Align stack using WCS not working on meridian flip images

[brandx]

Using Align Stack on WCs seems to work ok for each set of images provided they are in the same orientation but not if meridian flipped.  (I have 16 images some of which are flipped and each group of images is aligned ok if they are in the same side of meridian orientation but each of the two groups are not co-aligned)

[brandx]

So I aligned images using Pixinsight Star alignment process and, in Pixinsight at least,  I can blink the images and all are co-aligned.  Import those PI aligned images into AIJ and same issue  - those on same side of meridian flip are aligned but not whole set.  

[karenacollins]

Stack aligner currently only supports image translation in x,y direction to align images, but not image rotation. If you are running photometry, instead of aligning images, use the multi-aperture setup panel option to place apertures using RA/Dec (if your images are plate-solved) and the images will not need to be aligned. If your entire stack is not plate solved, then plate-solve at least the first image in the stack, and then the first image after meridian flip (but be sure in this case that the option in multi-aperture setup panel "halt processing on WCS or centroid error" is disabled).

If you are coadding images for other purposes, you'd need to load two separate stacks, rotate one of the stacks using the flip and rotate options in the Image Display > Process menu. Once they are in the same orientation, then you can use Image Display > Process > "combine stacks into single stack" to join the two. Then you can run stack aligner to align the resulting images using translation. However, if your images do not rotate exactly 180 degrees at meridian flip, or support image rotation throughout the stack, the stack aligner tool and AIJ may not currently be the best option to coadd your images (plus you and the alternate tool will need to deal with how to resample the discrete pixels when the images are rotated).

Karen

[karenacollins]

Also, regarding the second post: If the images are aligned in PixInsight and then saved and opened in AIJ, are they then already aligned, or do they still need aligning by AIJ?

[brandx]

Thanks Karen,  I noticed that despite them moving (viewing as a AIJ sliced stack) the aperture placement worked ok in the end