Photo tip of the week: How to take startrail photographs
As Earth spins under the sky, the stars appear to move. When a camera captures that movement with a long exposure, it is called a startrail. It is a cool technique that every photographer should try at least once and in this tutorial I will show you how it’s done. Lets get started.
Firstly, you’ll need a camera that has manual control over exposure settings and the ability to connect an intervalometer (or a built in intervalometer). Any basic external intervalometer will do. I recommend avoiding the in-camera intervalometers as configuration options can be a bit limited. The Nikon ones for example are limited to exposure times of 30 seconds or less.
You will need to choose a camera with reasonable noise performance as one of the critical part of this style of photography is being able to shoot at high ISOs (6400 and higher). It is difficult to change a battery mid shoot without moving the camera and as you will be running the camera for long periods of time, use a battery grip if you can. A good modern DSLR with a battery grip is usually sufficient.
With your camera sorted, it’s time to think lenses. I prefer ultra wide angles. These allow you to compose a shot with an interesting foreground while still capturing a good amount of sky in the frame. You don’t need the fastest lens you can get your hands on – f/2.8 isn’t required and an f/4-5 lens should be fine, but slower lenses will require a higher ISO setting to compensate, which might be an issue on camera bodies with poor noise performance.
You’ll also need a solid tripod. Bigger is better as smaller flimsy tripods can move around and flex in the wind which will result in startrails that aren’t smooth. Its also best not to fully extend the legs of the tripod to make it more stable. You can hang a weight (like a camera bag or bag of rocks) from the tripod to increase stability. Check to see if there’s a hook on the underside of the top plate for this.
Under certain conditions dew can form on the lens element. To prevent this you need to keep the temperature of the lens element slightly above the ambient air temperature. This can be done with a battery powered heating strap wrapped around the lens barrel, or by placing chemical hand warmer sachets (available from camping stores) on top of the lens barrel. I find the heating strap to be the best option, but you need to keep it powered for the duration of the shoot, which will usually require a few battery changes.
Finally use the highest capacity memory card you have. You’ll want to shoot in the highest quality setting available on your camera (ideally 14bit raw), and a high megapixel camera will easily fill a 64gb card if shooting right through the night. If your camera supports dual cards you may like to set it up for overflow onto the second card if required.
Planning the shot
Failing to plan is planning to fail and when shooting startrails there are a lot of factors to take into consideration. Once I have a location in mind I’ll start with a smartphone app called The Photographer’s Ephemeris. Using that I can look at an aerial view of the location I’ve scouted and get an idea of which direction I’ll need to point the camera to get the type of trails I want. In the southern hemisphere aiming south will give a circular pattern, east or west will give straight lines, and north will give a rainbow shaped pattern.
You also need to pay attention to the moon phase. It is possible to shoot startrails under moonlight but results are generally better under the darker skies you get around the new moon. I only shoot when the moon’s disc is below 40% illuminated.
If you are shooting on the coast look at the tides, as you don’t want to start a shoot and find your camera underwater a few hours later!
And finally keep an eye on the weather conditions. The website skippysky.com.au is good for cloud predictions.
I try to visit a location prior to the shoot to work out the best composition. It is much easier when the stars are visible in the sky as you can work out exactly where the centre of the star circle will be. You can use a compass to get an idea of this but it can be a bit hit and miss as the compass reading is influenced by the earths magnetic field.
The secret is to find the celestial pole (the center of the star circle). You can either do a test shot, five minutes should be enough, to show you where the pole is located, or alternatively you can calculate the location by using the southern cross and alpha & beta centauri (the pointer stars) as your reference points.
Disable autofocus, image review, and any processing options that can be performed in post such as lens correction. Doing this will help extend battery life. Many cameras have long exposure noise reduction enabled by default, this must be turned off to avoid large gaps in the trails. Shoot raw format at the highest available bit depth. A white balance setting isn’t important when shooting raw, as you can set it how you want later in post.
Finally the settings. Assuming you are shooting around the new moon and away from the city lights, a good starting point is 40 seconds at f3.5, ISO 800, with one second intervals between shots. Some intervalometers will allow less than one second intervals, so if you can, use the shortest interval available.
