Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

[MichaelG.]

This is not really relevant to the opening question, but it [unusually] gives some optical information about a range of condensers:
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2CD803A7-8EDC-4F4B-A2FA-805B719BDFFB.jpeg (171.72 KiB) Viewed 4499 times

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Ref. http://www.alanwood.net/downloads/olymp ... e-1977.pdf

MichaelG.

[viktor j nilsson]

This is not really relevant to the opening question, but it [unusually] gives some optical information about a range of condensers:
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2CD803A7-8EDC-4F4B-A2FA-805B719BDFFB.jpeg
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Ref. http://www.alanwood.net/downloads/olymp ... e-1977.pdf

MichaelG.

Wow, Michael! That is a great find, thanks for sharing. I've really tried to find information about what the typical focal length is of different condenser, but never found any data whatsoever. That is incredibly informative.

Edit: and I have, of course, looked at that catalogue before, but never noticed this information.

[MichaelG.]

Also worth noting the

CUTAWAY VIEW OF THE OPTICAL SYSTEM

Which presumably shows the AAC condenser and an auxiliary in reasonably accurate detail.

MichaelG.
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P.S. __ I’ve just found an ebay listing with decent photos of the auxiliary
https://www.ebay.co.uk/itm/382842736999

[patta]

Nice, I've never seen condenser focal length as well; I was expecting much longer focal lengths!
Also interesting from the brochure, they used an Abbe condenser for Phase contrast, that means that the aberrations are manageable
Is the "auxiliary" lens in the Olympus moveable? That would be a nice extra complication..

Below the result to visualize spherical aberration, from the "sour cream" setup of post #59. I've quite lost the exact calibration, and images are quite bad, but hope the method at least is clear. The images are the illuminated spots on a diffuser slide; each one taken by racking up the condenser in different positions. In order to focus the high NA rays, the condenser needs to be racked up to the max, almost touching the slide. 1mm thick glass slide, diffuser layer over it. Looking with a 4x objective may be better (here I've used 1x).

The offending condenser here is Swift polarization dry "Abbe".
As more or less visible, rays from different aperture do not focus at the same height. Also the amount of chromatic aberration can be estimated.

Edit: just came out another question: ANY lens has some aplanatic point, where (primary) SA is zero. Hmmm... another time.
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Condenser spherical aberration on object plane

Spherical aberration lettrømme.jpg (102.86 KiB) Viewed 4454 times

[MichaelG.]

Is the "auxiliary" lens in the Olympus moveable? That would be a nice extra complication..

Having never handled a BH, I can’t be certain, but: From the book, it appears that the auxiliary ‘plugs into’ the substage slide, and is therefore fixed in relation to the condenser.

As an aside: my interest in this area is piqued, because I have just bought a BH-AAC condenser [for use in some, as-yet unspecified, future project]

Keep up the experiments, patta … you have an enthusiastic audience !

MichaelG.

[patta]

Great, thanks for the support!
Let's wrap up and publish those things before moving to the next "project"

Here, I've used the "macro Bertrand lens" setup of post #59 to answer the question:
Does it work to put stops on the condenser filter tray? Like, a DIY phase annulus, or a darkfield stop.
A similar issue will be to use a condenser of brand A with objectives of brand B, as the condenser iris or phase annulus may be in different places for different brands/models.
In order to work, the condenser diaphragm needs to be neatly focused on the phase plate / iris of the objective. (apart from aberrations)

Answer: sometimes it works, sometimes not!
Racking up and down the condenser, we move the focus point of the image of condenser stop, inside the objective. So there is some room for adjustment.

- First case, a phase objective; I put a DIY annulus in the condenser filter tray. Yes, I could find a condenser position where the annulus get focused on the phase plate of the objective; so the DIY annulus works here. Ok in this example the carboard annulus is horrible and not even centered, but it is focused and may be get to work.
The DIY (or other brand) annulus needs also to be of correct diameter, it won't work well to enlarge/reduce it by defocusing the condenser.

