How I take a Star trail photograph with my Canon DSLR cameras
Notice I said ‘I’ in the article title? Well there are lots of different ways to shoot a star trail photo. If you speak to ten star trail photographers you will get 10 different techniques, you will also get a collection of tips from all of them. One of those tips might make the difference between an ‘ok’ star trail photograph and a great star trail photograph so it is always worth talking to other photographers.
2016 x 10 second images taken over 6 hours
The process that is best for you will depend on what equipment you have, where you are based, the light pollution and the atmospheric conditions:
- you can shoot single very long exposures if the light pollution is very low and I mean very low!
- with more light pollution or a bright moon, lots of shorter exposures can be used
- you can automate the capture process with external hardware connected to the camera
- if you have a zoom lens it’s possible to adjust the zoom, manually or electronically, during the capture process for creative effects
- you can use additional artificial lighting to paint the foreground to improve the composition of the final image
Remember you are only limited by the weather, your imagination and, to a lesser extent, the camera system you use. In the UK the weather is the dominant factor!
The first and second bullet points are the two main fundamental techniques for star trail photography i.e. single long exposures or multiple short exposures. The technique best suited to you will depend primarily on location, light pollution, moon phase and equipment. The basic idea for both techniques is detailed next but I will expand on them later as well.
Single Long Exposure Technique, typically 30 minutes or longer
In the good old days of film based cameras the single long exposure was almost the only way to capture a star trail image. It worked extremely well if the location had low light pollution and a moon close to its new phase and you get continuous star trail lines as a bonus. The early film based cameras also had one great advantage over modern digital cameras; they weren’t dependent on batteries and could shoot an 8 hour hour star trail without any battery issues!
a 3 hour exposure on a Hasselblad 503CW camera, CFi 4/50 lens and Fujifilm Provia 100F slide film by Oleg Novikov
Using a modern battery powered DLSR to shoot a single long exposure is still possible and is used but does require a very dark location with minimal light pollution. It also need some care as normally a dark frame is required for noise reduction. The camera automatically shoots this dark frame immediately after the main exposure so to shoot a one hour star trail the camera battery MUST be capable of working for at least two hours i.e. one hour for the main exposure and one hour for the dark frame. If the battery dies during the dark frame production you’ve lost the image and wasted several hours!
You can turn off the noise reduction in the camera but the resultant image will generally be HORRIBLE and almost unusable unless a very low iso setting is used which, as you will see, means less star trails in the final image. With the advent of the modern DSLR a new method of shooting a star trail has become popular and this is the;
Multiple Short Exposure Technique, typically less than one or two minutes per exposure
This technique uses multiple short exposure images which are blended together in post processing. The process means that the noise reduction can be controlled using internal camera processing or, even better, by producing a number of dark frames separate from the main image capture phase. It also allows higher iso values to be used to record more star trails in the final image and you can handle light pollution and even the presence of the moon to greater degree.
a star trail photograph under a full moon (spoilt by passing cloud)
Using this technique means that you only lose the very last image in the capture sequence if the camera battery dies or a few intermediate image might be lost because of external factors, maybe car headlights, an aircraft or a few clouds but you will still have the majority of the captured images to post process.
the effects of a helicopter captured in a few frames
The multiple short exposure technique is my personal choice and is becoming the main method because of the general increase in light pollution and I also prefer the flexibility it gives me during the post processing of the images. The method certainly requires a lot more effort in post processing and can result in small gaps in the line of each star recorded but there is software available which helps with the effort.
This is the technique that I will work through later in the article.
What are the minimum requirements for DSLR star trail photography?
- a DSLR camera, I normally use Canon 5D MkII and MkIII bodies
- ideally a fast wide angle prime lens, my personal favourite is a Canon 24mm f1.4L MkII lens, BUT you can use what you have and still get good results
- a good stable base for the camera, preferably a solid tripod
- fresh batteries and a good sized memory card
- a rubber band and a stone, yes I did say a stone
- patience, if you aren’t prepared to spent at least an hour shooting a star trail then, imho, you shouldn’t even bother trying.
The difference between a single 10 second exposure and over 1000 x 10 second exposures is very obvious
on left is a single 10 second exposure, on the right is 1080 x 10 second exposures
in addition to the minimum equipment list above I also use;
- dew heater straps or handwarmers and a sock with the toes cut off to stop any moisture forming on the lens during the night
- waterproof neoprene camera body sleeves
- an external 12v battery to power the complete system all night
- a DC battery adaptor for the camera
- a wired remote release or more versatile electronic timers that I’ve made or purchased over the years
- a small loupe for focusing
- a compass, basic inclinometer and notepad
- a powerful torch and/or a flashgun
- warm and wind-proof clothing
- and sometimes a laptop and a good book
Before we get into the detailed techniques for star trail photography let me clear up the rubber band and stone for you. If you don’t have a remote release for your camera you can use the continuous shooting mode of the camera to capture an image sequence. By putting a small stone on the shutter release button and wrapping the rubber band around the body you can keep the shutter depressed and the camera will take photographs until you remove the rubber band or turn the camera off. Sure there are limitations using a rubber band mainly the maximum exposure time will be 30 seconds for multiple exposures and there is no control on the delay between shooting images but if you forgot to buy a remote timer or left it behind at home then maybe you can still get a star trail with a rubber band.
