds and call it a day (or night 🤣).
That leaves us with ISO to consider. This one is harder because it changes depending on the night. Usually I find that the ISO should be around 3200, but it’s too variable to say for sure. Instead, get the aperture and shutter speed set and take a test exposure at ISO 3200. Once the camera finishes the exposure check the histogram. We want the majority of the data over on the left side but not clipping (touching the left). Something like the image below.
If your exposure is too bright or too dark, simply increase or decrease the ISO as needed. One thing to consider here is whether the night is going to get darker or brighter from that point on. If it’s dusk and there is still a lot of light in the sky, you might want to ensure that your images are a bit brighter than the histogram I have above so that when the sky darkens you will be at the correct exposure. Alternatively, if you know the moon is coming out in a couple of hours you might want to start a bit darker so that when the moon comes out you will have the correct exposure. Once you start your sequence you really don’t want to change any settings, so be sure! This is one of the hardest parts of the process to get just right, but it’s a super important thing to pay attention to.
Okay. We’re almost ready to start our sequence. We just need to get a couple more settings dialed in. First, be sure to disable Long Exposure Noise Reduction (LENR). This sounds counterintuitive as that should reduce our noise, but it also ties up your camera for 30 additional seconds after each exposure which will result in very large gaps in your star trails between each exposure. Let’s also be sure we are shooting in raw mode (please do this all the time). I also recommend formatting the memory card at this point. BE ADVISED that this will erase all the data on the card but it will also prepare it for new images to be recorded. Since we are about to take a bunch of images, this is a great time to do it, assuming your data on the card has already been imported and backed up. Lastly, let’s set our camera to continuous drive mode. This will help our camera keep the time between frames as short as possible. Again, we don’t want any gaps!
Now that we are ready to start a sequence, let’s summarize our settings.
- Manual focus (with perfect focus)
- Continuous drive mode
- Manual exposure mode
- Wide open aperture (low number)
- 30-second shutter speed
- ISO around 3200 (guess and check for the exact value for that night)
- LENR off
SERIOUSLY, TAKE A MOMENT AND DOUBLE-CHECK THESE SETTINGS!
Let’s start the sequence! Attach your cable release and lock down the button (all cable releases should have a way to lock them down). This will tell the camera to continuously shoot images until the card fills up, battery runs out, or you unlock the release. Awesome! I would strongly recommend practicing/testing this process before heading out to the field. It’s one of the most frustrating things trying to learn how to use a new piece of equipment in the dark, cold night.
As the sequence of images runs, keep an eye on the histogram, but don’t change anything. It all needs to remain the same for as long as you want the trails to be. The longer you wait, the longer each trail will be. I usually recommend at least an hour.
As the sequence is running, I recommend sitting back and looking at the stars. This is one reason I love shooting star trails; you get time to just sit and enjoy the night. If you have a second camera, this would be a great time to grab some Milky Way images or maybe do some light-painting.
Once you’ve shot for long enough, undo the cable release and stop the sequence. I like to look through the images on the camera and be sure the exposure stayed good throughout and that things are nice and sharp. Guess what?!? You’re done! All that's left is the editing.
To help you with the editing portion, I made a video. Here it is! It walks through the process in Photoshop!
So, there we go. Part I is complete. In my next article, I will be discussing how to create Milky Way images with nice round, crispy stars! Be sure to subscribe to blog updates to be notified when that post goes live. Happy shooting!