Hi all. My understanding is that in aplanatic objectives any spherical or coma aberrations in the objective have a magnitude on the order of <250 nm/2pi. Does anyone know how much residual spherical and coma (order of magnitude) to expect in an aplanatic objective (e.g. UPLXAPO100XO)? Would it be noticeable in the PSF of an isolated dye on a coverglass? Or is observed spherical or coma in the PSF of an isolated dye on a coverglass exclusively due to misalignment?
TLDR as a general rule residual aberrations should be much smaller than the PSF if you are using as designed. If you are seeing obvious aberrations, either the objective is damaged or else there is a problem with the rest of the microscope.
Most objectives are designed to perform within the diffraction limit. There are different criteria for “within the diffraction limit” but the criteria I have seen most often is that the RMS wavefront error is less than ~0.07 of the wavelength over the designed field and designed wavelength range (which is usually less than the transmission range). In some cases I think diffraction performance is traded away to improve other aspects e.g. chromatic effects, field of view, etc. Usually they don’t tell you what targets they use.
That is theory. Actual performance will be worse than theory (but hopefully still better than diffraction limit) because tolerances on internal elements and their placement in the assembly. High-end objectives will have been individually tested before leaving the factory but the manufacturers don’t say what exactly is tested and what the redline thresholds are.
If you ask very nicely maybe the manufacturer will give you some details on their design targets and/or testing process.
I have heard of picky customers that have special relationships with objective manufacturers purchasing a new objective by first receiving several nominally identical objectives, testing all of them, and returning all but the objective that performs the best.