First, you need a camera capable of capturing images of the stars. Some cameras just don't handle the night sky well. So play with your camera's settings to make sure that you can take a fairly sharp image of the stars in question. If you can't see the stars in the first place, you can't make star trails.
We used a Canon EOS 7D Mark II for these images, with a EF50mm f/1.4 USM lens.
Next, you want to go somewhere REALLY dark. Not just sort of dark. The darker the better. Go here to find the darkest area you can get to.
A new moon is best, for full darkness. You can also try shooting when there's a moon but it's below the horizon, although sometimes you'll get light leaking up over the horizon from it.
These photos were taken in a town which is fairly substantially light polluted. I'll be doing field trips to go to darker places soon.
Make sure you have:
* extra batteries (this sort of photography chews through batteries)
* a tripod, preferably with a weight
* a remote timer or a built-in intervalometer
* a flashlight
Now make sure you know where north is. Sure, you can use a cellphone app, but you might be somewhere without cellphone service! So figure that out beforehand. Also, figure out where Polaris is going to be. That's the north star. It's the tail in the little dipper. If you can find the big dipper, follow the lower right corner of the bucket up through the upper right corner of the bucket. That will point you to Polaris.
If you really can get to the internet from where you are, download a SkyMap app and use the 'search' button to find Polaris.
The stars are going to wheel around Polaris in a circle, so that will give you a cool star map.
Set up your camera with the exposure settings you find work best for the single shot. We use f/5.6, 125 ISO, with 30 second exposures.
Now it's time for the timer part. You don't want large gaps between your exposures - that will cause the star trail to look like a dotted line. But the camera is going to require some writing time for each image. Usually this is in the range of 1.5 seconds to 2.5 seconds per image. So don't set your camera to take photos every 30 seconds with 30 second exposures - there's no time for the image to write. If you do, the camera (at least ours) will therefore skip every other image while it's busy writing. We therefore set the interval time to 35 seconds. That leaves the tiny gap while it's writing.
The image above was taken during a 15 minute time period. It was 30 sequential exposures of 30 seconds each. By doing short exposures, each one can get a good balance of dark and light. Also, many cameras (including ours) have a 30 second maximum exposure time when doing work with the intervalometer.
Your aim is not to have the camera sitting out there for 24 hours getting each star to make a circle, although you could certainly do that. The aim is for all the stars to make an arc and when you look at them all together, circles are implied. Like this stock photo.
To set the Canon EOS 7D Mark II Intervalometer, follow these steps:
* Hit MENU
* Go right twice
* select Interval Timer and press the center button on the right
* highlight enable and click
* Push INFO to set the details
* Use the joystick to select which detail to change
* Press the center button on the right to select it
* Make sure you click OK
* Menu to leave
* When you're ready to start, just press the shutter button as you always do and off it goes
* If you chose 99 for infinite shooting, you have to turn the entire camera off to stop it. Otherwise it stops on its own when it ends.
Whatever you do, don't try pushing the button manually each time. Your finger press will shake the camera and blur the image.
OK whether you used an Intervalometer or a remote shutter, now you have a pile of images. How do you add them together?
I use PhotoShop. I open up PhotoShop, navigate to the folder in question, and open up all the images.
Next to get them into one image as layers.
I use Window - Tile to get them all on the scren. I make sure the layers window is turned on so I can see it. Then it's just a matter of clicking on image 2, clicking on the layer, and dragging the layer over to window 1. Click on image 3, click on layer, drag it to window 1. Click on image 4, and so on.
So, voila, now you have one image with all the various stages of the motion as layers.
Now you want to "screen" them all so the lower levels are visible. You can use "lighten" but that leaves small gaps! It's bizarre. It inserts gaps between the stars. So if you see gaps in your star trails it might not be the camera's fault. Those few seconds in between exposures is probably not the problem. It's probably part of the photoshop stitching progress. So in any case, set each one to screen. You should be able to click on a layer, click screen, click on the next layer, click on screen, and so on.
It's nice to get something in the foreground, for scale. This is Eight Lots School in Sutton, Massachusetts.
Note if you get over about 50 layers with "screen" the night sky gets lighter and lighter with each one and after a while it makes the stars fade out -
Here's the exact same set of images done with lighten. Now the sky is dark but the trails are jaggedy.
If you run into this problem you have some work cut out for you. Duplicate each layer except the top and bottom. Yes, really. For the top of each two, set it to screen so it gets the wider (connecting) aspect. Then merge it down into its duplicate. Now you can "lighten" this wider version.
Now you can edit to your heart's content! All I did as lighten this a bit so more of the star trails became visible. As I mentioned, this was taken in a light pollution environment. Note that these are the actual colors.
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