I think I might have found where the problem comes from...
It looks like the center triangle...is not centered !

here's what I did (tell me if something is not logical there)

i very precisely centered the secondary under the focuser, and the primary reflexion into it with the sight-tube. then i took...a laser !
ok so theoretically the laser spot might be right in the middle of the triangle now (am I wrong ? don't think so). and guess what. The spot is landing in one of the corner of the triangle (my laser is well centered, if i rotate it in the focuser, it's position doesn't move on the primary.)
ok, let's continue anyway...
Still with the laser, i'm centering the primary (this step is normally done with the blackcat cheshire.)
Now last step, if i look in the auto-colimator, i can see all the ghost images really close, and a minor secondary ajustement allow to stake the triangle and only one of it's ghost (of course not the 4 of them, cause the spot is not well centered, am i wrong again ?)

after that, i can check with the sight tube and the laser (and auto-colimator) everything's in place...

what do you think ? Did a miss something there or am i right ?

Renaud