This is a 30 second exposure of the Falcon 9 launch on March 6th. The trail is from right to left; the photo was taken north of the launch site, about 3 or 4 minutes into the flight. I think I caught the end of the first stage and the start of the second stage. But if you look closely at the right hand end of the trail there is a curious spiraling that gets tighter and tighter into the flight. Can anyone tell me what's happening and what is causing that spiral?
This is likely an effect called 'mirror shake' or 'mirror slap'. It also affects astrophotographers. What we are seeing is the camera is shakeing more intensely at the beginning of the exposure, but quickly settles down.
The internal mechanisms (the mirror that directs the light up to the viewfinder) of a DSLR are moving to allow the light to reach the sensor, and then cause the camera to shake just a little on the tripod. Most DSLRs that are better than entry level will have a 'Mirror Up Mode' or "Mirror Lock Up' which fix this problem.
A few links to further reading: