Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

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Hobbyst46
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Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#1 Post by Hobbyst46 » Fri Jun 21, 2019 10:27 pm

For a long time I produced darkfield images with LED rings and sideways illumination. Later on, I began using the darkfield port of my turret Zeiss condenser, nr. 465277. The Zeiss manuals (from about 1950, ?) claim that its performance is the same as the "Ultracondenser" dedicated darkfield.
In practice, the "D" condenser position indeed provides great DF, if oiled to the bottom side of the slide. For NAs up to about 0.8. So far so good.

Today, a dear friend presented me a gift - a Zeiss (West) Ultracondenser. "Carl Zeiss" mark on one side, 1.2/1.4 on the other side. The black paint enamel as well as the underside of the shining steel dovetail look great, no scratches. Looking from below, a perfectly reflective (mirror-like) small cap is visible.
I guess that this beauty was lying unused in a drawer, for years. These condensers are quite rare !

The disappointment appeared when I placed it into the condenser holder (perfect fit), raised, oiled to the diatom slide. The light beam appears to be OK, as far as I can tell. With the 40X0.8 Planapo (oil) objective, DF occurs, but not as contrasty as with the turret condenser. With the 63X1.25 Neofluar (oil, Ph3) objective, no DF at all. The whole field is very bright, and the diatoms are even less visible than in BF. I was hoping for DF with the 100X1.3 objective !!

I remember the lengthy posts about DF in the past, especially @Apochronaut (and others) explained that the NA of the objective must be significantly less than the NA of the condenser. Still, if not DF with 1.3, then at least with NA 1.25, or NA 1 (40X1 oil objective with iris open). It does not happen.

So I inspected more carefully the Ultracondenser under the stereoscope and noticed that the conical surface below the top glass surface is not uniformly coated with paint (see photo below). Rather, it appears that some irregular peeling had occured. This might have been caused by age, but more likely IMO is that the original owner applied aggressive cleaning to this front. Perhapps some antique immersion oil dried and had to be removed at all price.

Whatever the reason, could it be that the initially perfectly annular beam has become irregularly wider and negates the DF effect ?

Or alternatively, that I am expecting too much from that gadget (sorry, Zeiss, for the term)?

And if so, should I attempt to coat this conical surface with black paint ? of a specific type ?
All comments are welcome !
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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#2 Post by MichaelG. » Fri Jun 21, 2019 10:54 pm

I would certainly think it needs re-painting ... but, sorry I have no idea what paint would be best.

To confirm whether it's neccessary; maybe make a little 'hat' of black velvet.

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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#3 Post by wporter » Sat Jun 22, 2019 12:25 am

I'm not sure if it makes a lot of difference what paint you use, but you might go to a hobby shop and get a small jar of flat black paint that modelers use on model planes and trains.

Clean the surface with acetone, and stroke gently with a modeler's paintbrush upwards along the outside of the frustum towards the flat end. If you are precise, you will not get any on the flat. If you do, then after it dries, scrape the rim of the flat (gently!) with the edge of a plastic blade to remove any excess (almost zero pressure!), or wipe sideways across the flat gently with an acetone-moistened Q-tip (barely damp), so as not to remove any of your new paint.

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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#4 Post by MicroBob » Sat Jun 22, 2019 12:52 am

Hi Doron,
congratulations to your new rare dark field condenser!
I would expect it to deliver true dark field up to objective aperture of 1,0 or a bit more, after perfect adjustment. The 100:1 objectives were used with an iris to be used with dark field. Since you are already on the border of what is possible the condenser height, centering, light path and slide thickness have to fit just right. This condenser might show up a misalignment, that is not visible in bright field use.
It is very likely that the missing paint contributes to the problem. What I'm not sure is: Is the paint there to block light, to absorbe light from below, from above or does it play a role in the reflecting of light?
When it comes to the choice of paint I don't think that matte black paint reflects back into the glass better than ordinary black paint, but it absorbes stray light from above better.