Why does a Star Trail form?
If we look into the night sky several times during an evening we can see the stars appearing to move around us. This movement is caused by the earth rotating relative to the stars not the stars moving relative to us.
98 x 90 second exposures @ iso800 f/2.8
The Earth rotates around it’s polar axis, which goes through the north and south poles, relative to the stars overhead, so as we expose an image for longer and longer star trails start to form around the pole stars. You can use a compass to position this centre of rotation, in the northern hemisphere you’ll need to locate the North Star, Polaris or the South Star, Sigma Octantis in the southern hemisphere. All of the star trails will appear to form a circle around these pole stars and you can use this information to compose your final image.
The rate of relative movement, called the Sidereal rate, is approximately 15 degrees an hour. So a star trail of 1 hour duration will capture 15 degrees of star movement, this arc will appear significantly smaller nearer the pole star than the star trail formed nearer the horizon. The sequence of images show the relative size of the trails captured during a single 10 second exposure right up to 1080 x10 second exposures capturing a star trail over 3 hours. It is very obvious that the more time you spend capturing images the more impressive the final result.
The best star trail images are taken during the hours of astronomical darkness which is two hours after sunset and two hours before sunrise and before the moon rises or sets. Of course rules were meant to be broken and some of the best star trail images out there have the foreground or landscape illuminated by the moon.
the final image containing 1080 x 10 second exposures
Camera and Lens Settings
Firstly it’s important to realise that the number of stars recorded in an image is proportional to the Aperture and the iso setting chosen if we ignore the effect of light pollution or the moon. The light sensitivity of the camera is set by the aperture and iso setting, the higher the sensitivity the more stars will be visible in the exposure. The higher the iso setting and exposure duration chosen will also determine how much noise is generated by the sensor during an exposure.
Secondly, the exposure duration defines the length of star movement i.e. the star trail length captured in a single exposure. This trail length is also relative to the height of the star in the night sky.
It’s clear that there are a number of things we need to balance against each other to maximise the final quality of the photograph and I’ll try to offer up some tips to control them.
For the multiple short exposure technique I normally use exposures under 2 minutes because of local light pollution and my basic in-camera and lens settings are detailed below:
RAW vs JPG
I always shoot in RAW at maximum size, I don’t see any point in compromising on quality before you have even start shooting and the price of memory cards is so low that using Jpg quality to save memory card space is a poor excuse. Using Jpgs will save a lot of time later but the quality loss is completely unacceptable imho.
Noise Reduction OFF
Turn off the High iso and long exposure noise reduction. To manually reduce the noise in the final images I shoot 10 or more dark frames which are used in the final blending to remove the noise. The dark frames are created by putting the lens cap back on the lens and shooting 10 or more images (called ‘darks’) at exactly the same settings I used for the main images (called ‘lights’) These darks must be shot in the same ambient conditions as the lights i.e. at the same temperature, ideally you can shoot them while you are packing up the rest of your equipment.
Because I shoot in RAW what WB setting I use is less important, however, I would strongly recommend you use a static WB setting rather than Auto. I tend to use a Colour temperature setting of 5600K which is basically a Daylight setting for almost all of my photography.
The camera should be in Manual or Bulb mode if your setting dial has a Bulb position. If it doesn’t then the Bulb setting is normally after the 30 second shutter speed position. If the shooting conditions mean you have to shoot under 30 second exposures then you could use the built in settings rather than Bulb and use the rubber band and stone you’ve put in your bag. I prefer to always use a programmable wired remote release to give me more flexibility and it allows me to programme the shutter speed without touching the camera.
This option is down to the individual I think. On very long exposures the use of the mirror lock-up is debatable as any camera vibration is very short relative to the overall length of the exposure. As long as the tripod is solid and very stable I wouldn’t personally use it and I’ve never seen any quality reduction even when testing the benefits of mirror lock-up for astro photography. It also requires a bit more thought during the programming as two shutter button presses are required to take one image, some programmable remotes can handle the mirror lock-up function, most of the cheaper remotes can’t handle it.
Lens Manual Focus
The lens should be switched to manual focus. The infinity mark on most lens is rarely accurate so focusing should be checking using live view at x10 mag if your camera has it. I use a small eye loupe to improve this focus accuracy. If you don’t have live view and have a relatively slow camera lens it may not be possible to see the stars in the viewfinder but the moon might be around to help you or you can shoot a series of images adjusting the focus between shots and then review each image at maximum magnification to see the best position for the focusing ring and then put a small mark on the ring for use next time.
