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Testing high NA darkfield condenser

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#1

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by patta » Fri Feb 11, 2022 11:09 pm

Here follow the thread on the unsold "ultracondenser", darkfield for oil immersion.

viewtopic.php?f=16&t=14790&sid=99ac660a ... 641bf528d1

New/refurbished tools to check out how the illumination is coming out:

- slide with pinhole 0.3mm (aluminium foil with an hole made with a pin, mounted under a coverslip);

- test slide, UNI-Posca pencil ink under coverslip; it is composed of nice fine pigment grains;

- Ground screen, a BK7 glass block with one side covered with milky grease; to visualize the illumination;

- Prism apertometer, a BK7 45 deg prism, with a pinhole in the center of hypotenuse (downward in the photo) and graduated NA scale on the sides;

- Fake immersion oil bottle (refilled with frying oil, new);

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Attrezzi_controllo_illuminazione.jpg

Attrezzi_controllo_illuminazione.jpg (129.77 KiB) Viewed 13124 times

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Lenses used (dream team):

Lomo 60x 1.0-0.7 (actually goes to 0.2), good conditions;

CZJ 60x 1.40, clean but miscollimated;

Olympus Plan 100x 1.25 short, collimated but dirty;

Nikon PA 20x 0.75, supposedly good.

Eyepiece Olympus WHK 10x/20 goes well (only) with the Oly 100x;

The condenser to be tested, darkfield manufactured few days ago.

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Obiettive_test_condensatore_12_02_2022.jpg

Obiettive_test_condensatore_12_02_2022.jpg (101.98 KiB) Viewed 13124 times

Last edited by patta on Sat Feb 12, 2022 8:49 am, edited 2 times in total.

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Re: Testing high NA darkfield condenser

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#2

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by patta » Fri Feb 11, 2022 11:46 pm

According to the prism apertometer, the condenser emits a narrow cone with NA around 1.4 (I'd say 1.38-1.45), over a 1.2 mm slide and like 0.2mm of condenser-slide spacing; oil at all interfaces. The illumination does not pass though water (total internal reflection). This only for the center.

No photos for this, not much interesting.

Below the photo of the illumination on the ground screen; it is the hollow cone that hits the diffuser plane, after passing through the pinhole placed where the specimen should be.

It is a relatively neat stripe, with no color fringes up or down. The bright bands are from the condenser ring mirror, that is a thin aluminum foil glued and pressed over 3D printed (relief-banded) surface.

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Prisma_Tyndall_Ultracondensatore_5_9mm.jpg

Prisma_Tyndall_Ultracondensatore_5_9mm.jpg (74.51 KiB) Viewed 13110 times

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As comparison, below a refractive Abbe condenser (not achromatic), with a "darkfield patch" in the center; about 0.7-0.8; color fringes are visible

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Prisma_Tyndall_Abbe_07_patch_8mm.jpg

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Edit Saturday morning

A drawing that *should* illustrate the above photos; the "smile" in the photos is a vertical plane section of the illumination cone:

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Illustrazione_blocco_tyndall.jpg

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And managed to take a similar photo with the 45 prism apertometer.

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Apertometro_su_condensatore_5.jpg

Apertometro_su_condensatore_5.jpg (115.15 KiB) Viewed 13073 times

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Re: Testing high NA darkfield condenser

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#3

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by patta » Sat Feb 12, 2022 11:04 am

Now some photos.

The subject is acrylic paint ink from blue "posca" pencils, that is made of pigment grains about 0.5 micron in size. At the border of a line, the grains are few and separate. The black part is black because there are no scattering particles, just transparent mountant. If it's black, it's good, meaning the condenser+objective are working in darkfield mode.

No I don't have nice diatom test slides, just made this "grains slide".

Lamp is a 6W white led; imaging is afocal:

Visual eyepiece Olympus WHK 10x 20;

Objective Canon 50mm f/1.8

Sensor Canon 600D aps-c

It captures a field of about 12mm diagonal (that's my best)

The condenser is always the same.

Objective Lomo 60x at max aperture (NA 1.0), full field:

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Shoots_FullField_Lomo60x_10.jpg

Shoots_FullField_Lomo60x_10.jpg (95.51 KiB) Viewed 13063 times

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Lomo 60x with the iris a bit closed (let's say NA 0.50):

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Shoots_FullField_Lomo60x_050.jpg

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Center crop of said Lomo at 1.0

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Shoots_Centercrop2x_Afocal_Lomo.jpg

Shoots_Centercrop2x_Afocal_Lomo.jpg (105.4 KiB) Viewed 13063 times

Last edited by patta on Sat Feb 12, 2022 1:14 pm, edited 5 times in total.

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Re: Testing high NA darkfield condenser

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#4

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by patta » Sat Feb 12, 2022 11:15 am

Same afocal setup as above, with the almighty objective Zeiss Jena Apochromat 60x 1.40

This objective is a bit miscollimated because I've opened it for cleaning once. Not really visible in the photos, but visually it shows some issues on-axis.

Full-field, focused in the center:

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Shoots_FullField_Jena60_center.jpg

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Crop of above photo:

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Shoots_Centercrop2x_Afocal_CZJ.jpg

Shoots_Centercrop2x_Afocal_CZJ.jpg (91.76 KiB) Viewed 13063 times

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now this objective isn't Plan, definitely. Let's try to refocus it midway in the field:

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Shoots_FullField_Jena60_corner.jpg

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And that's the enlarged mash-up:

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Shoots_Centercrop2x_Afocal_CZJ_corner.jpg

Shoots_Centercrop2x_Afocal_CZJ_corner.jpg (94.24 KiB) Viewed 13063 times

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Pretty horrible eh? Anyway, the point here is so check that the CONDENSER is illuminating and is still dark away from the center.