When intentionally set wrong the condenser could be made to deliver oblique or circular oblique lighting.

Bob

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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#5 Post by zzffnn » Sat Jun 22, 2019 3:22 am

Hobbyst46,

I don't think you should expect good darkfield at NA 1.25, which is theoretically impossible for a 1.2/1.4 darkfield condenser (NA 1.2-1.4 is where you will have the illumination ring). You may see good to decent darkfield at objective NA 0.8, depending on subject/background and alignment.

But NA 0.8-0.85 is about the maximum for most 1.2/1.4 spec'ed darkfield condensers. Above that, you need an iris or funnel stop to reduce aperture.
I know theory says you should be able to get darkfield close to NA 1.1, but light spillage is often prominent in reality and prevents good darkfield even at NA 0.9.

To even get close to NA 0.8, you should have perfect condenser centration and good height (usually too high is better than too low).

I have used darkfield quite a bit with various NAs, subjects and different condensers (LOMO/old Zeiss clone cardioid, American Optical Spencer Cardioid, Bauch and Lomb Paraboloid and Leitz Heine). Only the Leitz Heine can provide darkfield at objective NA of over 0.95 (but remember, Leitz Heine is a very rare zooming mirror condenser and has an usually narrow illumination cone of around 1.3-1.4, based on what I have experienced). My Leitz Heine did ok with some subjects at objective NA of 1.1, but not at NA 1.23 (background was not dark with NA 1.23).

The my other 3 condensers all required objective NA to be stopped down to NA 0.8 or so, depending on the subject (thick subjects and busy dense background tend to require more stopping down).

I don't have NA 0.8 or 0.85 objective right now. But I did try NA 1.1 with or without funnel stop, NA 0.95 with or without back disc stop, NA 0.6-0.9 iris and lots of NA 0.65. NA 0.8-0.85 is what you shoot for when stopping down.

Do you know if your Zeiss "ultracondenser" is any different than other wide field cardioid condensers?

I am not sure that outside paint peeling matters that much, but any black paint that cannot be affected by immersion oil will work as repair.

You should really carefully examine the inside relfecting mirror surfaces though. Most darkfield condenser can be unscrewed from it's base to reveal it's inside better.

With objective NA of 1.3 and 1.2/1.4 darkfield condenser oiled to slide and nicely aligned, you will get good circular oblique light, which can resolve dot details from thin diatoms VERY well.

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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#6 Post by 75RR » Sat Jun 22, 2019 6:31 am

I picked one up a while ago on an impulse buy. Admit I have not done much with it as yet.

Looking forward to your results

Here is a what the Zeiss Optical System brochure has to say on the dedicated condensers and on the carousel darkfield condenser VZ 46 52 77 (I have an older version that just has a serial number )
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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#7 Post by MicroBob » Sat Jun 22, 2019 6:49 am

Hit together,
here I posted two images with objective aperture 1.0, taken with a Zeiss Jena pancratic condenser and dark field head swung in: viewtopic.php?f=28&t=5786#p51846. The n.a. range of this condenser is unknown to me, but it seems to start way up the range.
There is a german Wikipedia page on the "Ultramikroskop": https://de.wikipedia.org/wiki/Ultramikroskop.
Basically "ultra" is used as this illumination technique shows up detail that can't normally be resolved with a light microscope.
As far as I know dark field was used a lot in bacteria research until phase contrast became widely available. This probably explaine why this Zeiss West dark field condenser is seldomly found: The Zeiss Standard system started in the late 40s when phase contrast was already available.

Bob

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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#8 Post by zzffnn » Sat Jun 22, 2019 7:08 am

Bob,

Your Zeiss pancratic condenser is not a typical darkfield condenser. Its zoom mechanism is similar to my Leitz Heine, so it could have a higher NA range, like my Leitz Heine. The 3rd zoom condenser that I know of is the rare Reichert Polyphos condenser (supplied with some Zetopans).

