Banana-shaped PSF
Sometimes the Point Spread Function (PSF) looses the expected symmetry and adopts the shape of a banana. This effect is of course transmitted to all the acquired images:
YZ slice of a Nipkow Disk Microscope image with a banana shaped PSF.
This is not what you would expect from a Spherical Aberration. We have occasionally seen the banana shape to be strongly wavelength dependent. We believe it is either due to the scan lens, or due to a series of small, each other compensating misalignments. Both could induce wavelength dependency. Also, in both cases optimizing the pinhole position alone wouldn't help.