Also posted here
Last night I took some star trails photos. I set the shutter speed to 30", put the camera in burst mode and used my remote shutter release (with shutter button lock) to hold down the shutter button. I actually did this with two cameras at once, a Canon 7D and a Canon Rebel XTi. The 7D was mounted on my tripod, pointing up toward the sky, but low enough to include some trees in the frame. The Rebel XTi was sitting on the ground, pointing straight up into the sky.
I set both cameras to the exact same settings and had them running for the same amount of time. The focal length of the 7D was 17mm and that of the Rebel XTi was 18mm. I used a star trails program to stack the exposures together from each camera to produce two separate star trails photos. I'd like to stitch the two photos together (hopefully automatically) so that the resulting image covers a much larger portion of the sky than is possible with a 17mm focal length. However, when I run the two photos through either AutoStitch or Photoshop's Photomerge option, they fail to match the images, presumably since most of the image is a solid color with a few white streaks across it.
Is there a way to stitch these two photos together automatically? If not, is there a way to do it manually but also compensate for perspective distortion?