Monday, 20 February 2017

Making Time Lapse Motion Capture of the Night Sky

In previous blog posts, I’ve written about taking still pictures of the night sky, and how to take time exposures in daylight. Now, let’s look at the ten basic steps for capturing the motion of the stars:

1.       Leave town. I typically drive 1 to 2 hours north of the city to get away from its ‘light dome’.

2.       Make sure you have a sturdy tripod for your DSLR.

3.       Use as wide-angle a lens as you can get. For a full frame sensor, consider using anywhere from 14mm to 18mm focal length. For APS-C sensors (ie. Canon Rebel or equivalent), a 10 to 12 mm focal length will work. Camera brand lenses can be outrageously expensive, so consider much cheaper but still good third party brands like Rokinon or Tokina.

4.       Consider getting an intervalometer, or at least a wired shutter release for your camera. Either one will cost you less than $20 on eBay.

5.       If the air temperature is expected to drop below the dew point, be ready to clear condensation off your lens. You could use a microfibre cloth between shots, buy a battery operated fan at the dollar store to keep the air moving across the front of the lens, or look into some form of lens warmer.

6.       Set your ISO to about 3200 and your shooting mode to Manual. Since you want to keep exposure to less than 30 seconds to prevent the stars from creating noticeable trails on each frame you shoot, select a value between 20 and 30 seconds and set aperture to maximum (ie. f/2.8). I use f/4 with success, but it might force the choice of 30 seconds over 20 seconds. You may also want to use a fixed White Balance mode instead of Auto, but I generally have not found Auto to be a problem.

Normal Infinity Focus Setting
7.       If your lens has a focus scale, set it to the vertical bar near the infinity mark, but not at the infinity mark. Take a test shot and zoom in on the playback to see if you have optimal focus. If not, tweak it very slightly and re-check. The actual infinity focus point varies slightly with focal length, so leave the lens at your intended focal length when setting the focus point, and only zoom in on the playback to verify it.

8.       If using an intervalometer, set your camera to Bulb (may be found in Manual mode settings or may be a separate mode depending on the camera). Set the intervalometer shutter  ‘on’ time to 20 to 30 seconds (depending on your test shots), set its interval between shutter actuations to something like 5 seconds (to allow you to wipe condensation off the lens between shots), and set the number of frames to about 150. I normally shoot with RAW+JPEG, but you may want to consider doing such a long sequence in JPEG only to conserve memory card space. Elapsed time will be close to an hour and a half, so bring a coffee flask!

9.       If using a simple wired shutter release, set the camera to burst or continuous mode instead of single shot mode. You will not use Bulb mode in this case. Instead, select either 20 or 30 seconds for your shutter speed in Manual mode. When ready to start, use the lock on the wired release to hold the shutter button on. The camera will take one shot immediately after the other with no breather in between. This will get the sequence over with more quickly than when using an intervalometer with a 5 second breather, but you won’t have an opportunity to clean off condensation between shots. Depending on the camera, you may also have to manually keep track of the number of shots taken.

10.   Find an interesting object on earth that you can anchor your shots to. Put it in the lowe part of your frame, about a third of the way in from the edge. You can have it silhouette against the sky, or use a flashlight to do a little light painting. In a sequence, however, you may want to forget the light painting because it is difficult to make the object look uniformly lit in all 150 or so frames. Now, consider putting Polaris somewhere in your shots, so you can see the stars rotate around it in your final sequence.

That takes care of the capture portion. Now for the post-production steps:

1.       I import all 150 frames (images) into Lightroom. Then, if corrections for exposure, white balance, etc. are needed, you can make the changes to one frame and sync the changes to all 150. Similarly, I have set up a custom crop mode in Lightroom for 16x9, so that the resultant video will frame properly on television. Again, I can crop one and sync-crop all the others.

2.       A sidebar on ‘stuck’ pixels: You are going to notice that there will be bright spots on all frames, which don’t move with the stars. These are generally due to the long exposures and the number of such exposures, which result in heating up of the sensor. If these are left alone, it will look weird once you put all the stars in motion, since stuck pixels don’t move. You could use the camera’s long exposure noise reduction feature to mitigate this, but you will be out there for three hours instead of one and a half hours capturing the 150 frames. My approach is to use the clone brush in Lightroom on one frame and sync to all other frames.This is the most time-intensive and frustrating part of the process due to thenumber of stuck pixels my camera produces, and the way you clone them out may adversely affect other frames once you sync. There is a bit of an art to it, and I could write a whole treatise on it. But not now.

3.       Once I’ve cleaned up all the frames, I export them from Lightroom as TIFF files. At this point, you may want to decide if you simply want to make a video clip of the stars rotating around Polaris, or create progressively longer and longer star trails as they rotate. For the latter, you could use StarStax. This software creates another set of files in which the star trails get progressively longer. This is done by ticking a checkbox in the settings menu to force it to save a file every time it adds another file to the sequence. If you don’t check the box, it will only create one file that consists of all 150 frames forming a single long trail for each star.

4.       Now to put things in motion. There are a number of software packages that can accomplish this, but I use my Corel VideoStudio video editor. In that software, you simply right-click on an empty timeline and select the time lapse option(‘Insert Photo for Time-lapse/Strobe’). You then tell it where your frames are and how long you want to make the clip (I generally start with about 10 seconds), and the frames load onto the timeline. If you want to have the stars rotate without making trails, use the files you exported out of Lightroom. On the other hand, if you want the stars to create progressively longer star trails, use the files created by StarStax.

5.       Now you can create a finished video (Share tab in Corel). I prefer to save mine in .mp4 format. Have a look at my two samples on YouTube. One is without StarStax (Milky Way video) and the other is with StarStax (observatory video).  I hope these inspire you to try out this fun technique.