Thanks, everyone for the suggestions. Todd put me onto the following. As an astrophotographer I make lots of FLATS to calibrate my astro shots. Usually use the evening or morning twilight sky as a flat source. When I am in a bind and use the computer screen I also use a diffuser or fine vellum paper. The lens or telescope is focused at infinity so I get no artifacts at all - the file is very, very smooth. The computer file I have been using is like that. I have also made some flats with an expo disk on another lens (it is only 77 mm and doesn't fit the 105mm aperture) and as long as I am not trying to correct a nighttime sky image or daytime cloudless sky it works. It works on landscape images just fine. So the process does seem not to work on nighttime astro raw images, which is the only application I really have for Flat-Field Correction. A disappointment. However, I have discovered that if I convert the RAW astro to a TIF, it works on astro images. Now that is not an extra step I want to make since I usually stack 30 or more sky images. It seems Adobe needs to make this simpler. Don't know what they are doing behind the scenes but whatever it is it is not allowing the process to work on night sky shots. I will contact Adobe and put this info into their hopper and hope that someday something will come out the other end. Thanks, everyone for your suggestions.
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