Remember those zoom condensers were mainly designed for variable phase contrast, and phase contrast illumination rings are typically not that wide (could be even narrower, like 1.3-1.4, than the typical darkfield range if 1.2-1.4).

Mounted diatoms are also easier to image in dakrfield than a live ciliate like paramecium (I had to stopped down more on objective iris for paramecium in dakrfield). Your second diatom could gain even more contrast (in between frustules) by stopping down objective aperture; I have seen it many times in darkfield.

75RR,

Try to achieve objective NA of 1.0 in darkfield with a non-zoom darkfield condenser and you will see how easy it is in reality :mrgreen: Sometimes the pros achieve the ideal results by using lead containing mounting medium and iodine containing immersion oil; we mortal hobbyists don't usually have access to those reagents).

But why even bother trying it, as your DIC can produce better (halo-free) dark background without stopping down.

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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#9 Post by 75RR » Sat Jun 22, 2019 7:26 am

David Walker in this article: http://www.microscopy-uk.org.uk/mag/ind ... -bpae.html

mentions that he got a better Darkfield from the Ultra condenser than he did with the Darkfield 'D' condenser setting on the carousel Phase condenser.
But why even bother trying it, as your DIC can produce better (halo-free) dark background without stopping down.
They do say that one tends to abandon the other illumination techniques once one has DIC, and my experience so far is that that is the case.

On the other hand each illumination technique has something to offer - so it would be a shame if it were always true.
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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#10 Post by MicroBob » Sat Jun 22, 2019 7:52 am

Hi 75RR,
thank you for posting the Zeiss documents! It is interesting to see that the achromatic-aplanatic revolver condenser and the ultra condenser are stated with the same upper objective n.a. limit on 1.0. The revolver condenser is the high end model, probably was quite expensive, but still it can help to explain why there a so few untra condensers on the market.

Bob

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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#11 Post by Hobbyst46 » Sat Jun 22, 2019 10:19 am

Thanks all, for the rapid, kind and highly informative responses! and the link (thanks 75RR) ! and the hard data (thanks zzffnn) all are much appreciated !

I Looked again at the schemes and specs of Cardiodids, described for Zeiss or B&L.
I believe that the Zeiss Cardiodid is similar to others, but am not sure.

So I had been over-optimistic...ignored the printed specs NA=0.75...1.0. Well, I do not own iris objectives other than the 40X1 Planapo (oil), nor funnel objectives.

Yet, there are reasons to continue with it:
1. Such rare (now I understand why), (and costly) plaything should be functional.
2. Oblique beam for high NA would still be a virtue
3. Darkfield fluorescence. The real world has long ago shifted to epi-fluorescence, but DF-FL could be great fun ! an opportunity to check both the DF level and the light source intensity !

So the frustum coating should be mended. Schemes of cardiodids suggest that the main (or only) function of the black coating on the frustum is to prevent the projected light beam from exceeding the annulus size. Seemingly it is not a reflecting coating (reflecting the rays inside, and opaque on the outside), since light rays are traced as going parallel to the frustum on their way out to the specimen slide.

I thought at first to apply a reflective coating to the frustum, I know how to silver glass and make a mirror, but its seems too much of a challenge.
So, I will try black paint, perhaps it will help some. Carefully of course. Need to verify that the paint is compatible with immersion oil.

The Ultracondenser can be dismantled, I will post a view of the parts later on.
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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#12 Post by 75RR » Sat Jun 22, 2019 10:35 am

I suppose then that this is Condenser Holder 'Z' rather than 'S' ?

It would seem then that it was possible to buy just the Darkfield Condenser and insert it in either the 'Z' or 'S' holder.

