I have only begun to use the Atik Infinity OSC, so consider I'm a greenhorn.
But I've been saving the images as TIF format files and processing in Adobe Photoshop Elements 12.
While admittedly it is a lowly form of Photoshop, it was affordable to me. And works for my simplistic processing.
My PS version will not open a FITs file. So yours actually opening the file is surprising.
I've only recently graduated from being happy with JPG files. But still, Elements has to convert 16 bit TIF files to 8 bit in order to use any of the Guided functions. So I bounce between Expert, and Guided modes sometimes.
But TIF files afford me with all the processing perks, without the staleness of antiquated FITs files. Please pardon my distaste for FITs files.
So my suggestion would be to step back to (or forward to) TIFF files and give it a go. You might be well pleased.