It is important to know what do you want to have on your video: if you want to share it, you have to be aware of copyright issues: if you use commercial music, you will need to ask for permission. You can find several musicians that offer their work for free, as long as you credit them. My favorite author is Kevin McLeod (http://incompetech.com/music/royalty-free/index.html). Please read carefully the conditions before sharing in order to avoid copyright claims from the original authors.
Once you have the pictures, and you have edited them in a manner that you are satisfied, it is time to start putting everything in a video editing program:
For Windows users, the program by default is windows movie maker. This program works perfectly for this purpose, it just needs to be taken one step at a time.
For Mac users, there is a free software called Time-lapse assembler (available on cnet). It has very basic features, but it is the easiest program to start with.
For Linux, there are several options, but you can find simple tutorials in ubuntuforums (for example, this one). Note that they explain also how to take time-lapses, but with a small difference: we are working with northern light time-lapses, not regular ones.
In any case, there are some general rules we have to think about:
We first need to think of how much are we willing to speed up the reality. As we have seen in previous posts, we will be using long exposure times for each picture. When we mount the time-lapse, we will decide how many frames we want to have every second: a minimum number of eight is necessary to get a proper motion. Lower frame rates will make the video be "jumpy", and the higher the frame rate, the more smooth will the video be.
If we have a series of pictures taken with one second exposure time, for example, and we put 10 frames per second, each second in the video will represent 100 seconds (1 minute 40 seconds) in reality. This will make the northern light to look a lot faster than what it really is. The next videos show the same northern light with different frame-rates.
Here there is a small time-lapse. I have used the same pictures, with different frame-rates. See the difference!
Technical characteristics: ISO 4000, f=2.8, 14mm, 1 second exposure.
Look at the people at the lower left side, how fast they move in each video. There you can see how much the acceleration is.
First, at 5fps (each second in video equals 5 seconds in reality):
At 10fps (each second in video equals 10 seconds in reality):
And at 20fps (each second in video equals 20 seconds in reality)
Note: I did not add audio to the videos, they only serve the purpose of showing the differences between the aforementioned frame-rates.
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