Banana shaped PSF

Hello

A customer of our's using a nipkow-disk confocal microscope reported recently a banana shaped PSF in z (vertical axis). He measured this by running a X-Z scan of fluorescent beads. In a well-aligned microscope system a X-Z scan normally produces a cigar-like shape of the PSF. What is the most probable cause of that cigar shape bending to one side creating a banana shape? Could it be simple spherical aberration or rather an oblique lens in the optical path?

Thanks for suggestions.

Franco
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