Torii-san has measured the DLC sample and discovered a small unsymmetry between "positive" and "negative" angles.
I will try to search for en explanation.

Regarding the ghost beam. Torii-san wants to create a conical shaped baffle (blackend by candle-soot) to avoid an influence of ghost beams and, at the same time, keeping the symmetry. I think that the ND filter is in a first glance OK and we should focus on the sample holder for solblack. After this, we can think of this baffle.