What might cause
this behavior in a (low magnification 2x) objective lens. There seems to be a image separation in different axes as the object goes traverses through the focus plane (working distance plane) from one extreme to the other?
Current theories:
1. Target is not orthogonal to optical axis - creates a double image as object (USAF1951) traverses through the range of [-Z ... focus ... +Z], thus causing a double image of the chrome pattern?
2. some parallel plate within a converging beam path is giving rise to what looks like astigmatism ? (There is a parallel plate in a converging beam path within the objective lens)
Looking for possible references/journals/papers on why this happens. Is it possible to model this in zemax?
PS: Looking at group 4/element 4 on a USAF1951 target if that helps.