An Introduction to Star Trail Photography

Method I: Exposure Stacking

So, if we don't have dark skies, how can we expose long enough to create decent star trails without completely washing out the sky?

One way is to stop down your lens, or turn down the sensitivity. This will allow you to take longer exposures before the sky lightens too much. Unfortunately, you will end up sacrificing all the dimmer stars you're trying to save, because you aren't letting enough light in to capture them in the first place

A better approach is to take several shorter exposures back to back, and then combine them digitally later:

Figure 7. A single long exposure (A) washes out the sky when dealing with light pollution. Combining multiple short exposures (B) allows you to build long trails without sacrificing contrast.

Each short exposure has high contrast in the sky, allowing dim stars to stand out. When you combine the frames later, this contrast is retained, circumventing a gradual brightening of the sky. You can combine any number of short exposures you want. In the past I have added well over 100 for a final image, but you could do many more.

Pros to this method:

• Helps retain contrast when there is light from the moon or cities
• Allows editing of individual frames to remove airplanes, or other incidentals
• You can choose to use only a fraction of the exposures in post (useful if there are intermittent clouds or you decide you don't want your trails to be too long)
• Offers a way to minimize blurring of swaying vegetation in a final image
• There is generally no need for dark frame subtraction

Cons:

• Need to deal with many files
• Very labor-intensive post processing
• In darker locations, landscape can be grossly underexposed, requiring artificial light

Preparing to shoot:

I like to arrive on site before dark has fully set so that I have time to fine-tune my planned composition. If you have not scouted your location in advance, be sure to arrive by sundown to give yourself some extra working time. The darker it gets the more difficult it will be to assess your composition, so it pays to do it while there's still a lot of light. Once you're composed and focused, ensure that autofocus is disabled so that it will not be activated between successive exposures.

You will need to decide the exposure time for the shots in your series. I generally recommend taking a few trial photos with different settings until you're happy with your exposure, but a good starting point I like to use is around ISO400, f/4, 30 seconds. Tweak to your satisfaction or to fit the purposes of your image.

Once you've done this you will need to set the camera up to take the shots successively without any pauses between them. The first step is to disable Dark Frame Subtraction (DFS).

Many cameras feature automatic DFS, which is initialized on exposures lasting longer than a certain length of time. Digital sensors begin to produce copious amounts of noise when they are activated for long periods of time. These noise patterns are unique to individual sensors in individual cameras, and are reproducible if you expose under the same settings and conditions for two shots.

This reproducibility is what DFS takes advantage of. Immediately after your exposure ends, a second begins, but with the shutter closed, preventing light from striking the sensor. Any bright pixels are therefore inferred to be noise, and are digitally subtracted from the original exposure. Since exposing the Dark Frame occurs immediately and automatically after each long exposure, you will want to disable this feature in your camera. If you do not, you will end up with gaps in your star trails from when the camera was too busy cleaning up anomalies to expose your subject of interest!

(If you are worried about the noise your short exposures will produce -- don't be. Modern cameras and RAW editors are pretty good at eliminating this type of noise for the types of exposure lengths we're going to be dealing with for this technique. However, if you're really worried, you can perform DFS manually. Once you're finished taking shots for the night, simply make your own Dark Frame by exposing for the same length of time as your other exposures. Subtract this layer out from each of your frames later. In any event, you will still want to turn automatic DFS off so that it doesn't create hiccups in your star trails.)

On a Pentax K-7, open up your MENU; navigate to C -> 3, and select setting #18, "Slow Shutter Speed NR". Set it to "Off". On the K-7 and several other Pentax dslrs, DFS will still occur on exposures longer than 30 seconds. Those of you with K-5s can experiment with longer exposure times in Bulb mode, as DFS can be completely disabled on that camera. If you are using a camera from another manufacturer, check the user documentation to see whether it has a similar feature, and disable it following their instructions.

Next, you'll want to set up the automation for taking back-to-back shots. I strongly recommend using a remote for this purpose. Many remotes will allow you to lock the shutter release button down. If you are in Manual Mode with a pre-set shutter speed selected, this is the way to go. The remote will constantly attempt to trigger the shutter, ensuring your camera fires off the next shot as soon as it finishes the last.

If you are shooting in Bulb Mode, you will want a programmable remote. These are more expensive, but will enable you to designate the desired exposure length, the interval time, and the number of exposures you wish to capture. Your camera may have options governing the behavior of the remotes in Bulb Mode. On my K-7 you can choose between requiring the remote button to be held down for the duration of the exposure (the behavior my remote expects), or it can be set so that a press of the remote button begins an exposure with a second press ending it.

