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Tutorial: star trails photography and timelapse

on 28/10/2012 | 0 comments

With sunny days and if you have the opportunity to move away from large urban centers and light pollution, summer is particularly adapted to watching stars and thus to taking night shots!
In line with last month post about this amazing timelapse of our planet seen from ISS, I wanted to share with you my first experience of star trails photography and time-lapse, a 2 in 1 tutorial somehow. Besides being a very pleasant exercise, you will learn a lot about how to use your camera in extreme conditions, how to shoot at night, and most importantly how to manage noise in extreme conditions.


You may have already seen this kind of images, where the stars leave a beautiful colored circular trail, this is called a circumpolar star trails.

Star trails around Polaris

Photo1: 1h20 (163 photos – 10 mm – 400ISO – f4.5 – 30s)

Star trails around Polaris

Photo 2: 30 min (15 photos – 100ISO – f4.5 – 2min)

If you want to shoot a nice star trails, there are basically two techniques:

  • take a single shot with a loooooong exposure time (like> 40 min)
  • take lots of pictures at regular intervals and then assemble them ‘( stacking method ).

Both methods have their advantages and disadvantages. The first requires less hardware and finally less work in post-processing. However, you have to keep in mind that it can very easily fail and make you waste hours (a car headlight, a blow of wind, a battery issue and your photo is ruined…) and most importantly, this technique generates a lot of noise in your image.

We will concentrate on the second technique. You will need a control device (not tested but is should be manageable by hand if you are patient and disciplined) and a post-processing software. In addition to not having the disadvantages of the first method, this technique also allows you to have a foreground subject properly exposed and to make a movie of your pictures, isn’t life great?

Digital noise is photographers worst enemy. Noise is the presence of random information in your pic. It is typical of low light areas where the signal / noise ratio is low, but also in areas of uniform textures. We usually try to avoid noise by choosing the lowest possible sensitivity (low ISO) as high ISO will amplificate the noise/signal ratio. With a very long exposure, your digital sensor will tend to heat which will alter its inner properties and generate another form of thermal noise. This form of noise is very sensitive to your shooting settings but also to your environment (temperature, humidity …). To reduce it, it is possible to subtract the so-called hot pixels. These hot-pixels are identified by shooting with a closed shutter in the same exact condition as your real shot. As no light is entering your camera, pixels sending a signal are those that must be weakened in the photograph. It is this process that is used by camera bodies when you set the automatic correction of noise for long exposures.

Some basic requirement for shooting stars

What equipment is needed for star trails photography

For this exercise, you will need:

  • a tripod (essential)
  • a camera with a manual mode or at least that lets you choose a long exposure time (> 15s). Ideally, a camera with a BULB mode.
  • basic notions of astronomy
  • a control device for triggering the camera. In my case and as I don’t have an intervalometer, I used my laptop, a USB cable and EOS Utility, the software that came with my Canon. It basically gives you the possibility to control and program your camera.
  • a flashlight which is convenient for orientation in the dark and that could also be used to lightpaint your foreground.
In ‘bulb’ mode, the mirror of your reflex remains open as long as you hold the shutter button. With a remote control, a computer or simply with a bit of paper and a rubber band, it is possible to take a photograph with a potentially infinite exposure time or more precisely as long as you have batteries.

Astronomy 101 or how to find the fixed point ?

It is not necessary to have a PhD in astrophysics, but some very basic concepts of astronomy are quite useful. The apparent motion of the star is due to the Earth rotation. In the Northern Hemisphere, the North Star (Polaris) is in the axis of rotation of the planet, therefore, it will appear as a fixed point around which all other stars will appear to rotate. In the southern hemisphere, locate the Southern Cross. Here’s how to find the Northern Star.

  • You have a smartphone: Download an app (randomly Sky Map on Android, Star Map on iphone). It works great and it takes 3 seconds to find what you need.
  • You are more the adventurer/romantic/poor: type. Polaris is the brightest star in the Ursa Minor constellation. If you know roughly where the North is located, this is the direction to look. First locate the Big Dipper. In early summer night (2-3h after sunset), it appears rather low in the horizon. It is recognizable thanks to its famous pan form. Once found, simply follow an imaginary line that would pass through the two stars forming the tip of the bowl. At about five times the distance between these two stars, you’ll come across Polaris. Polaris is the tail of the Little Dipper, or in other words, the extreme point of the handle.

The Southern Cross is more difficult to locate, and I did not have the opportunity to exercise, have a look here for more information.

Advantage of a wide angle lens and the 500 rule for photographing stars

Any lens can be used to photograph the sky but it is best if it has a wide aperture (low f) in order to reduce exposure time (and hence noise), and it is much easier to take picture with a wide angle for two reasons:

  • It is not always easy to compose a picture in complete darkness. With a wide angle lens, you can be almost sure that everything will be in your photo. You can then crop your shot in post-processing. This is basic but it is true.
  • Stars move very quickly! And the further it is from Polaris, the faster it moves. It may seem to be counter-intuitive at first, but I assure you that depending on your viewing angle, if you take a 30s shot with your kit lens, the stars will not stay still. OK and the wide angle then? Well it’s simple, the closer you look, the faster it seems to move…If you’ve ever watched the moon with an amateur telescope, you can see exactly what I mean.

There is a simple rule to determine the maximum exposure time you can set if you want to freeze the stars. Divide 500 by your focal length in 35mm equivalent (multiply by 1.6 if you have a Canon croped sensor, for other model check it here).

For instance, with a 35mm kit lens on your EOS 500D (small sensor), this gives 500 / (50 * 1.6) = 6.25 seconds! It’s fast…

If you want to make a beautiful timelapse, where time for each image seams to be “frozen”, it is necessary to comply with this rule.

