Kaczorek wrote: I would say it's typical pitfall ;-) Exposed frames always look totally black before you shift histogram to right. No matter before or after stacking.

Yes, I know. ;-)
I've been imaging for a few years already, just on Windows, had a 4 year break (other priorities) and I'm now starting again with the whole observatory control moved to Astroberry / KStars.
I've never used FITS files before though, only CR2.

I know that light frames are usually very dark. But not completely black. ;-)
When I load a CR2 file with the same exposure and everything else the same into DSS, I see the image. It is very dark indeed, but I do see stars and a hint of the galaxy / nebula. When I load the FITS it is completely black. No stars. Not even hot pixels.
The same for flats. When I load a CR2 flat in DSS, I see a rather light image with the gradient. When I load a FITS flat, I get a completely black image.

But since you guys have shown that there is actual data in these FITS, I'll just put it down to a difference in how DSS displays CR2 and FITS in the preview. Or something like that. ;-)
And I'll probably stick with CR2 for now. ;-)

Thanks for all your help. I've definitely learned something here.