Thanks! Happy it helps. As below I got advice from community here. And no: I did not use flat field correction as darktable seems not to be capable of this. But I learned that disabling the base curve is essential.
@chris With respect to the collimator lenses: I had exactly the same thought and tried it with them: but you can stop down aperture heavily and still see the glowing lamp as a spot. So the small sheet of teflon realy does the trick (and you shoudln’t overestimate the quality you got in former times from projection - that means the projector-method to me seems quite comparable to projecting. Scanning will be of course superiour in quality).
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