Start by taking a few shots about 30 minutes after sunset to capture the foreground details while there is still some light in the sky. Shoot this like a normal landscape shot with the camera at base ISO and the lens stopped down. If you use ND filters, make sure the holder isn’t done up too tight so it can be removed without moving the lens.
Once you are happy with your detail shot, change settings to bulb mode, with an aperture as wide open as you can (I use f3.5) with an ISO of 800. Then, once the sky is completely dark, start the intervalometer. Aim for 40 second exposures at 0.1 second intervals (most intervalometers will only go down to one second intervals, which is fine). Set the number of exposures so the sequence ends about 90 minutes before sunrise, I usually end up with between 700 and 1000 images, depending on what time of year I’m shooting. Then sit back and relax!
Stacking and cleaning the images
I do everything in Adobe Photoshop & Bridge CC, although the process is similar for the older CS versions of Photoshop. I prefer to work in 16bit mode, which gives more processing headroom but uses more system memory. To set this, open an image in camera raw and click on the text below the image which shows the color space, bit depth and image resolution.
The number of images you select in the following step will be determined by how much ram you have in your system and the size of your images. I have 64gb ram and my images are 36mp 14bit losless RAW (from a Nikon D810). I work with 100 images at a time. Loading too many images at once will cause the system to use the scratch disk which will slow the process down considerably.
First, select your images in Bridge, hold down SHIFT and click on the first and last image you want in the selection. Right click on one of the highlighted images and select ‘open in Camera Raw’.
In Camera Raw click the ‘select all’ box at the top left. I like to apply a white balance tweak and a small saturation boost (do not apply lens correction here, as it will result in moire patterns in the final stacked image). Don’t worry about noise reduction, even though the individual images might be a bit noisy, it will be eliminated by the stacking process. Click on syncronize at the top left and then done at the bottom right and you will be taken back to Bridge.
Now go to the menu bar at the top and select Tools> Photoshop> load files into Photoshop layers. Loading the images into Photoshop can take a while, especially if the images are stored on a mechanical hard drive, you can try storing them on an SSD to make the whole process go a lot quicker.
When the images are finished loading, hold down the SHIFT key and click the first and last layer in the stack to select all the layers, now go to the drop down menu at the top of the layers pallete and select ‘lighten’.
The images are now stacked but we have some unwanted trails from planes flying through the scene. To remove these you’ll need to go through the layers one at a time and paint over them using a small black brush. Start by holding ALT and clicking the eye icon next to the top layer, this will show that layer and hide all the other layers. Make the brush size just big enough to cover the trail, then paint over it.
Try not to paint out any bright stars when doing this as it can cause a break in the trails, although you can use the spot healing brush/clone stamp later on the stacked image to fill in any gaps. The black lines will not be visible once we enable all the other layers.
This process can take a very long time, I live to the north of an international airport and I usually end up with between 600 and 1100 images for a startrail shoot, and usually around half of them will contain plane trails.
When you get to the last layer, right click on the eye icon and select ‘show/hide all other layers’, all layers should now be visible and the plane trails removed.
Finally flatten and save the image, now go back to bridge and start again at step one with the next batch of images. When finished you can now stack all of your flattened images together, then finally flatten & save.
Now open up the twilight image you shot and the final stacked startrail image as layers. The blending method I use for combining these depends on the scene, but shots taken over the water often just need a basic mask, while others can require more complex masking techniques such as contrast or luminosity masking.
Once you have the masking done you can start the final editing stage. I usually apply a saturation boost and white balance tweak to the startrails to bring out the colours, and then tweak the colour balance and exposure of the foreground image so it doesn’t look out of place with the sky. At this point you can do as much or as little additional processing as you want.
If I am planning to print the image I’ll go over the entire sky at 100% view and clean up any gaps left when removing plane trails. I’ll also spend quite a bit of time applying colour, contrast and exposure adjustments. The choice is yours.
Based in Bendigo, Victoria, Lincoln Harrison specialises in astrophotography. See more of his work at lincolnharrison.com ❂