Hi There - and welcome to the PixInsight Forum!
PixInsight can handle mosaics 'natively', with tools such as the GradientMergeMosaic Process and the MosaicByCoordinates and MosaicPlanner Scripts - as well as the CatalogStarGenerator Script that can be used to create a 'background image' onto which you can align your mosaic.
There is also the StarAlignment Process (amd the DynamicAlignmnet Process) that can be used to align images, including those panels that will be used to build an overall mosaic image.
Be warned though - it will take you a little bit of time to get to know how all these processes and scripts actually work. However, the end result should be worth all the effort.
You might also want to re-think 'how' you acquire the image data. PixInsight doesn't need you, for example, to take 9 images, with three images being acquired and stacked for each 'panel', with a 33% overlap between panels. Instead, you could take the same nine images (in this example, as a 'row' of images) but with an 8/9ths overlap between panels. Even better would be to add an extra two images at the start position and an extra ywo at the end position to allow you to stack data from a full three images right across the whole mosaic. The ImageIntegration Process then has far more statistical data to work with for every pixel in the final mosaic. (And, obviously, in a 'perfect world' you would take far more images, with far smaller steps between panels).
The point I am making is that PixInsight does not limit you to 'one method' of dealing with your data - leaving you the ability to explore different methods that suit you and your requirements.
Hope this helps.
