Photographing star trails

Photographing star trails can be very beautiful. But also challenging. Hence some tips!

Choice of location
First of all, find a pretty dark place. The darker the place, the more contrasty your star trails will stand out against a darker/blacker background. If necessary, consult the so-called light pollution maps (see info section below). If you really want to get the most out of your images, you can use filters to magic away excess background light. Search for "light polllution filters for cameras" on Google, for example. However, this is not a must when you are in a sufficiently dark location.
What can also play a role for your location is your choice of subject matter. Pick an attractive foreground. Now yes, anything you photograph on Earth is de facto foreground and could be attractive. For example, choose an attractive beach, a picturesque mill, a tower, and so on. Unleash Your Creativity. Sometimes it may be necessary to lighten your foreground a bit. For this purpose, during the actual shooting of the star trails, you can illuminate a nearby foreground with the flashlight of your cell phone or a specially brought LED-lamp.
What is often neglected but in my opinion also very important: choose a place where you can work at ease. After all, you need to be able to concentrate for hours on end. You certainly do not want to be disturbed by overzealous police who come to check what you are doing and/or by passers-by and/or neighbors. It may be necessary to warn these people in advance if you do not want hours of recordings to be lost. A major culprit may be that these people are carrying lamps that come to disturb your image. Or they might accidentally bump into your tripod, and so on.

Choice of time and duration of shots
It's best to check the weather forecast beforehand. For the Hauts-de-France region, this can be done on the METEO France website. For Belgium, you can consult the RMI. It must remain cloud-free for the entire duration of the exposure. Also, you will only be able to make good contrast images if it is really dark, i.e. after dusk and before dawn. If you want to take very long exposures (I'm just saying: from 6 hours or more), in our region this will only be possible during winter. For example, consult this website to find out how long the twilight will last for a certain date.

Tip: It can also be especially creative to start photographing just as dusk falls. This has the nice advantage that your foreground is already well exposed. In later shots of the series this will no longer be the case. If you bring all shots together, you will have both a well-exposed foreground and beautiful star trails.

Use of a tripod
It goes without saying that one should use a tripod because otherwise one can never keep the camera pointed precisely and still for long enough. Which tripod will then make you wonder. If you use wide angle lenses then a simple and inexpensive tripod will suffice. As you use longer focal lengths (anything over 50 mm), you will need a sturdier tripod if you want to control the vibrations (caused by the wind, for example) sufficiently. Is your tripod too light (meaning mechanically unstable, because actually it has little to do with the weight as such - although more stable tripods are usually heavier tripods).

Choice of photographic lens & aperture
Next, choose a pretty bright photo lens. The larger the aperture is open, the more light you get in. Examples of such lenses include those with a maximum aperture ratio of f/1.8 or f/2.8. If you use a lens of somewhat lower optical quality, you may still need to make your aperture a little smaller. That is, work at f/2.8 or f/4.0 when your lens can handle an f/1.8 or f/2.8 aperture. This will allow you to utilize more of the central portion of your lens and thus lens errors will be less apparent in your photographic shot. Lens errors can be that the shape of a star is no longer pointed(astigmatism) or that, for example, the star image is split into colors(chromatic aberration). Lens errors are especially evident in the corners of your images. Go into your test shots especially there to see if you see any.

Tip: If you are working with zoom lenses, then once your field of view is fixed, it is best to clamp that lens at that position (focal length). Otherwise, during hours of shooting, zoom lenses can shift a bit (sometimes under their own weight) and that is something you (normally) do not want. Do not forget to check the focus again after clamping!