Let's assume that the rainbow CA above arises from a deep misunderstanding between the Deutsch objective and the 日本 eyepiece, and not from my beloved condenser.

Ultra-resolution, I'm not seeing much of that. Let's blame the sunflower oil immersion.

Last edited by patta on Sat Feb 12, 2022 8:33 pm, edited 4 times in total.

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Re: Testing high NA darkfield condenser

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#5

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by patta » Sat Feb 12, 2022 11:21 am

Here finally a proper Plan with more or less proper eyepiece, Olympus 100x 1.25 HI Plan

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Shoots_FullField_Oly100_125.jpg

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Shoots_Centercrop2x_Afocal_Oly.jpg

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Last edited by patta on Sat Feb 12, 2022 11:33 am, edited 1 time in total.

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Re: Testing high NA darkfield condenser

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#6

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by patta » Sat Feb 12, 2022 11:29 am

The above images were Jpegs, resized straight from the camera - no adjustments. The background appears pitch-black, but visually ain't so, it is slightly bluish.

Here a photo with phone camera over the eyepiece, HDR. That's actually a bit exaggerate in term of exposure, the scattering grains are fully burnt in this image. Anyway, there is some stray light and haze, but not so much. I think was the Oly 100?

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Oly100x_phone.jpg

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The Olympus 100x in particular had, visually, a lot of haze. Looking at it with "phase telescope", revealed that indeed the top lens was "a bit dirty". Believe or not, the photo at the previous post has been taken through those geological deposits.

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Oly100x_top_lens_full_of_crap.jpg

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Last edited by patta on Sat Feb 12, 2022 12:47 pm, edited 5 times in total.

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Re: Testing high NA darkfield condenser

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#7

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by Hobbyst46 » Sat Feb 12, 2022 11:49 am

Impressive project, patta!!

I would try some monochromatic light with it, perhaps a bandpass filter with the same LED.

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Re: Testing high NA darkfield condenser

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#8

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by patta » Sat Feb 12, 2022 12:19 pm

Great, thanks. Now I don't have any strong monochromatic source or proper filter; oh, wait, a weak blue one; one of next days.

Last for today, let's check how much large field the condenser is covering, use the Nikon Planapo 20x 0.75; mounted on the newest, exclusive CFI stand. Black enamel is back in fashion! The field is limited by a 23mm eyepiece bore. No eyepiece, direct imaging with a camera objective as tube lens (either 90 or 125mm)

.

This photo is from the day after, with different tube lens. For imaging had to remove the Analyzer and Bertrand lens holders, as they were vignetting the large field

This photo is from the day after, with different tube lens. For imaging had to remove the Analyzer and Bertrand lens holders, as they were vignetting the large field

Nikon_CFI_on_horseshoe.jpg (75.67 KiB) Viewed 13049 times

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The tube lens isn't working perfectly, but more or less a 1mm field is observable. The subject are the lil' points of grit on black background.

Here the slide is oiled to the condenser; the objective is dry.

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Nikon_20x_Tamron_90.jpg

Nikon_20x_Tamron_90.jpg (152.77 KiB) Viewed 13049 times

Last edited by patta on Sat Feb 12, 2022 1:32 pm, edited 12 times in total.

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Re: Testing high NA darkfield condenser

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#9

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by patta » Sat Feb 12, 2022 12:27 pm

I've put the 3D design of the condenser here:

https://www.thingiverse.com/thing:5241935

Uploaded also the FreeCAD model that is a bit messy, can be adapted by modifying the Master Sketch.

Materials:

3D printed parts. about 15 grams total

Aluminum foil

One half-ball lens 9mm diameter (a disk may also usable?)

One round coverslip 18mm

Canada balsam or cement to glue the coverslip

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Re: Testing high NA darkfield condenser

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#10

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by patta » Sun Feb 13, 2022 12:02 pm

wait a minute, monochromatic LED.. the old trick of the phone screen; it is composed of OLEDS, if one visualize a green image, well' that's emitting green.

Unfortunately turned out too weak for this inefficient darkfield; I've managed only with the Green color, since the screen is more powerful in green and the camera is also more sensitive there. Anyway not strong enough for convenient live-view focusing. It looks sharper indeed, but the extreme low brightness spoils the result.

Hope the Germans sort that out https://www.mikroskopie-forum.de/index. ... ic=43184.0

Tested only with the Olympus 100x Plan:

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Shoots_FullField_Oly100_125_green.jpg

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Attachments

Shoots_Centercrop2x_Afocal_Oly_Green.jpg

Shoots_Centercrop2x_Afocal_Oly_Green.jpg (148.26 KiB) Viewed 12917 times

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Re: Testing high NA darkfield condenser

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#11

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by Hobbyst46 » Sun Feb 13, 2022 12:22 pm

It is very nice to achieve true DF with an 1.25NA objective , without an iris that lowers the NA.

I have two old 100x1.25 Zeiss achromat objectives. One of them is 160/-, the other bears no mark of tube length and coverslip (yes/no/both). Unexpectedly, the first one yields a not-too-bad DF with the oil 1.2-1.4 DF cardiodid condenser. But not nearly as nice as your DF above !

My second 100X does not yield DF at all - as expected.

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Re: Testing high NA darkfield condenser

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#12

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by Chas » Mon Feb 14, 2022 11:18 am

This is quite stunning (literaly) ... it seems to work well. Before you posted the photos using your mobile phone screen as a light source, I guessed that it had to be a pretty inefficient system.

Soooo the question is; why didnt anyone make a df condenser like this, before ?????

(When one reads the descriptions of old oil immersion df condensers most of them say that a funnel stop must be used with oil objectives to limit the numerical aperture).

Apochronaut mentions chromatic effects, do you think that might be a reason?

