icon under the left image — this briefly switches between the two images. After non-linear correction, images of the original and rotated orientation are supposed to match each other well. Chack that by You can by clicking the small icon under the left image — this briefly switches between the two images.
I they match, you may want to sum both images to improve statistics or smooth small imperfections not fully corrected. Simply press the processing button — .
At this step, the frame size cannot be changed; only the minimal common area (with no out-of-image pixels) is used, thus the 
icon is deactivated.
For several reasons, the last step of correction might not result in perfect match of two images. It is recommenned to perform additional cross-correlation before merging them. You can control it via Menu:
Menu Setting → Merge → Correlate.
Menu Setting → Merge → shift limit.
The shift limit value should range from 0.0 to 1.0 defining the maximal shift in the fraction of the total image size.
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