How can I de-rotate and align star-field images for stacking without an equatorial mount?
Asked 11/15/2015
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I shoot night-sky photos on a fixed tripod and don’t use an equatorial mount. I’d like to stack multiple exposures, but the stars rotate between frames. Is there a reasonably simple software workflow to de-rotate and align the images for stacking, preferably something easier than Iris for a beginner?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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The Hugin panorama stitcher can be used to align pictures. The align_image_stack executable is part of Hugin and can be run as a standalone command line program. To align pictures im1.tif im2.tif, im3.tif,..., you can give the command
align_image_stack -a al -C -t 0.3 -c 20 im1.tif im2.tif im2.tif ....
which will output the remapped aligned pictures al0000.tif, al0001.tif, al0002.tif, etc. Here the -a option sets the prefix of the remapped image files, chosen to be "al" in this example, -C means that the output files are all cropped to the same size, "-t 0.3" sets the maximum tolerance for the control points to be 0.3 pixels (the default is 3 which is too large if there are no good reasons why you shouldn't get almost perfect alignment) , "-c 20" sets the maximum number of control points in each 5 by 5 subdivision of pairs of images to be 20, the default value is 8.
If you add "-p test.pto" to the command, then you can open the file test.pto with Hugin and then modify the control points, do the remapping with different option or make other changes.
Originally by user28357. Source · Licensed CC BY-SA 4.0
user28357
10y ago
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Generated from our catalog & community — verify before relying on it.
Yes. One practical option mentioned is Hugin. Although it’s known as a panorama stitcher, its align_image_stack tool can align rotating star-field images well enough to prepare them for stacking.
Example command:
align_image_stack -a al -C -t 0.3 -c 20 im1.tif im2.tif im3.tif
This creates aligned output files such as al0000.tif, al0001.tif, etc.
What the key options do:
-a alsets the output filename prefix-Ccrops all outputs to the same size-t 0.3uses a tighter control-point tolerance for more precise alignment-c 20limits the number of control points
So, yes: you can de-rotate/align night-sky frames without an equatorial mount using Hugin’s alignment tool, then stack the aligned files in your preferred stacking software. The answer provided is command-line based rather than fully graphical, but it is a straightforward workflow compared with more complex astrophotography packages.
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