BTW was the half-spherical lens exactly half or was it a lens that fell out of the top of a condenser, that might be slightly more than half?

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Re: Testing high NA darkfield condenser

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#13

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by patta » Mon Feb 14, 2022 12:25 pm

Yes the phone screen it is very dim and inefficient in terms of energy - but quite effective in terms of setup time (search "screen green image" on the phone, and there you have it :lol: )

This high NA could be obtained also (possibly better) with a Cardioid design. I'm sure has been done in the past; the fancy "Colonel Woodward" illuminator in the previous thread, looks like it's capable of doing such high NA oblique darkfield. Today, I think it is not common because of practical issues:

- not many suitable subjects. The subject needs to be very thin, mounted in oil or balsam. No live ciliates in water mount! No live blood analysis!

- Darkfield haze: from the images above, the field is really dark; but near the particles, it gets hazy. my guess is that haze comes from the nearby particles out of focus; since the illumination is a very shallow cone (high NA) a lot of haze is produced. With smaller NA, less haze. See sketch attached below; and in post #3 above, the objective with iris closed, cuts off haze. This issue can get very bad with complicated, packed biological subjects.

- Chromatic aberration, after above testing I'd say it is not so troublesome, and if it is, one could go monochromatic, like in phase contrast. The violent rainbows of the 60x 1.40, I'd say is improper eyepiece compensation rather than condenser itself- but not sure yet.

- Modest increase in resolution: sad truth; going from NA 1.25 to 1.4 is a big hassle; but resolution improvement is just some 10% (while price increase of objective is some 1000%)

.

Attachments

DF_highNA_haze.jpg

DF_highNA_haze.jpg (47.54 KiB) Viewed 12855 times

Last edited by patta on Mon Feb 14, 2022 1:58 pm, edited 4 times in total.

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Re: Testing high NA darkfield condenser

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#14

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by patta » Mon Feb 14, 2022 12:42 pm

Ah forgot, the half-ball lens is really half-sphere, I bought a bunch for another project from Aliexpress Mowen/Bohr (they were 1.5$ each, but they doubled the price last week :x well, still 1/10th than other suppliers.) I chose it for this condenser because it's a standard piece available in stock, only spec is its radius. There are also made of Silica if one fancies UV.

Ideally it should be a bit less than half sphere, such that spherical surface is concentric with the subject.

That was why initially the condenser was made for use with thin slide (the subject isn't much off-center); Now, over a 1.2mm slide, the subject is pushed up off-center, the light isn't impinging orthogonally on the sphere surface, so there is some refraction and hence there must be some CA.

Last photos for this testing campaign: Radial polarization

Normally polarization is linear; with this condenser it is easy to get "radial" polarization, meaning the polarization is different along the hollow cone.

It seems to be used in lasers, or to improve resolution.

This polarization can be achieved for a straight beam, either by combining slices of linear polarizers, like a recombined cake; or with some swirling liquid crystals.

With this modern half-ball condenser, the radial polarizer can be done over the converging cone by rolling a film of linear polarizer:

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Azimut_polarizzatore_disegno.jpg

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Anelli_polarizzazione_cilindrica.jpg

Anelli_polarizzazione_cilindrica.jpg (104.82 KiB) Viewed 12854 times

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Photo results: Nothing! :cry: Not even with a linear pol analyzer after the objective. Well It's darkfield, we see only the scattered light; Hopefully will be more interesting with other subjects or with COL.

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Shoots_FullField_Afocal_Oly_radial_pol.jpg

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Shoots_FullField_Afocal_Oly_azimuth_pol.jpg

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Shoots_FullField_Afocal_Oly_aziumt_analyzer.jpg

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Last edited by patta on Mon Feb 14, 2022 1:32 pm, edited 1 time in total.

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Re: Testing high NA darkfield condenser

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#15

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by patta » Mon Feb 14, 2022 1:07 pm

About rainbows, here's how the calibration slide looks like in DF (plated chrome over glass)

The same effect can be seen with naked eye, the 0.01mm ruler of the calibration slide looks colored from some angle, like diffraction grating.

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Shoots_FullField_Afocal_Oly_Calibration.jpg

Shoots_FullField_Afocal_Oly_Calibration.jpg (88.6 KiB) Viewed 12849 times

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Re: Testing high NA darkfield condenser

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#16

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by Chas » Mon Feb 14, 2022 8:57 pm

I was looking at the description of Watson's immersion paraboloid:

Watson immersion paraboloid resized.jpg

Watson immersion paraboloid resized.jpg (53.08 KiB) Viewed 12803 times

The text made two interesting comments:

a] That if one shuts down the iris with this condenser head, then the lowest NA light is blocked first

b] This condenser prefers a brilliant source of light (in contrast to the Cassegrain where a pearl-frosted bulb is recommended)

I am sort of wondering whether your condensers behaviour might, in someways, be more similar to the paraboloid's than the cassegrain's.

?

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Re: Testing high NA darkfield condenser

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#17

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by josmann » Mon Feb 14, 2022 10:13 pm

Would a high index ball lens be helpful here at all? You can get some 2.0 index balls and half balls from Edmund Optics. Pricey, but not insane pricey.

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Re: Testing high NA darkfield condenser

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#18

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by apochronaut » Tue Feb 15, 2022 2:49 am

Chas wrote: ↑

Mon Feb 14, 2022 8:57 pm

I was looking at the description of Watson's immersion paraboloid:

Watson immersion paraboloid resized.jpg

The text made two interesting comments:

a] That if one shuts down the iris with this condenser head, then the lowest NA light is blocked first

b] This condenser prefers a brilliant source of light (in contrast to the Cassegrain where a pearl-frosted bulb is recommended)

I am sort of wondering whether your condensers behaviour might, in someways, be more similar to the paraboloid's than the cassegrain's.

?

