Thanks Wouter for your reply. However, I know that the image needs to be stretched because it is linear, but after stretching it, it remains like that. Only one channel can be stretched, the other two remain black. In the meantime, I think, I have found the reason - it seems to be necessary to activate "3D Cube" option in the FITS setup of the program. I tried it without activation and stretched all three channels - and only the red one resulted in an image - in red of course.The blue and green remained black.
Have you ever heard about that before?
Thanks and regards
Georg

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