I’ve left aperture, iso and shutter speed selection until last because they interact with one another and there are some rules of thumb and other factors available to help with the right choices.
You’ve spent a lot of money on a fast f1.4 lens for star trail photography so you’re going to use the lens wide open at f1.4 to maximise the camera sensitivity and maybe reduce the iso setting to control the sensor noise. In general this is a good idea but most lens produce chromatic aberrations when used wide open which cause the stars in the image corners to have some blue coma artefacts. If you process all your images into a final star trail these coma affected stars produce some very ugly wide lines. It is worth reducing the aperture from f1.4 to f2.8 or smaller to minimise this effect, however, it does mean increasing the iso setting to get the sensitivity back up. One significant benefit of the faster lens is that stars are easily seen during the focusing.
The higher the iso setting the higher the sensor noise and the higher the number of stars you can record. That is a statement of fact so you need to try to keep the iso setting down but still have enough sensitivity to record the stars. I typically use iso 800-iso 1600 and control the noise with dark frames.
It is possible to capture too many stars in a very long session which, when combined in a composite image, can look very overexposed and the trails contain little colour. If you prefer to capture less stars and retain more colour in the star trails then think about reducing the aperture towards f/8 and use an iso of 200-400. In post processing you will be able to produce star trails which are more colourful but there will be less of them.
I’ve left this variable until the end because it tends to be the last variable I set when I’m preparing to shoot a star trail. With everything else setup i.e. aperture and iso plus all the in-camera settings I will adjust the shutter speed until I have a reasonable image to work with. What I look for on the image histogram is most of the data across to the left but not black clipped and then increase the length of the shutter speed to get a small amount of background sky colour. It’s very important to realise that adjusting the shutter speed has minimal effect on the stars, they are all pretty much burnt out already which you would expect. Some colour can be retained/recovered in post processing if you keep the main bulk of the image data to the left of the histogram.
Once you’ve got the shutter speed set you can choose whether you need to use Bulb or an internal shutter speed for the duration of the night.
If you are using a programmable remote release you can also decide on whether you will introduce a slight delay (1-2 seconds) between exposures. This delay will appear in your final image as regular gaps in each star trail but it will also allow the sensor to cool slightly between exposures. As the sensor temperature increases the noise also increases, if the ambient temperature is low, say less than 5C, then the sensor temperature isn’t likely to be an issue so shoot the images as fast as you can with no delay to minimise the length of the gaps.
Aperture test with a Canon 5D3 and 24-70 f2.8L II USM lens. 15 second exposure @ 1600iso with varying aperture
With all the settings complete it’s time to get settled in for a session with the camera on the tripod
I use a Gitzo carbon fibre tripod with a hook mounted under the gear head so I can hang my camera bag on it to increase the stability. If you try this the camera bag should be just touching the ground to avoid the bag swinging if there is any breeze. If you are looking to purchase a tripod please don’t buy one with a geared centre column, they look like a good idea in theory but in practice the centre column is very flimsy when raised up and will cause no end of problems and loss of image quality. For the same reason I try to keep my tripod as low as possible and rarely use the smallest diameter leg section.
Extra Camera Hardware
For long sessions I always use a DC adaptor in my cameras, this adaptor allows me to use an external 12v battery to power the camera all night
I also use a neoprene cover over the body which keeps the camera dry if any moisture forms and I use a small dew heater strap wrapped around the lens, this dew heater prevents any moisture forming on the lens optics if the ambient temperature reaches the dew-point.
Post Processing your Captured Images
This part of the star trail photographic creation will be a personal choice so I’ve detailed how I do things and why I do it.
I use a batch process in Capture one Pro to convert all of the RAW files into 16bit tiff files. During this process I do not change anything and I also process the darks to tiffs using the same ‘recipe’ to avoid mismatching the lights and darks.
With all the images in Tiff format I then look at each image in Photoshop to see if there are any artefacts like aircraft trials that I want to edit out, again I do not alter anything else. Doing the changes on the individual images rather than on the final processed image does take a lot of time but in my opinion the end result is better.
There are several software programmes on the web which will blend all of the lights and darks to produce a final star trail image and they can also produce a star trail movie in avi or mp4 format.
The main programmes I use are Startrails.exe or StarStax, at the moment I prefer StaxStax but there is little to chose between them.
The final processing of the star trail image is done in Photoshop to remove any artefacts I’ve missed and to improve the star trail colour and clarity.
Vignetting is the darkening in the corners in an image, it can be a, a natural effect of the optics within a lens or b, can be the result of a filter or filter holder being used which is too small for the lens field of view. Assuming it is the lens optics a photographer can use ‘flats’ to remove the darkening effect in the corners or use the Lens correction in Lightroom or Photoshop resulting in a better final photograph.
Luck plays a big part in getting a great star trail photograph, the photographer can spend a lot of time surveying a location and waiting for the perfect conditions but the more they go out and shoot the higher the chance of luck dropping into their camera giving them a very unexpected, but great, result.