Paraboloids are like this one , single reflecting condensers, so the illumination beam gets reversed with the iris diaphragm first closing on the part of the light cone that ends up being the interior or low N.A. Double reflecting condensers such as Cassegrain or Cardioid or Spherical reverse that again.

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Re: Testing high NA darkfield condenser

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#19

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by patta » Tue Feb 15, 2022 2:22 pm

Yes the condenser here is in practice like a Paraboloid. Below the examples, the first one is the half-ball of this thread.

It would be nice also to remake an old style dry paraboloid, like the dry Wenham.

Cassegrain, I think there there are 2 different:

- Cass - "Schwarzchild" like a telescope, with two mirrors; also like the "reflective" microscope objectives. It is still in use, with the reflective objectives; don't know if it is used also for darkfield.

- Cass - "Nelson", I've seen it only on Watson's catalogue, with two pieces of glasses and a nasty double internal reflection; in the tiny characters below write NA 1.37 - 1.47, oh yeah.

I cant understand the why of the high NA Nelson-Cassegrain; a normal Paraboloid or Cardioid should be capable as well of high NA. Maybe at the time the glass available had too many air bubbles in it, so DF was impaired; while the thin top of the Cassegrain could be blown or stamped to remove bubbles? or the second reflection further reduces stray light?

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Vintage_DF_condensers.jpg

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Vintage_DF_condensers_cassegrains.jpg

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Images from microscope-antique, Quekett, Archive.org etc.

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Re: Testing high NA darkfield condenser

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#20

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by josmann » Tue Feb 15, 2022 7:16 pm

patta wrote: ↑

Tue Feb 15, 2022 2:22 pm

or the second reflection further reduces stray light?

That seems like a good explanation. The way it’s drawn, it looks like that second reflection is designed to be right near the critical angle. So any light which would tend to go into a lower NA cone would not TIR?

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Re: Testing high NA darkfield condenser

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#21

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by Chas » Tue Feb 15, 2022 8:47 pm

"a normal Paraboloid or Cardioid should be capable as well of high NA"

Here is a table of the performances of presumably modern implementations of the designs from the Olympus site:

DF condensers from olympus site.jpg

DF condensers from olympus site.jpg (41.25 KiB) Viewed 12662 times

https://www.olympus-lifescience.com/en/ ... darkfield/

Or another one, attributed to Needham, ('The microscope a practical guide'??):

DF Table orginaly from Needham.jpg

DF Table orginaly from Needham.jpg (46.14 KiB) Viewed 12660 times

Taken from:

https://www.chem.uci.edu/~dmitryf/manua ... oscopy.pdf

Which also contains some drawings of the Spot Ring Bicentric et al

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Re: Testing high NA darkfield condenser

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#22

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by Chas » Tue Feb 15, 2022 9:25 pm

And..as a little distraction; attached is E.M.Nelson's obituary:

Nelson obit.pdf

(229.73 KiB) Downloaded 1993 times

And just in case you are wondering about 'Norbert's 19th band' here is a further distracting link:

http://www.microscopist.co.uk/about/fav ... opy-paper/

:-)

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Re: Testing high NA darkfield condenser

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#23

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by apochronaut » Wed Feb 16, 2022 1:41 am

Chas wrote: ↑

Tue Feb 15, 2022 8:47 pm

"a normal Paraboloid or Cardioid should be capable as well of high NA"

Here is a table of the performances of presumably modern implementations of the designs from the Olympus site:

DF condensers from olympus site.jpg

https://www.olympus-lifescience.com/en/ ... darkfield/

Or another one, attributed to Needham, ('The microscope a practical guide'??):

DF Table orginaly from Needham.jpg

Taken from:

https://www.chem.uci.edu/~dmitryf/manua ... oscopy.pdf

Which also contains some drawings of the Spot Ring Bicentric et al

Those sites, such as the Olympus Lifescience .com appear to be prepared by several authors under contract. The same information appears on a Nikon page as well as others, slightly specific to the host corporation's product line but highly generalized to explain various principles of microscopy. The chart of DF types is also highly generalized. Several of the types, Paraboloid and Cardioid for instance, have many permutations from many manufacturers and the specifications are different, depending on which condenser is being considered. A Cassegrain type design on the other hand does not seem to have been embraced by too many companies, possibly only a couple of designs exist, with each originating with the same inventor.

The array on that site has not much to do with Olympus. It is a historical compendium.

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Re: Testing high NA darkfield condenser

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#24

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by apochronaut » Wed Feb 16, 2022 3:58 pm

patta wrote: ↑

Tue Feb 15, 2022 2:22 pm

Yes the condenser here is in practice like a Paraboloid. Below the examples, the first one is the half-ball of this thread.

It would be nice also to remake an old style dry paraboloid, like the dry Wenham.

Cassegrain, I think there there are 2 different:

- Cass - "Schwarzchild" like a telescope, with two mirrors; also like the "reflective" microscope objectives. It is still in use, with the reflective objectives; don't know if it is used also for darkfield.

- Cass - "Nelson", I've seen it only on Watson's catalogue, with two pieces of glasses and a nasty double internal reflection; in the tiny characters below write NA 1.37 - 1.47, oh yeah.

I cant understand the why of the high NA Nelson-Cassegrain; a normal Paraboloid or Cardioid should be capable as well of high NA. Maybe at the time the glass available had too many air bubbles in it, so DF was impaired; while the thin top of the Cassegrain could be blown or stamped to remove bubbles? or the second reflection further reduces stray light?

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Vintage_DF_condensers.jpg

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Vintage_DF_condensers_cassegrains.jpg

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Images from microscope-antique, Quekett, Archive.org etc.

Optical glass was worked and worked to remove air bubbles and then shattered and inspected for imperfections. I can't see that glass with air bubbles would have been used to make anything in microscope optics, after a certain point in time : mid.19th century or before.