Noise reduction in long shots: deactivate it!
Some better cameras allow you to automatically correct pictures longer than 30 seconds for noise. They do this by taking a second picture each time with the shutter curtain closed. They then obtain a second basically blank image (in technical jargon, we speak of a "dark frame") containing only noise and so-called hot pixels. Hot pixels are image elements of your sensor that constantly emit a signal, while they should not do so. This dark frame is then subtracted from your actual original image, resulting in a better, noise-free image. However, you cannot use this technique when taking star trail images. Why? Because you would lose half of the tracks. Suppose you make base shots of 5 minutes, you would have 5 minutes of image, then 5 minutes of nothing (because the camera then makes the dark frame), then another 5 minutes of image, and so on. Conclusion: this noise reduction for long-exposed shots should be deactivated.

Advanced tip: It may well make sense to take one "dark frame" yourself: place the cover on your lens and take an image that is exactly the same length as the base shots of the star trail sequence you will be taking. Once you have the series of exposed shots, you can always then use that one dark frame yourself to remove your hot pixels from your final composite image.

Take your time to focus correctly
Pay due attention when focusing on the stars. Enlarge your test image well on your camera' s built-in LED screen or on a separate large computer monitor. Make a few test shots anyway. This way you avoid that hours of shooting would be unusable due to unsharpness. Focusing can take some time, do not neglect this step. Also make sure that your focus does not change during the total shooting time. Best to set the lens to "manual focus" once the correct focus position is chosen.

Advanced tip: What's nice about photographing star trails is that you also make the colors of the stars visible. With the naked eye this is a bit more difficult, but photographically it can certainly be done. You don't have to do anything special for this. There is even a technique in which the lens is specially defocused to blur the star trails. By smearing the star trails a bit, the star colors come out more. You can only use this technique when you have nothing in the foreground (because otherwise it will also be out of focus, of course). You can possibly solve the latter by focusing your foreground in a different way and afterwards mount it in your final shot. But as I pointed out: that's for advanced photographers 😉.

Choice of ISO value & test shot
Next, do a test shot. Try a slightly higher ISO value than what you would normally use: for example ISO 800 or ISO 1600 instead of the more common ISO 100 or ISO 500 when shooting in sufficient daylight. Try to take as long an exposure as possible at the set ISO value: that way you get a good signal-to-noise ratio. The exposure time should not be too short, because then you will hardly get any starlight, but also not too long because then the sky background may become too bright. The trick is to find the best possible combination of ISO sensitivity and exposure time.

Setting a longer exposure time
By default, most cameras can only handle a maximum shooting time of 30 seconds. Longer is possible but then you have to keep it open with a special (sometimes electronic) remote control: we are talking about "bulb mode". Once you have found a good ISO value and shooting speed, the only thing that remains is to take several such shots one after the other. It is then these individual shots that are brought together in image editing software, making "star trails" fully visible in the end result.

Determining how long you will photograph
Keep in mind that the Earth rotates around its axis in a single day. So if you would like to obtain a star trail that covers one full circle , you should be able to photograph for 24 hours at a time. Due to the seasons (twilight you know) this is normally impossible: an early rising Sun will prevent you from taking pictures with a sufficiently high signal-to-noise ratio. Dusk will wipe out the stars, so to speak. If you follow a patch of sky for 4 hours at a time, you will obtain arc circles of 360°/24 * 4 = 60°. If you succeed in photographing for 12 hours at a time (in winter, for example), then you could obtain half arc circles.

Determining the direction in the sky
The further your camera is pointed awayfrom the North Star, the more the stars will seemingly move and the longer the corresponding star trails will be. Also note that the longer the focal length of your lens, the longer the star trails will be. This is because you are magnifying the image that much more.

Avoid condensation
Another thing to keep in mind is that your camera, and especially the lens, can cool considerably as the night progresses. This can result in condensation starting to form on the lens. You can counteract this by keeping the lens warm with a heating ribbon: this is a piece of fabric with a heating element built into it. This ribbon then keeps your lens at a temperature just high enough to prevent condensation from forming. Don't put the ribbon too warm because heat can always cause turbulence (and thus imperfections in your image). Here is an example of such a heating ribbon.

