MicrobeHunter.com Microscopy Forum

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.

That's astigmatism.

A very interesting video … I have downloaded it for frame-by- frame inspection, as I think some study of the best-focussed frame might give us some clues.

MichaelG.

It needed some ‘faffing about’ to look at the individual frames; because the linked “video” is an animated GIF

Here [having converted the GIF to a movie] is what I believe to be the overall-sharpest individual frame [at 00:09]
. .
There has been a lot of digital processing involved, but it looks pretty good.

Scrolling through the frames from 00:07 to 00:11 reveals the double-imaging quite well, especially on the number 4

MichaelG.

I'd vote as well astigmatism (and/or coma)
it happens on the center of the field, so there may be miscollimation or misalignment of some lens.

You can raytrace with zemax and other, shift or tilt one of the lenses off-axis

Forgive my ignorance, please … but can you describe how ‘astigmatism (and/or coma)’ produces a distinct double image.
I have always thought of their effects as quite subtle, in an ‘Analogue’ sort of way.

Many thanks
MichaelG.