One thing worth considering Patta, is the effect that sunflower oil is having on the N.A. of your entire optical system. With a maximum refractive index of 1.473 but likely less , your objective for starters is no longer a 1.4 N.A., plus the glass to glass homogeneity between the upper condenser element and slide is being broken, reducing the angle and N.A. You are working with lower angles than if homogeneity was intact. Maybe you should calculate this out and see what the angles actually are? Maybe closer to 1.25 for the objective?

I have a Reichert glycerin immersion 63X objective, which I occasionally used to use for DF along with an oil 100X. Sometimes, I have absent mindedly dipped the oil objective into the glycerin before I cleaned off the slide with a resultant dramatic, reduction in resolution. Glycerin has pretty close to the same N.A. as sunflower oil.

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Re: Testing high NA darkfield condenser

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#25

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by Chas » Thu Feb 17, 2022 8:33 pm

I have only just noticed this... but Watson's immersion Paraboloid was sold with a "supply of suitable slips"

And the thickness of the slides "...must be within 20% of thickness engraved on each of each of the paraboloid mountings"

And they werent very generous with the slides ! :

https://collection.sciencemuseumgroup.o ... condensers

(that one says 1.10 mm on the side)

This one says 1.6 mm on the side:

https://www.microscope-antiques.com/HIP.html

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Re: Testing high NA darkfield condenser

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#26

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by apochronaut » Thu Feb 17, 2022 9:09 pm

The principle of DF requires that the rays entering the slide do not pass through the upper surface, they reflect off of it and undergo TIR in the slide. Essentially the slide becomes like a fiber optic. If the slide thickness is too thick or thin, then one gets a partial DF with an uneven background and if the thickness is too far off the field brightens. Most DF condensers are quite specitic regarding slide thickness , with some stamping the specifications right on the condenser.

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Re: Testing high NA darkfield condenser

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#27

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by patta » Thu Feb 17, 2022 10:31 pm

On both tables above there is the word "aplanatic"; and also in the Nelson's obituary! So should be important for high NA, even if the illumination is just a thin ring. At NA 1.4 and 1.5 the angles are so shallow, aberrations likely get gigantic. So the condenser needs to be well made and relatively sharp, like a high NA objective - with its shallow focus depth, hence tight slide thickness. Two mirror surfaces are needed to get aplanatic, one isn't enough.

Below an illustration of how I think the current condenser get around this issue, with cheats:

-The mirror ain't conic nor parabolic, but is rough and wobbly, so it actually diffuses the light in many directions, instead of focusing it;

- The baffle stops that determine the aperture are after the mirror, so final NA isn't affected by aberrations (assuming that the glass half-ball has no significant effects)

So in practice this condenser is working as a "diffuse ring light + thin annular slits"; very inefficient in terms of light use, but no worries about aberrations. For illumination it needed 6 Watts LED at full power, which in brightfield would be "scorching sun".

Yes I'll get asap a gallon of proper immersion oil and test again (more rational choice than getting a refractometer for vegetable oil); extra perk, the runny sunflower oil was giving much less bubbles than the thick "B" imm oil.

Working NA, the "apertometer" used (photos in post #1 &2 of this thread; it is a prism from binocular) it's measuring the angle at which light cone comes out, after slide and oil layers; at the time I've had measured its RI, and checked it against objectives, it was working ok; so the aperture shouldn't be too much off; however that's also true that the NA of the condenser came out always higher than designed...?

I've stopped working on this condenser, will just use it and test a bit more. Well, ideas about modifications always welcome.

Thanks everybody for the historical contributions, now I know much more about condensers than two weeks ago. Hopefully some manufacturer will start again making the "Nelson Cassegrain"!

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Condenser &quot;raytracing&quot;

Condenser "raytracing"

Condensatore_talmente_baracco_che_funziona.jpg (96.66 KiB) Viewed 12493 times

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Prism apertometer profile

Prism apertometer profile

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Re: Testing high NA darkfield condenser

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#28

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by Chas » Sat Feb 19, 2022 1:27 pm

This was the Mk.I soft drinks can Cobb slide holder:

drink can cobb holder.jpg

drink can cobb holder.jpg (65.51 KiB) Viewed 12395 times

-It has a 3/4 inch punch hole in the middle and and 'half a hole' puched each side of it.

The carboard ends do help some mechical stages to 'get a grip'.

Mistakes (apart from trying to flatten the holder whilst the coverslips were inside) :

1] The slightly jagged edges can break the corners off the slips, so a more continuous scissor cut after getting the size about right, would be an improvement.

2] The can material was just a bit to springy, I think I should have attempted to anneal it first.

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Re: Testing high NA darkfield condenser

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#29

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by patta » Thu Mar 03, 2022 1:43 pm

Apart coverslip holders annealing (and maybe a compressor..)

I'm working a bit more on the condenser, will make the base to fit other stands.

Could somebody help me with the dimensions? I have only the ancient Swift and the Leitz dovetail, and can't find reliable data on the sizes of other makers.

Need Fit Diameter and the approximate height of condensers, as drawing below;

The more the better; at least the "standard 37mm"; maybe also other more sophisticate fittings like dovetails

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Condenser_fit_sizes.jpg

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Re: Testing high NA darkfield condenser

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#30

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by Hobbyst46 » Thu Mar 03, 2022 4:50 pm

Does "Standard" refer to Zeiss? if so, I can provide some dimensions later this evening.

Re: Testing high NA darkfield condenser

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#31

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by apochronaut » Thu Mar 03, 2022 4:53 pm

AO had two, that you would need. 1945-1980 and 1980-about 2002. Ditto, Reichert, Buffalo. There would slso be Reichert, Austria. Bausch & Lomb Balplan is another to consider but it has a unique arrow shaped registration arm. Probably not difficult to do once you have a sample.

