In previous articles I covered the early processing steps, like cosmetic correction and linear defect removal. I talked about stacking but missed mentioning star alignment.
For the stars to literally line up, they must be aligned. Without this alignment, when the frames are stacked or combined you will be seeing stars; that is at least where they are not supposed to be. Shifted slightly from the original image will be another ghost image, the duplicates of the stars next to each other. Sometimes in different colors, depending on where in the process the alignment was mismatched.
In the old days, each frame had to be aligned visually by shifting the frames around on adjacent layers until all the stars did line up.
Software, like Maxim DL or PixInsight, allows you to click on two or more stars (the more, the better) in different parts of the frame and it would automatically position those stars atop one another, as if a pin were placed through them. This is still awkward, manual, and somewhat inaccurate, as your cursor crosshairs could be off by a pixel or two.
Ultimately, a better solution arranges the frame position based on astrometric data. Astrometric data is the position of the stars as measured very accurately to one another and those values stored in a catalog of star positions.
Within the FITS header stored with your image frame are values for RA, DEC, and Position Angle and other measurements. When the Star Alignment process is invoked, it writes additional offset measurements into the FITS file. You will see an entry of “Registration with StarAlignment process.” The frames are mathematically, rather than physically rotated. When the frames are combined, they appear to be in the same position.
In the final image it is sometimes possible to see the remaining fragments at the edges of the frames. These will have to be cropped out. This is especially true, if the imaging occurred on different instruments, or at different times.
For a complete, detailed, exhaustive, and technical explanation of how PixInsight handles Star Alignment, go to : https://pixinsight.com/.../StarAlignment/StarAlignment.html