- Second case, an objective made for darkfield with built-in adjustable iris. I put a stop disk on the filter tray to get darkfield illumination. In this case, no, I could rack up or down the condenser all the way, but couldn't get both the condenser stop disk and the objective iris in focus. So, darkfield won't work properly, need to find a way to mount the stop disk at a different height wrt the condenser lens, so to get it neatly focused in the objective's iris plane.
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Phase and darkfield plane matches.jpg (112.67 KiB) Viewed 4414 times

[patta]

Now, again with a similar setup as the previous post (Macro Bertrand, aka micro fisheye or apertometer)

For brightfield, we have learnt at kindergarden that the optimal condition is when the NA of the condenser is the same of the NA of the objective (critical illumination?)
Now, NA is chocked by the aperture. For the condenser, we know where the aperture is: it is the adjustable iris.

Instead, where is the aperture baffle of an objective? In the middle of it, but where exactly?
I would expect the objective aperture to be conjugate to infinity; is that so?
So, with the objective focused at infinity with the "macro Bertand lens", let's see.

Below the "aperture" image of 8 different objectives of random brands & ages, all finite-conjugate, 10x and 40x.
The focus is sharply at infinity in the center (trees in the background). We look at the round border of the image, that should be the objective's aperture stop.

Results are random: some objectives have a sharp baffle visible there, while other not, it is defocused, meaning that the main aperture-chocking baffle is at another place, not conjugate to infinity. That is baffling.
And (un)surprisingly, the phase ring of phase objectives is also out-of-focus, not conjugate to infinity; that may be the same thing observed for the condenser phase ring of posts #53 and #60.
So the corresponding condenser and phase annulus, are in reciprocal conjugate position, but do not need to be "telecentric" or conjugate to infinity. Brand A and brand B may have placed them in different positions.

Interpretation: microscope designers put the aperture wherever they find suitable, not necessarily conjugate to infinity. That was too easy!
Using phase objectives of brand A with phase condenser of brand B has randomly low chances of success. Even making a new annulus of right size, won't necessarily work well (see also previous post).
Brightfield objectives do not always have a well-defined aperture stop (hence no exactly defined aperture!)


The images are a bit small rescaled, if you want bigger, we need to find sponsors to pay for forum storage space. Or, I could post a long row of separate images...
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Aperture_objectives_infinity.jpg (110.03 KiB) Viewed 4397 times

[patta]

Last, those condenser aperture issues are getting out of hand.

From post #32, there is this fear (real or imaginary) that the condenser may not project parallel cones of light across all the field, and this will give big troubles for critical illumination, phase and darkfield. If the illumination is not parallel (or, well, not matched with the objective aperture), it may work in the center but on the corners we won't get proper illumination. It is more or less the same issue as from previous posts.
It should get worse for small aperture, small magnification, large field objective.

So, question, is this a real threat or a mental masturbation?
With the usual "Macro Bertrand" and a 4x objective; plus I've put the slide with three pinholes spaced 1.2mm on the stage. Condenser Swift "Abbe".
We're going to look at the three "bokeh balls" from those pinholes, and check if the condenser aperture matches up with the objective stop(s), if those bokeh balls are separated or merge in one.

The 4x objective in question, has two visible baffles inside, not sure which one is the official aperture (see previous post).
I've taken photo at the object plane (the pinholes), then focusing the image of the condenser iris, and then at those two stops.
For the two configuration, condenser with its top lens, and without.
Edit: to be clear, condenser, stage and objective don't move; only the macro lens refocuses (like a focusable phase telescope)

Results:
With the cap on, the focus plane of the condenser aperture is very near to the objective baffles; there is no visible clash between those.
With the cap off, the condenser aperture is focused far away; at the objective's aperture(s) the bokeh balls are widely separated and get severely vignetted by the objective baffles, making "cat's eyes".

So, result, with the cap on, it is all fine, we can have good, parallel critical illumination across all the field tested (3mm with 4x objective)
With the cap off, illumination parallelism goes pretty much ashtray; correct in the center, but on the border of the field it becomes basically oblique illumination.
I'm not sure how serious the effect would be in real use; in the photo below, for example, all 3 pinholes appear of same brightness, although the central one should be brighter (its aperture is not vignetted).

That is a bit weird as we are supposed to remove the condenser top lens when using low-power objective.
I won't do that anymore. Instead, to get a large field illuminated, seems better to keep the top lens and use a large, uniform diffuser below. Or, buy an expensive, dedicated low-power condenser.