Do not panic
It is possible that your star trails are not equally bright everywhere: this is the result when, for example, clouds pass by. At that moment the star weakens in brightness and you will see that in your image of course. It also happens that you sometimes see straight star trails in your image: usually this is then a meteor (this is a light trail formed by a falling meteorite on the Earth). Of course, you can also have aircraft or satellite tracks in your recordings.

Sometimes you will see that your composite image shows a shiny, sallow band of light. Don't panic, that's just our Milky Way coming into focus 😉.

Additional Tip: No one obliges you to take hours of shots. Take a telephoto lens and place something in the foreground (a tree, for example). This can already produce nice results. See this example: here the red giant star Betelgeuse was photographed behind a tree branch. It resulted in a moody shot. The image was realized on February 11, 2005 in Grandpré (in the French Ardennes where there is still quite little light pollution ). It was also windless at the time so the tree branch remained motionless. EXIF Data: Canon EOS-1D Mark II with 30 mm lens, f/4, ISO 400, one 30-minute exposure.

Information on light nuisance / light pollution / light pollution / darkness areas:
Info on Wikipedia on Lichtverontreiniging: Dutch-language - French-language - English-language
Internet media of the VVS Werkgroep Lichthinder: Website - Facebook
Want to know where it's still pretty dark? Consult the VVS light pollution map for Belgium.
Light pollution map for the work area of Top.Vlaanderen.
Light pollution map for Europe: red is bad, blue is good.
Internet media of the Dark Sky Organization: Website - Facebook

Useful materials (and other) checklist:

Absolutely essential:
- camera
- lens
- tripod
- choice of location: dark, quiet, nice foreground
- clothing to keep you warm: hat, scarf, windbreaker

Optional attributes:
- heating ribbon
- light pollution filter
- extra lighting option (could be your cell phone)
- table with seat
- hot or cold drinks

Example: image taken on Sept. 19, 2020 at a wind turbine on the Ypres industrial park

Camera: Canon EOS 5D Mark IV - Lens: EF35mm f/1.4L II USM.
Partial shots were taken with the following settings: f/5.6, ISO 200, 5 minutes exposure time per shot

The camera was positioned next to a busy road, just in front of a windmill.
Because heavy traffic sometimes passed by, its head- and tail lights caused horizontal light stripes at the bottom of the image.
The surrounding light fell on the rotating wings of the windmill in such a way that it formed a pale circle of light.
You can even see the shadow of the windmill mast on this round plane of light. Special shot, isn't it?

Example: image taken Sept. 20, 2020 in the Ypres industrial zone

Basic shots of 7 minutes each were taken at ISO 400 and f/5.6.
Camera: Canon EOS 5D Mark IV. Lens: Canon EF 14mm f/2.8L II USM. With tripod.

Example 2: recording made Sept. 15, 2020 at the Irish Peace Park in Messines

There were 7 basic shots of 10 minutes each at ISO 400 and f/5.6.
Camera: Canon EOS 5D Mark IV. Lens: Canon EF 35mm f/1.4L II USM. With tripod.

Sample: shots taken in Equihen-Plage (Opal Coast, France) on Sept. 28/29, 2011

Camera: Canon EOS-1Ds Mark III with 14 mm lens
- f/8, ISO-160, 10 minutes, 57 exposures - so total is 9h 30 m exposure
The horizontal light trails are from fishing boats that were sailing on The Channel between France and England.
In this wide-angle image, you can clearly see that some of the star trails are curved upward, others downward. A quasi-straight line also appears in the middle of the image: that is the so-called celestial equator. It separates the Northern Hemisphere from the Southern Hemisphere. I give here two versions of the same shot: one finished a bit brighter than the other.

Examples: recordings made on Feb. 7 and 8, 2005 in Grandpré (French Ardennes)

Camera: Canon EOS-1D Mark II - Lens: 16 mm
- f/5, ISO 250, 20 minutes per partial shot
- f/5.6, ISO 250, 60 minutes per partial shot
- f/5.6, ISO-160, 1 minute per partial shot