Aberrant PSF Diagnosis

Hi Everyone,

I manage a core facility, and am concerned that several of our oil objective lenses have become damaged. Well, they have become damaged, and I am trying to figure out if I can determine the underlying cause (damage to the front lens element, something on the back focal plane, etc).

Is there any kind of gallery of PSFs that can be related to specific damages to an objective lens? I’ve tried googling this several times, and come up with pretty much nothing.

Any advice would be much appreciated!

Chris

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It might be worthwhile posting the PSFs here to see what thoughts others might have.

There is some discussion in the Pawley book about different types of PSF aberrations, but they are not really related to damage.

The one case I have seen at our facility was primarily chromatic aberration due to the front lens being damaged (far red and blue showed a much greater difference in position than expected for a bead) after being run into the stage when someone was trying to focus between wells on a plate.

I may have a contribution for anyone who wants to curate a gallery.

Upon seeing these projections, a Nikon rep said “That objective looks like it has been dropped or run into the stage.”

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Hello,

It could also be a comma error due to a tilted sample holder. Check also if your sample holder is flat.
A damage or drop one should see it. Also try a different/new bead sample.

Greetings

Antonio

Probably most users (not manufacturers) have not enough damaged objectives that it is worth for them to establish a precise measurement concept and to collect and interpret such PSF data.

Theoretically this issue is discussed for a long time in a lot of articles.
Maybe you can find helpful information in

Smith: Modern Optical Engineering
Chapter 11.12 Point Spread Functions for the Primary Aberrations

or

Malacara, Optical Shop Testing
Chapter 11 Star Tests

How do you measure the PSF?

Hi Chris,
You should try to replace this objective with an equivalent one and take a PSF again. If it is banana-shaped again, it can very well be that something is wrong with your scope body (it could be dichroics, tube lens, etc.). If the PSF is good, then it’s your objective.
Then see if there are scratches over the surface/front lens of your objective. If one is clearly visible, it is most probably that. If not, you can try to clean with different solutions, several times, the objective and try again. Some dirt is not visible to naked eye and tend to stick firmly. We have had in the past bad PSFs for a little while for a give objective and then after 2 or 3 weeks it got better again. I guess it is because we finally managed to get rid of the very sticky but invisible dirt on it.
Good luck and best regards,
Laurent.

In my experience, the banana PSF is usually a sign of unsealed front lens. Here are the same beads with two different Zeiss 100X, 1.46 objectives, on intact and the other with a moving front lens:

ezgif.com-video-to-gif

(source https://twitter.com/christlet/status/1068189330072571904)

This “banana” aberration appears to be coma. There are many potential sources of coma. Keller’s chapter in Pawley’s book is an excellent resources for info on aberrations.

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There are of course a lot of possible sources for coma. Nonetheless in my experience on a range of commercial microscopes in a facility setting, it is most often coming from an unsealed objective front lens. A good trick to know if this comes from the objective (if you don’t have another, similar good one like in my gif above) is to unscrew the defective objective half a turn or a full turn and see if the defects rotates.

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