Odd performance from an Olympus SPlan Apo 20x

Linden.g · Post by **Linden.g** » Tue Sep 09, 2014 7:48 pm

I'm really puzzled by what I'm seeing from an Olympus SPlan Apo 20x. I've used this objective many times in the past and now I'm seeing degraded performance. The image of a diatom test slide is shown below with full DIC shear. I have a full set of Apo's and UPN DIC prisms. Only this object shows the extreme flare. If I use a 20x Splan achromat with the same UPN there is also no issue. I've not dropped or knocked the objective and it looks optically very clear when inspected. Its really puzzling, does anyone have any suggestions?

discomorphella · Post by **discomorphella** » Tue Sep 09, 2014 9:01 pm

Hi Linden,
Is the front surface clean? A film, surprisingly thin, of "crud" on the front surface could contribute a lot of flare. Did it get any immersion oil on it, or perhaps something evaporated onto it from an incompletely dried slide? Or some less-than-reagent grade solvent filmed it?
Just a thought. Surface degradation or "crud" films often give us flare issues in both microscopy and lithography.
Hope you figure it out, the 20X/0.75 is a very useful tool. Too useful to have it down for flare...
David

Linden.g · Post by **Linden.g** » Wed Sep 10, 2014 2:07 pm

Thanks for the advice David. The film would need to be very subtle but I will give it another clean. The flare rotates if I rotate the objective by loosening. There is some wear on the front mask but this is no worse than when I bought the objective used. I also made a mask to cover up the warn black mask and the flare remained. Can you recommend a more stringent cleaning solvent for removing synthetic immersion oil in case its that which is the problem?

Linden

Choronzon · Post by **Choronzon** » Thu Sep 11, 2014 5:47 am

One of two things can be wrong with this lens, if the external elements are clean.
1, it has taken a very hard knock or has been dropped, maybe even the front element is tipped
2, there is fluid inside the lens. Immersion oil, or cleaning fluid.
How can this happen? In the case of cleaning fluid, regular lens cleaner just doesn't evaporate quick enough, and if you use too much, well, dry lenses aren't sealed.
Oil can get in in the same way; if you bought the lens used from a multi-user lab and this was used on an inverted microscope, immersion oil will move down into everything if left to it's own devices. I've seen immersion oil inside a 4X objective, and that was on an upright. Oil will lurk inside a lens for quite some time until it creeps from the lens cell to the actual lens surface. Sometimes just suddenly overnight.
Judging from the off axis image, it's quite serious and needs to be opened up, at least it's not a high NA immersion lens, so the element spacing is a little easier to deal with (shims)

Choronzon · Post by **Choronzon** » Thu Sep 11, 2014 5:53 am

Looking at it again, I'm guessing the front element has been crashed and it's no longer on axis or perpendicular. That's a fairly huge off axis diffraction pattern.

Linden.g · Post by **Linden.g** » Thu Sep 11, 2014 5:18 pm

Thank you for the insight, um doesn't sound good! just disappointing as I know I haven't knocked it around.

Thanks

Linden