Had some issues, so results are not very neat:
- there appear to be severe miscollimation with my microscope, condenser iris is quite off - centered!
- With the 4x objective, the "macro bertrand" could visualize only an (object) field of ~3mm, which is only the central part; the objective is capable of larger field, say 5mm. What happens at those extreme borders of the filed, is still unknown. Dragons? Cosplayers? Cheshire cats?
- The pinhole distance, may be 1.2mm or 1.5mm, depending on mood. Let's settle for 1/16 of an inch.
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[patta]

Ok one more
the question to answer here: what is the point of the previous posts? This thread was about condenser aberrations, not about aperture position!

Below two LARGE images that may provide some motivation. Usual macro-Bertrand, focused on the phase ring of two different phase objectives. (Wild Phase 40x, Leitz Phaco1 10x), mounted on the modern Swift & son microscope.
I've put in the condenser tray a filter with many pinholes. Cap lens is on. We look at the aberrations and focus of those pinholes.

The first image is the 40x: the pinholes are imaged more or less sharply together with the objective's phase ring. Ok, we can see some aberrations and colors, but isn't so tragic after all (it's a condenser, not a Planapo objective); it would be sharp enough to project properly a phase annulus illumination. Note, this condenser is just an Abbe, not aplanatic, nor achromatic at all!
Do you think that those aberrations are too large? Look at the next image...

Second image, the 10x objective. Similarly as observed by Hans, the phase ring is put in another place... so the pinholes are not focused, they make those big bokeh balls, and I really couldn't put them together with the phase annulus, however I rack up and down the condenser, or try to move the pinholes diaphragm (hm, maybe is feasible, with a lathe. or a hammer. anyway)
Now, this out-of-focus issue is much much larger than the aberrations of the previous image; I won't be able to project a phase annulus in the right place, too out of focus. ( the balls are larger than the phase ring).
Translated in field term, we can get phase contrast in the center, but it will fade away rapidly as we move toward the borders. Tis issue is caused by the condenser and objective apertures that are not conjugate.

So, aperture position and its focus are more dangerous than aberrations.
It seems better to have an Abbe condenser with the right spacings, rather than an Achromat-Aplanat from another brand that has the iris/annulus in the wrong positions.
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40X objectives pinholes in focus

Aberration_vs_defocus0001.jpg (109.59 KiB) Viewed 4340 times

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10x objective pinholes out of focus

Aberration_vs_defocus0002.jpg (116.33 KiB) Viewed 4340 times

[MichaelG.]

The images are a bit small rescaled, if you want bigger, we need to find sponsors to pay for forum storage space. Or, I could post a long row of separate images...

… or you could [but not saying you should] open a free account with Dropbox, and load the images into a shared folder, viewable by anyone with the link.

MichaelG.
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Ref. https://www.dropbox.com/en_GB/basic

[patta]

For reference, just found this article from Ted Clarke, about spherical aberration of the condenser.
His conclusion, the SA issue can be patched by keeping the field diaphragm fully open (in Köler illumination)
Or, I'd say, by using a non-kohler large and diffuse illumination before the condenser.

https://www.mccrone.com/mm/evaluation-o ... denser/

[apochronaut]

For reference, just found this article from Ted Clarke, about spherical aberration of the condenser.
His conclusion, the SA issue can be patched by keeping the field diaphragm fully open (in Köler illumination)
Or, I'd say, by using a non-kohler large and diffuse illumination before the condenser.

https://www.mccrone.com/mm/evaluation-o ... denser/

That's the way it was done prior to the invasion of the wire filament using critical illumination with a large planar source. The wire filament or any small source of light throws a monkey wrench into it, though. Köhler had his stimulus from that problem and evolved a method for diffusing the light. Diffusion screens can also work but they tend to absorb energy reducing intensity and lowering contrast.

Regarding an abbe condenser patch being implied from the Ted Clarke article. I don't see that from what us being said and from the ray diagrams. Clearly the abbe condenser axial and angular rays cannot be brought to the same point of focus, whereas with an aplanatic condenser, it happens. What is implied though, is that correct focusing of an abbe condenser when using a DF stop, will allow one to use the angular rays for DF. That doesn't take into account the problematic ca of an abbe at those large apertures. An achromatic aplanat is necessary for high quality microscopy in all cases.