The maximum exposure time that will give you still stars is obtained by dividing 500 by your focal length in 35mm equivalent.

How to shoot stars trail

Composing your photography

  • Locate your still point and position it in your composition.
  • Make sure your tripod is stable.
A useful technique to check what you have in your field of view to make your composition is to set your camera to max aperture and max ISO and a few second!

Setting your camera and choosing a lens

  • You should prioritize short focals, if you have a wide angle, use it.
  • Set your lens in manual mode (MF) and set the focus to infinity.
In the dark, setting the focus at infinity is not so obvious…It may be useful to do some tests of manual focus to infinity during the day and to identify the coresponding position on your focus ring. On most lens, you will notice on your ring that infinity is not a point but a range. If you put your focus ring to its max, it is very likely that you are too far (beyond the infinite!), The stars will not be punctual but slightly blurred as seen in picture 2. That might be a desired effect though.

First shot

Take a shot with your foreground properly exposed. This image will be used to combine a well exposed foreground with the star trails. You can use your flash or maybe your flashlight to illuminate your subject.

Settings for correctly exposing the stars

  • All your images will use the same settings, so set on the Manual mode. The idea is to have an aperture as large as possible (small f), ISO as low as possible and exposure time respecting the 500 rule. Do not worry about your foreground, we want the stars now. In a moonless night, 400ISO, f4.5 and 30s worked quite well but you can set higher ISO (800 or even 1600 if your camera device is recent) if you want to reduce the exposure time. Try different combinations, there really is no magic formula. The only certain thing is that high ISO and long exposures will give you more stars and more noise…
  • Set off the automatic noise reduction for long exposures if your camera has this feature. On one hand, the noise control is always more effective in post-processing and secondly, on the other hand, because this features works by doubling all your picture by a ‘negative’ equivalent, your device will be unavailable between each shot. In other words, if you leave this option, your trails will be dashed.
  • Instead of using the in-Camera noise reduction function, we’ll do our own hot-pixel detection and use it at post-processing. Without changing your star settings, put the cap on your lens and take picture.
RAW or JPEG? It is not absolutely necessary to shoot in RAW for this type of exercise. With hundreds of photos of more than 30MB, shooting in RAW can become a nightmare if you do not have a lot of storage space and a good computer. One of the main advantage of RAW in this case is to properly control your white balance. If you make your shots in JPEG, I suggest you set the white balance to tungsten. I found this setting to be the one achieving the best results

Sampling

  • It is not necessary to make complex calculations to find the best combination between your number of shots and the timelapse framerate. A very fluid timelapse is at nearly 20 images / s, however, I found the final result to be quite decent at 2 frames / s.In all cases, it is important to caliber your session based on the real time you’ll be shooting the stars and to adapt your sampling exposure time accordingly. The longer the session, the longer the trails. With only 30 minutes, effects are quite pronounced as you can see in the picture 3.
  • Minimize the time between each shot! With just 6 seconds of delay between two shots, the streaks will not be perfectly continuous.
  • Launch the sequence and do not touch anything, read a book, watch the shooting stars …
  • Once your photo series is finished, take another hot-pixel picture, ie, put the cap back and take a photo with the same settings.
Example: you want a 2 hours star trails (real time) and you achieved good results setting your camera at 200ISO-30s. With these settings, you’ll get 2 (hours) x 60 (minutes) x 60 (seconds) / 30 (exposure) = 240 photos. At 12 frames / second, your movie will only last 20 seconds! If you think this is too short, adjust the exposure of your photos to eg 400ISO-15s in order to double the size of your sampling.

Post-processing, stacking and timelapse

Creating a stacked image

  • Make your adjustments (brightness, contrast, balance …) with your usual software on a photo of your series.
  • Replicate your adjustments on all photos. Most programs allow you to do these mass corrections. In Aperture for example, find the option for Taking and Applying metadata , with Canon DPP, the option is called Copy recipe . Look at your manual if you don’t find the option.
  • If you worked in RAW, export all your photos to JPEG.
  • Put all the post processed pics in the same folder.

There are several stacking software, I used StarStax , it is a simple, multiplatform and free software.

StarStax

The software can’t be easier to use:

  • Load the folder containing your star trails photos but not your foreground.
  • Load your hot-pixel negatives (2nd icon from the left ).
  • Specify the export folder.
  • Make sure that the Lighten and Subtract Dark Images options are checked.
  • Start the conversion and watch the magic!
  • Now, you just need to add your well exposed foreground, either with StarStax or with any other photo processing software.

Creating a timelapse

Again this is quite easy. With software like Timelapse assembler on Mac OS or Virtual Dub on Windows, you’ll just be asked to select your images and to define the video framerate.

Note that StarStax has a Cumulative saving option, which allows you to save each stacking step. If you make a timelapse with these outputs, you’ll get another cool effect.

timelapse of circumpolar startrails around Polaris

The standard timelapse

Stacked timelapse of circumpolar startrails around Polaris

The stacked timelapse

OK, I agree, this is not exceptional from an artistic point of view but it was not really the purpose of the first session. You have to admit though that the basics are definitely there !

Finally, and for inspiration, here is a small selection of Flickr photos taken with this technique. Note that a photo shot at dawn or dusk can add a wonderful supernatural effect .

  • 11-03-10(320p)

    Eung-soo

  • Startrails

    Vedran Matica

  • Star Trails in Garden City Beach, SC

    Jason Barnette

  • Weir Lake Trails

    Bill Church

  • Starry Night Over the Shore

    Moniza

  • Under The Spell of Polaris

    Nuno Serrão

  • Port Macquarie Lighthouse

    Yuri Propopenko

  • Moon-mill

    Michael Menefee

  • waiting for the perseids

    wunderskatz

 

Time to practice now…between two shooting stars …

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