If you do not have access to a remote, you can still take a series of photos back-to-back assuming your camera features an intervalometer. On the K-7, set your desired exposure length in Manual Mode and then open up the MENU. Move to the second tab and select "Interval Shooting". You will need to designate the number of frames you wish to shoot, and the interval. Importantly, this interval does NOT refer to the length of time between shots. It refers to the length of time between the beginnings of each shot. Therefore, if you are shooting images with 30-second exposures, your interval length should be set above 30 seconds.

Interval shooting is not ideal, because it does not "wait" for your camera to be ready. If you are shooting 30-second exposures with a 31-second interval, your first few shots will go off fine, but eventually the camera's buffer will be full and it will start missing shots. Sometimes this causes the Interval Shooting to abort altogether in the middle of a set. If you are doing Interval Shooting, make sure your interval is high enough to account for this. Know, though, that this will introduce noticeable gaps in your star trails later -- even a second or two can create an obvious hole if you are shooting with a longer focal length or far from the Celestial Pole!

Once you are set up, begin your exposures and continue shooting until you're satisfied. I would encourage you to shoot in RAW so that you are given some leeway to make exposure adjustments later.

Post Processing:

Star Trails over Greenlake (See it on flickr)

I will now take you through my processing steps for the above image. It is composed of approximately sixty individual exposures, each with the following settings:

• 30-second exposure
• 15mm FL
• ISO400
• f/5.6

Although specialized software exists that will allow you to automatically merge a stack of star trail frames, I prefer to do so manually myself in Photoshop from the RAWs. This allows me to tweak a variety of settings. In particular, the 8-bit space of a jpeg will work against you if you need to push the exposure substantially. Hand editing in Photoshop also gives you the ability to remove incidental features from troublesome frames, such as passing airplanes, satellites, boats, windy water, or swaying vegetation.

I like to work with ten frames at a time. I select my first set of files, and open them with Photoshop, launching Adobe Camera RAW. I make several adjustments to them, most importantly to the temperature and the exposure. Be sure to make note of any adjustments you've made so that you can apply the same ones to the next batch of images.

Figure 8. Process your images in batches, making note of any changes you make in ACR

(You'll notice there are some light clouds passing through the sky. These kill off the dimmest stars, but still leave a pleasing number of brighter ones).

Open the images and stack them as layers into a single file. Keep track of any frames needing further correction (such as the top layer in this example, which contains a streak of light from a passing plane on the left edge):

Figure 9. The first batch of images stacked as layers in a single file

From here, change the blending mode of all but the bottom layer to "Lighten". This will tell Photoshop to add only those pixels that are brighter than the layer below. This will include passing clouds, moving reflections on the water, airplanes, and most importantly: your stars. Everything but clouds can be corrected later, and of course you want the moving stars added!

Figure 10. Change the blending mode to "Lighten"

Go back to any frames that needed attention and deal with them before you move on. In the case of this set, I remove an airplane from one layer with a mask:

Figure 11. A layer mask is used to remove the airplane from the top frame. You can visualize your layer mask as a red highlight by pressing the backslash key

At this point you can merge the layers into one and open your next set of ten frames. As I work, I merge each set of ten into one layer, and add it on top of the rest. It is tedious process, but the satisfaction of watching your star trails grow bit by bit keeps you going. Eventually I run out of new frames to add and I examine my product:

Figure 12. A stack of approximately sixty 30-second frames

The wind was very calm that night, but there were kayakers moving across the lake, breaking the clean reflections visible in most of the frames. I'm unhappy with the ripples and sheen that accumulated on the water when it was disrupted, so I identify the layers responsible for the turbulence and mask those sections out:

Figure 13. Masking allows us to remove undesirable features present in only a subset of our frames, one of the flexible aspects of this processing technique

If there had been any wind, we could expect blur on the foreground vegetation over the course of those thirty minutes. However, so long as I had one frame where they were still, I could easily correct swaying by masking in the frame with the still foliage.

Before we finish this image, there's something else I'd like to do to it. Back in the field, after finishing my exposures for the sky, I had decided to take a few photos of myself in the frame with the same settings. I want to add this in now to offer the photograph a human element and a sense of scale. On my computer I review my options and choose my favorite:

Figure 14. I ran out and posed for 30 seconds in this frame after finishing the rest of my exposures for the evening

I place it in a new layer over my merged star trail background, and then create an opaque layer mask to hide it. Using a white brush in the mask, I paint in only my figure and the shadows I'm casting on the dock:

Figure 15. Using layer masks you can add in additional features or lighting effects that only existed in one of your original frames.

At this point I usually do a number of other minor edits, including adjustments to the color balance and contrast, noise reduction, and some sharpening. A note on sharpening in particular: I have found that the star trails themselves are very sensitive to sharpening algorithms. It's easy to overdo it on them, so I will only perform heaviest sharpening on a duplicated layer with the sky masked out.

The final result (See it on flickr)