Re: Problems with image alignment
Posted by karenacollins on Aug 18, 2016; 8:50am
URL: http://astroimagej.170.s1.nabble.com/Problems-with-image-alignment-tp448p449.html
Hi Bernardo,
I assume your images do not have stars, so I think the best approach for you is to use the image stabilizer mode that can be found in the menus above an image stack. Make sure you have loaded your 8 images into a stack (use Toolbar->File->Import->Image Sequence) rather than loading them as 8 separate images. Then select Process->"Align stack using image stabilizer". Watch the lower part of the toolbar to see the progress so that you will know when it is finished (the process can be slow). This feature can do translation only or can attempt a full translation + rotation alignment (for that, select the affine mode option).
If module does not work for some reason, an alternative is the image alignment module, which is best for star fields, but it may work for you if there is a bright feature in the image that an aperture can be placed around and centroided on. Access it using the stack aligner icon (
) above the image display. There are three options available, and the best depends on how much image shift you have and whether or not your images have WCS headers (I assume
not in your case). I think option #3 below may be the best alternative for you.
Alignment modes typically used for star fields (stack aligner):
1) If your images have WCS headers (i.e. have been plate solved either outside of AIJ or using AIJ), the easiest method is to choose the "Use only WCS headers for alignment" and then click okay. The images will then be aligned based on information in the FITS headers.
2) If your images do not have WCS headers (and you do not want to plate solve them), and if your image shift from one image to the next is less than the size of a (possibly large) aperture that can be placed around a relatively bright isolated star, then set the appropriate aperture size (similar to multiaperture), deselect all options (except maybe the "Align only to whole pixels.." and/or "Remove background ...", click OK, the click near one or more stars to place alignment apertures. When you are finished placing apertures, press <Enter> or right-click, then the images will align.
3) If your images shift more than apertures can track, you will need to use the same setup as #2, except enable the option "Use single step mode". After clicking OK in that mode, place one or more apertures around one or more stars in the first image. Then press <Enter> or right-click. Then when the image is aligned and the new image in the stack is displayed, click near the first star you clicked in the first image. Continue clicking on the same star until all images are aligned.
NOTE: Image alignment using the stack aligner module is current limited to translation only (i.e. shifts in x or y only - no rotation or scaling correction is done).
NOTE2: The black regions after an image has been shifted are unavoidable. If you want to crop those regions, you will need to crop the same region in all images of the stack. The crop option will crop all images in a stack, so that should work out correctly if you load your images as a stack rather than individual images.
Hopefully one of the above options will work for you,
Karen
URL: http://astroimagej.170.s1.nabble.com/Problems-with-image-alignment-tp448p449.html
Hi Bernardo,
I assume your images do not have stars, so I think the best approach for you is to use the image stabilizer mode that can be found in the menus above an image stack. Make sure you have loaded your 8 images into a stack (use Toolbar->File->Import->Image Sequence) rather than loading them as 8 separate images. Then select Process->"Align stack using image stabilizer". Watch the lower part of the toolbar to see the progress so that you will know when it is finished (the process can be slow). This feature can do translation only or can attempt a full translation + rotation alignment (for that, select the affine mode option).
If module does not work for some reason, an alternative is the image alignment module, which is best for star fields, but it may work for you if there is a bright feature in the image that an aperture can be placed around and centroided on. Access it using the stack aligner icon (
) above the image display. There are three options available, and the best depends on how much image shift you have and whether or not your images have WCS headers (I assume
not in your case). I think option #3 below may be the best alternative for you.Alignment modes typically used for star fields (stack aligner):
1) If your images have WCS headers (i.e. have been plate solved either outside of AIJ or using AIJ), the easiest method is to choose the "Use only WCS headers for alignment" and then click okay. The images will then be aligned based on information in the FITS headers.
2) If your images do not have WCS headers (and you do not want to plate solve them), and if your image shift from one image to the next is less than the size of a (possibly large) aperture that can be placed around a relatively bright isolated star, then set the appropriate aperture size (similar to multiaperture), deselect all options (except maybe the "Align only to whole pixels.." and/or "Remove background ...", click OK, the click near one or more stars to place alignment apertures. When you are finished placing apertures, press <Enter> or right-click, then the images will align.
3) If your images shift more than apertures can track, you will need to use the same setup as #2, except enable the option "Use single step mode". After clicking OK in that mode, place one or more apertures around one or more stars in the first image. Then press <Enter> or right-click. Then when the image is aligned and the new image in the stack is displayed, click near the first star you clicked in the first image. Continue clicking on the same star until all images are aligned.
NOTE: Image alignment using the stack aligner module is current limited to translation only (i.e. shifts in x or y only - no rotation or scaling correction is done).
NOTE2: The black regions after an image has been shifted are unavoidable. If you want to crop those regions, you will need to crop the same region in all images of the stack. The crop option will crop all images in a stack, so that should work out correctly if you load your images as a stack rather than individual images.
Hopefully one of the above options will work for you,
Karen
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