"Even more surprising as this creature is completely sharp ! "
I'm not the least surprised. We all too often consider the Heligons as poorly performing lenses because there are lots of chromatic and spherical aberrations present in our images. What we concomitantly forget is that these lenses are dedicated for a very special task, namely, acting as a relay system from an X-ray screen to a film, cine, or CRT device. In their time these lenses were very expensive and use a lot of high-refractive, high-end glass as well. However, also due to their strictly defined optical configuration and high speed, the prize in optical deterioration and aberrations is very significant once the lenses are used well outside their dedicated conjugate distances. I remember a film capture of a hydrodynamic lab model done in real time with the Heligon 50 mm f/0.75 on a high-speed lab camera and the TV images were high contrast and tack sharp.