I can send you those plus PZO, Baker early next week.

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Re: Testing high NA darkfield condenser

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#32

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by patta » Thu Mar 03, 2022 5:42 pm

Thanks, no rush, I was hoping that there is just "one" most common condenser mount... instead there is one "Standard", another "Universal", and many more :lol:

Any model is good; it is just to make the 3D design ready for future generations; it doesn't take much time for me to draw a couple more dovetails.

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Re: Testing high NA darkfield condenser

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#33

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by Hobbyst46 » Sat Mar 05, 2022 10:57 am

Here is a drawing. Measured with calipers.The 4mm thickness of the trapezoidal section of the dovetail is an estimate.

It fits my Standard GFL.

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Zeiss Ultracondenser darkfield drawing.jpg

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Re: Testing high NA darkfield condenser

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#34

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by Chas » Sun Mar 06, 2022 2:23 pm

The Swift's 38.7mm is I am pretty sure, the standard 'English' mounting size (I dont know if its exactly 38.7 ..but I reckon it is an RMS standard ).

EDIT: Or could the 38.7 mm be 38.6mm ? - A CTS catalogue says that the outside diameter of their condenser mounts is 38.6 mm.

???

Edit: from Stephanides (1947) " The Microscope and the practical principles of observation " A bit more exacting than my wobbly calipers :-) :

Condenser fit from Stephanides.jpg

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Re: Testing high NA darkfield condenser

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#35

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by patta » Wed Mar 16, 2022 4:43 pm

with some procrastination, I've added the add-on Zeiss dovetail - the worst and fastest design.... must be glued on the bottom of the "RMS" condenser

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Zeiss_add_on_1.jpg

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Re: Testing high NA darkfield condenser

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#36

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by Hobbyst46 » Wed Mar 16, 2022 6:44 pm

Looks nice. What is needed to fabricate a real working darkfield condenser ? will it be dry or immersion (the Ultracondenser is immersion type) ?

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Re: Testing high NA darkfield condenser

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#37

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by patta » Wed Mar 23, 2022 4:53 pm

Hobbyst46 wrote: ↑

Wed Mar 16, 2022 6:44 pm

Looks nice. What is needed to fabricate a real working darkfield condenser ? will it be dry or immersion (the Ultracondenser is immersion type) ?

It is oil immersion (or also water for those with NA<1.3)

It needs a 3D printer, some aluminium foil, a glass half ball as optics, a round coverslip and some mountant.

I'll publish the detailed assembly instruction later

Reverse-engineered also the Leica (DM) condenser dovetail from photos on internet - turns out to be

of same size as the much older Leitz ones!

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WIN_20220323_17_48_03_Pro.jpg

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Re: Testing high NA darkfield condenser

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#38

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by blekenbleu » Sat May 28, 2022 9:32 pm

AO 1094A "flip-top" condenser:

Max dovetail diameter is 1-7/8" (47.3mm), 7.8mm [dovetail + (45mm) cylinder] depth to flange.

Flange to lens top height is about 1.55" (39.5mm), for a total height of 7.8+39.8 ~ 47.6mm.

AO1094A.jpg

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Metaphot, Optiphot 1, 66, 150; AO 10, 120, EPIStar, Cycloptic

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Re: Testing high NA darkfield condenser

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#39

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by patta » Wed Jan 25, 2023 7:38 am

AO dovetail coming...

I've taken back this project, and, dismal discovery, wasn't really NA 1.4 :oops:

One of the testing objectives - CARL ZEISS JENA 60x 1.40 - upon closer observation turned out to have NA 1.3-ish, not the stated 1.40.

I'm in good company about NA cheating...

If somebody has a calibrated Abbe Apertometer to loan...

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Re: Testing high NA darkfield condenser

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#40

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by BramHuntingNematodes » Thu Jan 26, 2023 2:03 am

You can make a thin slide act like a thicker one with a more viscous immersion oil applied to the bottom

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Re: Testing high NA darkfield condenser

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#41

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by apochronaut » Thu Jan 26, 2023 2:50 am

Two questions I still have about this condenser is regarding it's focus and chromatism?

In every other DF condenser I have come across, great lengths were taken to pass the light through using finely polished mirrors and glass components, whether lenses or blocks. In this condenser the reflecting surface induces a lot of scatter. Why would other designers of condensers be so persnickety about ray scatter and the sillvering of mirrors etc., and at great cost, if those details were not important?

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Re: Testing high NA darkfield condenser

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#42

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by patta » Thu Jan 26, 2023 8:00 am

apochronaut wrote: ↑

Thu Jan 26, 2023 2:50 am

Two questions I still have about this condenser is regarding it's focus and chromatism?

In every other DF condenser I have come across, great lengths were taken to pass the light through using finely polished mirrors and glass components, whether lenses or blocks. In this condenser the reflecting surface induces a lot of scatter. Why would other designers of condensers be so persnickety about ray scatter and the sillvering of mirrors etc., and at great cost, if those details were not important?

I think here it gets away with polished mirror because "the center stop is after the mirror":

A Cardioid has a stop patch before the mirror, so the mirror needs to direct the light truthfully, no scatter, no aberration, or it will result in haze

Here the annulus / baffles are after the mirror, so light rays out of trajectory are stopped anyway. See post #27 above. There is a substantial waste in light available, that is not efficiently collected, but who cares we have powerful LEDs that consume few watts.

