617 exposures blended together
The most important thing to watch for when planning a Star Trails shoot is a clear sky. In order to get a spectacular Star Trails image, you need to have a completely clear sky for the duration of your shoot. I would highly recommend using Clear Dark Sky charts to help you pick a good night. These charts can be a bit difficult to understand at first. Basically, the more rows of dark blue squares, the better.
Next, you will need a location that has relatively dark skies. I recommend using this Dark Sky Finder map to find a dark sky near you. Keep in mind, unlike Milky Way photography, you do not need a completely dark sky. You can still take good Star Trails in light polluted areas, you just won't see as many stars. You can also do Star Trails during a Full Moon.
Capturing star trails doesn't require expensive gear. All you need is a camera with an intervalometer and a wide angle lens. Any wide angle lens will do, even ones that have bad coma. Since all of your exposures will be blended, the star distortion is not an issue. Personally, I would recommend using a Full-Frame camera with a 14mm lens. This allows you to capture a large portion of the sky and a foreground. Alternatively, you can set your camera to capture 90% sky for the star trails. For the foreground, take 1 photo at the beginning or end of the shoot. You can merge the two exposures in Post-Production.
A large, fast SD card is recommended. You will be shooting potentially hundreds of Long Exposure photographs one-after-the-other for up to 6 hours straight! You cannot afford to have your SD Card get bogged down, as it will completely mess up the interval.
The SD Association has created a graph that clearly explains what all the numbers on the card actually mean. As you can see, you will need a Class 10 or UHS Class 3 card to be able to successfully capture Star Trails. You can always test your camera out before leaving the house, to see if your current SD card is fast enough.
I use these 32 GB SanDisk cards in my D750. The 32 GB card holds roughly 1,000 uncompressed 14 bit RAW images at 24 megapixels. If you do not use a fast enough SD Card, your camera may need extra time to process each RAW file, potentially causing gaps in your star trails.
Before you take your Star Trails photo, you need to decide how you want the final image to look. If you shoot at a high ISO (1600+) you will have tons of stars in your image and your Star Trails will be very dense and bright. If you shoot at a lower ISO (100-800) you will have far fewer stars, more star colors, and your Star Trails will have some space between them. Click here to see the difference between ISO 100 - 6400, it should give you a good idea of how many stars you can capture. Therefore, there are no "default camera settings" when photographing Star Trails, it's entirely up to you! With that being said, here's some settings to get you started:
The trickiest part of doing a Star Trails photoshoot is setting up the Intervalometer properly. I shoot with a Nikon D750, so I have this function built into the camera menu. Look for the "Interval Timer Shooting" in the "Photo Shooting Menu." Nikon should have guides on their website for different camera models. Keep in mind, many of the cheaper camera models, like the D3300, do not have a built in Intervalometer. You will need to purchase an external remote that plugs into the camera, like this (expensive) Nikon Brand Remote.
Lonely Speck has compiled a list of cameras that have built-in intervalometers, head over to their website to see if your camera has this function.
Using a Nikon D750, my built-in intervalometer has a couple different parameters:
The Interval we choose is entirely dependent on the Shutter Speed. You can set the interval to any combination of hours / minutes / seconds. For this example, I am using my Rokinon 14mm with a 20 Second Shutter Speed. If I set the Shutter Speed to 20 seconds and the Interval to 20 seconds I will have problems. The camera needs time to save each exposure to your SD card and close the shutter. Therefore, the Interval needs to be longer than the Shutter Speed. I have found that setting the Interval 2 seconds longer than the shutter speed is usually fine. Remember, if you have too big of a delay between exposures there will be gaps in your Star Trails. There are ways to fix this in Post - Production, but it's better to have this done correctly in-camera. So, basically just set your Interval to 2 seconds longer than your Shutter Speed.
I've noticed that the longer the shutter speed is, the more time the camera needs to process the images. For example, a 30 second shutter speed takes 3-4 seconds to process. Therefore, when using a shutter speed of 30 Seconds, set your Interval to 34 or 35 seconds. A 20 second shutter speed usually only takes 1-2 seconds to process.
If your card is near full, this can also slow down the processing time. Be sure to sit with your camera for the first few exposures, you will be able to hear if the camera is in sync or not. If your Interval is too short, the camera will still be processing when it should take another photo. This will cause the camera to lose sync between the Shutter Speed and Interval. I recommend testing out your interval before you leave the house. Make sure Long Exposure NR is turned Off! Otherwise the camera will be unable to do Star Trails properly.
Sit with your camera and listen, you will be able to hear if things are working properly. The camera shutter should open, take in light for 20 seconds (or whatever your shutter speed is), then close the shutter. Almost immediately after the shutter closes it should open again and repeat the process. If there is a long delay between photos, your Interval is not long enough. The camera ends up waiting for another interval before taking the next photo. If this happens, increase the Interval / Shutter Speed Gap. (20 second Shutter Speed with a 22 Second Interval -> Increase to 24 Second Interval)
You also need to determine the Number of Intervals, or photos, you want. If you have all night and a fully charged battery, you can set this for a couple hundred exposures. 300 photos (with a shutter speed of 30 seconds and Interval of 32 seconds) will take about 2.5 hours. The amount of exposures will also change the look of the Star Trails. The more exposures you take, the longer the star trails will look. I normally set my camera to take 100 - 200 Intervals.
Star Trails over Mosquito Lake, Ohio
Star Trails give you a lot of freedom to create a unique image. Depending on your Shutter Speed and ISO you can create a lot of different effects. For example, if you shoot at a lower ISO (400-1000) or a shorter shutter speed (10-20 seconds) you will pick up less stars. If the Full Moon is out, many stars will be washed out from the bright light. You may need to adjust your settings to account for the extreme amount of light, compared to a dark moonless night. The image below was taken during a Full Moon, which exposed the foreground nicely.
Star Trails under a Full Moon
Finally, the direction you face will completely change the look of the photo. Facing North gives you the cool circular effect, while facing South produces a wall of Stars. Depending on your Latitude, the North Star (and resulting circle) will be higher or lower in the sky.
Star Trails over the Great Sand Dunes, Colorado.
Star Trails facing South
Star Trails facing East
Star Trails facing West
Facing South-West in Boulder
StarStaX is a free program that allows you to easily create a Star Trails photo. This application will take all of your photos and blend them together. You can choose from a variety of methods including Gap-Filling and Comet Trails. StarStaX will also allow you to use a Dark Frame, if you remembered to take one after shooting all of exposures. This Dark Frame will help to eliminate noise and hot pixels.
StarStaX also has one of my favorite functions, Cumulative Blend. If you toggle this mode, StarStaX will process each photo and save the composite image. This will allow you to create a Star Trails video. See the video below for an example.
I find that Photoshop creates the highest quality Star Trails. However, this is by far the hardest and most time-consuming option. I would recommend most people stick with StarStaX. You will need a powerful computer (SSD, 16 GBs of RAM) to handle this method, as you will be layering dozens of photos together. Watch my Star Trail tutorial for a complete guide to post-processing your images.
Star Trails Tutorial