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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'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?

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Your problem consists of two parts.

Star trail stitching
First you want to combine the photos of each camera separately into star trail images. Your result will be a star trail image from the 7D and one image from the XTi. This answer has more details on astrophotography. The relevant part is under the heading Capturing Star Trails. For the stacking you need software like DeepSkyStacker, IRIS or StarStaX

Combining two images
Once you have got your two star trail images (one from the 7D and one from the XTi) you can combine them in a panorama with Photoshops Photomerge. For the panorama tool to work a slight overlap of the images is necessary.

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  • Sorry, I guess I wasn't very clear in my original question. I already used a stacking program (startrails.de/html/software.html) to stack all of the exposures from each camera. Then I took the two stacked images (one from the 7D and one from the XTi) and tried to use Photomerge to merge them, but it couldn't find a match (even though there's a substantial amount of overlap). Is there a more hands-on way to create a panorama?
    – Justin
    Sep 3, 2012 at 23:10
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    You could try adding features to the images. Find common stars and make them bigger and brighter. If that does not work you can do it by hand: put both images on the same canvas. Use the Right Click -> Transform -> Warp to distort the images until they overlap. Usually a ( ) shape is what works.
    – Phil
    Sep 4, 2012 at 5:48

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