That's what I could manufacture; if I had Mr. Huygens Optics at my service, sure would prefer finely polished mirrors:

https://www.youtube.com/watch?v=A0bysBIj0FA&t=9s

I do believe that standard designs (cardioid etc) can be made to higher NA; just was found inconvenient because of haze, light scatter, or low working distance (drawing in post #13). I'm a high NA freak but objectively, the gain in resolution between 1.25 and 1.4 is quite risible, while cost and hassle skyrocket. I tried to stop down high NA apo objectives, in particular a dry 40x 0.95 stopped to NA 0.65, and results are really clean - and better than an equivalent achromat. I wish all apochromat ad an adjustable iris! It may be that (slightly higher) NA is mostly a marketing hype, to motivate a 10x price increase. Would you pay 10x more for an objective that looks identical to an achromat, and has also identical specs 100x 1.25?

Chromatic aberration, the main reason are the mirrors - mirrors don't have chromatic aberration, and are mirrors that do the "heavy job" of bending the light almost 90 degs in this condenser and cardioids.

Yes there is also some refraction but not at heavy angles like in the Abbe condenser. See the designs at post #19, the light rays always enter/exit the glass almost at right angle (apart from the top, that it is oiled so no refraction).

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Re: Testing high NA darkfield condenser

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#43

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by apochronaut » Thu Jan 26, 2023 10:26 am

DF oil immersion condensers have a maximum and minimum aperture, let's say 1.4 and 1.2 for instance.It is impossible to achieve DF if the objective aperture is above 1.2 in and in practice scatter precludes DF at 1.2 in such a condenser rendering a not so dark field : blue, gray, brown or even silvery., depending on the hue of the incoming light but other pollution in the scatter as well, reflection off of surfaces etc. In order to achieve true DF, which has a black background the objective then must be stopped down somewhat below the minimum aperture of the condenser or to about 1 in the case above. It is impossible to achieve DF at an objective's full aperture unless the condenser has a minimum aperture , higher than the objective by some margin.

How is does your condenser differ in this regard. You mention that your background is blue?

Last edited by apochronaut on Fri Jan 27, 2023 1:10 am, edited 1 time in total.

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Re: Testing high NA darkfield condenser

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#44

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by patta » Thu Jan 26, 2023 11:20 am

My condenser ain't anything special, and don't think it is better than a normal cardioid.

It is what I could make quick by myself with the materials at hand (half-ball lenses) instead of getting gouged on ebay for some antique Ultracondenser.

Perk is that it can be made with very high NA, the last photo in post #2 is an "oil condenser apertometer" (handmade, of dubious calibration) that indicates the hollow light cone has max aperture ~1.45 and minimum ~1.40.

The model tested gives DF with all oil objectives I have - but the one with largest aperture, 1.40, turned out to be a cheat, with actual NA more like 1.30. So let's assume that DF worked up till 1.30. Geometrically it seems possible to get it higher by tweaking the design. The main tradeoff are working distance (thickness of the slide) and diameter of the top (it needs to be larger and won't fit in the stage hole).

Yes the DF isn't perfect and there is some bluish background light; on camera looks black but by eye it is not. It happens also at "low" NA, so it must be some haze or scatter, maybe in the condenser, or maybe in the objective and other optics.

This background light got reduced a lot (but not disappeared) when I've blackened one in the condenser surfaces, right under the specimen - the background light was coming from the specimen itself, that glows light in all directions, then this light is reflected again upwards unless care is taken to absorb it.

I liked a lot oil with DF because it seems to get much darker; I think it is because oil reduces all scatter from a dirty slide and the dusty and scratched top condenser lens.

Maybe I should improve my marketing skills and just boast wonder features - instead of going into those dirty details? :lol:

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Re: Testing high NA darkfield condenser

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#45

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by apochronaut » Thu Jan 26, 2023 12:11 pm

Not all Oil DF condensers are expensive.

AO 214F ( probably bispheric) are around 100.00, sometimes less. Lots of them around. 37mm sleeve , normally carrying an AO chrome on brass series 15/35 - series 20 (35 year span) dovetail. The dovetail is removeable in the time it takes to find a 2mm flat screwdriver by loosening 2 set screws. Any other dovetail needed could be printed ,slipped over the sleeve and attached the same way. The optical section is threaded in and can be adjusted over about a 10mm range for a precise adjustment to the stage.

The previous AO/Spencer condenser has a different size sleeve and came with various dovetails to accomodate various microscope models going back to the first w.w. The optics are the same with the same 10mm adjustment, just that the older condenser is a little physically shorter, about 10mm.

Both have polished silver mirrors. The older condensor is often very cheap. I sold one recently with only a restriction on the height adjustment for 40.00.

Both will cover the field of a 25X objective and up with a 20mm 10X field.

With very tall substages, the condenser body can be mounted on top of the dovetail.

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Re: Testing high NA darkfield condenser

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#46

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by BramHuntingNematodes » Thu Jan 26, 2023 10:48 pm

I think you might have the difference in NA between achromat and more highly corrected objectives backwards. With a 100x objective design you would start at NA 1.4, then drop it down after you realize the design of an achromat will throttle the color resolution anyway and you might as.well get a little bit more working distance

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Re: Testing high NA darkfield condenser

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#47

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by BramHuntingNematodes » Thu Jan 26, 2023 10:50 pm

Also you can use a funnel stop on lots of high na objectives without an iris, works well even with paraboloid condensers which can be had by the bushel used.

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Re: Testing high NA darkfield condenser

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#48

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by apochronaut » Fri Jan 27, 2023 1:22 am

If you are referring to my post, there was a typo I fixed. It confuscated everything so as to confusedly confuse the unconfused.

With higher N.A. objectives that have optics filling the barrel, which are most these days, a funnel stop is even simpler because it can take tthe form of a washer on the rear diaphragm, just slightly smaller than the rear diaphragm.

I have a Reichert 63X 1.0 N.A. planachro glycerin. It doesn't quite give DF with the AO cardioid condenser ; kind of a gray field but the thread in rear baffle/diaphragm from the 1.25 planachro has the same thread and a just slightly smaller hole. The 1.25 has an iris, so no loss and it's diaphragm works perfectly as a funnel stop.

