Hi Freddy,
Do not worry too much
The FIT file extension is very much a universal standard - PixInsight will most likely read your unprocessed image files without any issue. I say 'most likely', because there are some manufacturers (of capture software) who use a very slightly different format when they create their FIT files during the capture process (and sometines you may even encounter this if you perform any software processing outwith PixInsight).
But, the important thing is that PixInsight has the power and flexibility to read these formats (yes, you 'may' need to experiment with some conficuration settings, but once you have done this you will not need to repeat things thereafter).
Then, as you work through your PixInsight processing, the choice of how
you save any further data is up to you. Many choose to use the XISF format - it is a PixInsight-prorietary format, but the details of the format are extremely well documented, and have been made available for all software vendors to include in their programs.
Alternatively, you can simply choose to save the images in FITS format - if you have some reason to continue processing outwith PixInsight itself, or (perhaps more likely) if you with to share images with other image processors who have not yet made the move to PixInsight.
Obviously, you might also wish to save a duplicated copy of images (especially your final images) in web-friendly formats such as PNG or JPG. All of this can be very easily achieved without having to leave PixInsight.
On the subject of XISF images again - why would you save in this format? Well, the XISF format is more flexible and more efficient than the FITS format. The FITS format is now several decades old and, although it is a very capable data storage format, XISF has shown that there was room for improvement. It would seem quite possible that XISF may yet take over from where FITS stands today.
When I image, I am capturing the data in FITS format, reading the image data into PixInsight without any issues or concerns, then processing the data thereafter and saving all interim, and final, images in XISF format. I create a JPEG version of my final result as required, and would - for example - email either the JPEG or a transformation of XISF back to FITS if I needed to pass my data to anybody else.
I hope this helps to clarify the matter.
