This guide will take you through the steps required to create a star trail photo using photo stacking.

Why stack

Why not just take a very long exposure? Well you can do this and with film it works great, as film doesn't have the noise issues that digital has.  The longer the exposure you have on a digital sensor the more noise that gets introduced.  Noise is the digital photographers trade-off for the super convenience of instant results and price.  Plus with a long exposure on film you never know if the exposure was right till after it's developed.  Also the foreground might end up overexposed compared to the sky.

How does stacking work? Well the stacking software takes two frames and creates one new frame from the brightest pixels in each of the frames.  It then takes that new frame and does the same process on that new frame and frame 3 and so on.  Until it has processed all the input files.  Once it has done that it will subtract the pixels in the dark frames from the image.  So any hot pixels or sensor noise will be removed as well.  The end result being a your star trail.

What you need

  • A camera capable of a manual exposure settings with up to a 30 second exposure or best a BULB setting (i.e. shutter stays open as long as you keep shutter depressed)
  • Fully charged camera batteries
  • Tripod
  • Intervalometer - device for doing multiple timed exposures (available for most DSLR camera from eBay for about £10)
  • A flash light or light source - useful for filling in details in the foreground and seeing what you're doing.
  • A lens cap for getting your Dark Frame shots
  • A night when you have a clear sky and can see plenty of stars
  • Warm clothes, plenty of time and a flask of hot drink
  • Software to stack the images - I use StarStaX

Setup

  • Switch off Long Exposure Noise Reduction This option on your camera takes an image with the shutter open followed by a second exposure with it shut. It then uses the second one to remove any hot pixels or sensor noise from the original. The result will be gaps in your star trail.
  • Switch off any image stabilisation in the lens and the camera. The camera should be on a firm tripod but the camera will still try to stabilise the image so it will actually end up less sharp and drain your battery.
  • Set your ISO to something low like 100 and not Auto You don't want the camera to be changing it half way through.
  • Switch off the Auto White Balance and set it to something like cloudy Again you don't want the camera changing it between shots.
  • Switch the lens to manual focus and focus at infinite Those stars are pretty far away :)
  • RAW or JPEG? Some people are purist and always insist you must shoot RAW but it's going create a whole load extra work.  Firstly the image stacking apps don't take RAW so you are going to have to convert them in your own software anyway.  Plus you are going to be creating a lot of frames and those RAW files sizes soon add up.  I just use the finest JPEG quality setting.

How To

The first step is to work out the right direction and exposure.  Any direction will give you nice trails but if you want one of those circular style ones you need to be looking North in the northern hemisphere as everything will rotate around the pole star.  Normally about 30 seconds at about f4 is a good starting point.  Take a few test shots at different shutter speeds and apertures until you get a shot that shows up as many stars as possible while maintaining a darkness of background sky you like.  Once you are happy it's time to start.

Setup up your intervalometer with the shutter speed selected and set the delay between shots to the minimum value (normally 1 sec).  If it has a delayed start facility put that to 10 seconds just to let any initial shake settle before the process beginning.  Set up the number of pictures, the one above was 120 at 30 second intervals, so 1 hour total time.  Then start the time and sit back and let the camera do it's work.

Once all the exposures have been taken put the lens cap on the lens and take 2-3 more shots at the same interval.  These will act as the dark frames that help remove any hot pixels or sensor noise in the post processing.  That's the photography done - time to pack up you kit and get back into the warm.

Post processing

Once you have your images on the computer it's time to stack them.  I use StarStaX to do my images and full details can be found on the website.

The process is as simple as selecting your batch of photos, adding in your dark frames and clicking the start processing button.  The results just pop out the end.  You can then import that final picture into your favourite photo editing package for any final tweaks or adjustments.  I run it in Gap Filling mode and tweak the sliders to ensure that the trails look nice and smooth.

Light Painting

Using a torch to fill in details in the shed on the first few frames.

Very often the right exposure for the stars leaves the foreground dark and void of interest.  To get some punchy details in the foreground you can do light painting.  This is nothing more than using a flash or torch to light up the foreground.  It takes trial and error knowing how much light to use and how much to wave your torch around and is worth doing as a project on its own.  For more information just Google Photography Light Painting.

Do it either on the first one or two frames only or the last few or both.  That way if it doesn't work out as expected you can exclude them from the stacked images and not end up with gaps in your trails.