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Re: Testing high NA darkfield condenser

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#49

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by BramHuntingNematodes » Fri Jan 27, 2023 4:28 am

Oh I was speaking to pattas earlier post about the expense of app objectives along with the wish about all app objectives having irises. Also, hello what's this about a coke can Cobb slide? Now that's a diy project with some promise...

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Re: Testing high NA darkfield condenser

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#50

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by BramHuntingNematodes » Fri Jan 27, 2023 4:30 am

And thank you, apochronaut. I have been meaning to test a triple alliance df condenser to see if it is any different than the later ones and this trick may come in handy.

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Re: Testing high NA darkfield condenser

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#51

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by apochronaut » Fri Jan 27, 2023 7:35 pm

BramHuntingNematodes wrote: ↑

Fri Jan 27, 2023 4:28 am

Oh I was speaking to pattas earlier post about the expense of app objectives along with the wish about all app objectives having irises. Also, hello what's this about a coke can Cobb slide? Now that's a diy project with some promise...

When the Reichert Univar came out and continued with the Polyvar, all objectives with an N.A. above .75 were built with an integrated iris diaphragm. Each objective could be used for all contrast trchniques, even phase because the phase annuli were placed in a separate console above the nosepiece.

With later models, such as the Polylite, they departed from that, no doubt due to cost. That is probably why my 63X 1.0 N.A. immersion objective has no iris.

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Re: Testing high NA darkfield condenser

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#52

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by patta » Sat Jan 28, 2023 10:39 am

About funnels, I take the bait to advertise one of my earlier DYI - 3D print creations

https://www.microbehunter.com/micro ... 8&t=14262

Sure brass would be better than plastic, but now I have funnels for almost all objectives.

The centre stop/inverted darkfield/dispersion staining funnel turned out to be a messy diffraction crap.

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Re: Testing high NA darkfield condenser

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#53

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by Chas » Sun Jan 29, 2023 4:10 pm

About funnels, I take the bait to advertise one of my earlier DYI - 3D print creations

I dont kmow if this is still the case, but when Beck made their pretty extreme (focusing) condenser they sold a 1.25 N.A objective for it and commented that in order to obtain a dark field using a stop placed behind

an ordinary objective's lenses, the stop needs to cut the aperture down to about 1.05. So maybe keep your condenser and leave the stops in the draw ;-)

Last edited by Chas on Sun Jan 29, 2023 9:56 pm, edited 1 time in total.

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Re: Testing high NA darkfield condenser

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#54

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by MichaelG. » Sun Jan 29, 2023 8:22 pm

Chas wrote: ↑

Sat Feb 19, 2022 1:27 pm

This was the Mk.I soft drinks can Cobb slide holder:

.

Only just spotted this great project, Chas & patta … I will be following with interest now

Here’s a slick little demonstration of making a Cobb Slide : https://youtu.be/od5ARpqGmbw

Nice, if you have the right tools !

MichaelG.

Too many 'projects'

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Re: Testing high NA darkfield condenser

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#55

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by patta » Mon Jan 30, 2023 9:09 am

Voila, here too, making those Cobb slide from a beer can, sorry for disregarding the previous suggestion by chas, and thanks MichaelG for the video link!

the aluminium is a bit too thin, like 0.1mm, while the one used in the video linked above looks much thicker (0.5mm?) It is difficult/impossible to get them flat, with the coverslip sitting parallel.

Anyway, they're really quick and easy to make.

Slides_Cobb_can.jpg

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Last edited by patta on Mon Jan 30, 2023 3:27 pm, edited 1 time in total.

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apochronaut

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Re: Testing high NA darkfield condenser

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by apochronaut » Mon Jan 30, 2023 1:41 pm

The top and bottom of larger food cans , contain a flat section tn the middle that would be large enough and would not curl. The material is now, plated steel though and a little thicker., although still thin enough to cut with large scissors.

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User avatarpatta

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Re: Testing high NA darkfield condenser

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by patta » Thu Feb 02, 2023 3:00 pm

Too late, the 3D print bug hit again: Plastic dummy slide, coverslip holder 2.0

They're plastic, not so stiff, and they soak up water and oil. But sit flat and the slide can be put flush over the condenser or the objective.

Two model, the one on the photo need a dab of glue to fasten the coverslip.

I've loaded the models for 20mm round, 20mm square and customizable OpenSCAD:

https://www.thingiverse.com/thing:5822468

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WIN_20230130_14_57_18_Pro.jpg

WIN_20230130_14_57_18_Pro.jpg (138.43 KiB) Viewed 9878 times

WIN_20230130_15_32_27_Pro.jpg

WIN_20230130_15_32_27_Pro.jpg (150.66 KiB) Viewed 9878 times

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MichaelG.

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Location: North Wales

Re: Testing high NA darkfield condenser

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by MichaelG. » Thu Feb 02, 2023 7:32 pm

That’s neat !

MichaelG.

Too many 'projects'

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Chas

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Joined: Tue Jan 19, 2021 3:11 pm

Re: Testing high NA darkfield condenser

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#59

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by Chas » Thu Feb 02, 2023 9:30 pm

Thanks. Downloading something pre-designed is a real breeze !

:-)

(It goes without saying that after printing I discovered that I have every other shaped coverslip but the one that I need ).

A question; what are two round holes on the 20x20mm slide for ?

Here is a link to Cobb's description of his slide (and a lot of curious nematode stuff):

https://www.biodiversitylibrary.org/pag ... 6/mode/1up

Maybe there is some sense in using two different coverlip sizes/shapes, as he did, especially if one is thinking of holding one to the holder with glue.

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