There are probably some of these about, with resellers particularly not quite sure what they are. Be nice to have a set.
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Last edited by 75RR on Sat Jun 22, 2019 6:24 pm, edited 1 time in total.
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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#13 Post by Hobbyst46 » Sat Jun 22, 2019 12:56 pm

75RR wrote:I suppose then that this is Condenser Holder 'Z' rather than 'S' ?.
I suppose so, since it has a dovetail that precisely fits the centrable condenser carrier. The same carrier that I use for the Abbe or the turret phase contrast condenser. My Ultracondenser does not bear any Zeiss cat nrs, except the serial nr, it is an older product.
Here are the components. Upon taking it apart, it is clear that immersion oil can penetrate inside between the exposed tip and the outer frustum. Probably it is not important.
Ultracondenser components.jpg
Ultracondenser components.jpg (27.39 KiB) Viewed 3390 times
The ring (2) holds 1 and 3 together. Part 3 is simply inserted, friction-fitting.
I believe that part 1 is what Zeiss calls "holder".
The bottom of part 1 is the dovetail that fits the carrier.
In part 3, the mirror is evident.
Part 3 decomposes into a housing (4), retaining ring (5) and the "heart" (6; a cardiodid must have a heart).
The bottom panel shows the bottom side of part 6, not the frustum surface. It shows an irregular pattern of light transmission, due to peeled coating.
Part 6 is a sealed unit, cannot be disassembled. I am not sure that the internal reflective surfaces are perfect, but clearly the conical surface coating is damaged. Although the outer part of the conical surface of part 6 is effectively protected under the conical surface of part 4, so ... :? :?

Furthermore, the phase condenser does not have a Cardiodid element - only a circular light stop for darkfield, but, there are the lenses. Whereas the Ultracondenser has no lenses.
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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#14 Post by Hobbyst46 » Sat Jun 22, 2019 1:26 pm

Note: I forgot to mention the role of the flip-out auxiliary lens that resides under the condenser. The lens is part of the condenser carrier. For DF, I always try to check if the effects are better with or without that lens.
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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#15 Post by wporter » Sat Jun 22, 2019 2:00 pm

So, I will try black paint, perhaps it will help some. Carefully of course. Need to verify that the paint is compatible with immersion oil.
Good point; I now retract my suggestion to use hobby paint, unless it claims to be oil-resistant. Radio-controlled airplane paint might work, if for use on planes that use non-electric engines. Or automobile engine enamel (usualy aerosol but you could spray some on a brush or Q-tip to apply).

(This is a very interesting discussion, regarding darkfield condensers.)

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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#16 Post by 75RR » Sat Jun 22, 2019 2:01 pm

Note: I forgot to mention the role of the flip-out auxiliary lens that resides under the condenser. The lens is part of the condenser carrier. For DF, I always try to check if the effects are better with or without that lens.
I always assumed that the auxiliary condenser existed to help low power objectives fill the field of view or at least cover most of it while permitting Köhler. The alternative would be to remove the top condenser lens and use critical illumination.
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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#17 Post by Hobbyst46 » Sat Jun 22, 2019 2:39 pm

75RR wrote:
Note: I forgot to mention the role of the flip-out auxiliary lens that resides under the condenser. The lens is part of the condenser carrier. For DF, I always try to check if the effects are better with or without that lens.
I always assumed that the auxiliary condenser existed to help low power objectives fill the field of view or at least cover most of it while permitting Köhler. The alternative would be to remove the top condenser lens and use critical illumination.
Yes, I do the same, and I prefer to leave the top condenser lens on all the time, since unscrewing it requires removal of the condenser from the carrier. For DF, when the condenser is oiled to the slide, and depending on the objective, I also try with and without the flip-out lens.
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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#18 Post by Hobbyst46 » Sat Jun 22, 2019 2:42 pm

wporter wrote:...Radio-controlled airplane paint might work, if for use on planes that use non-electric engines. Or automobile engine enamel (usualy aerosol but you could spray some on a brush or Q-tip to apply).
(This is a very interesting discussion, regarding darkfield condensers.)
Thanks wporter. I hope to do something about it and report back (only if positive...).
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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#19 Post by Hobbyst46 » Sat Jun 22, 2019 8:25 pm

zzffnn wrote:...My 40w LED barely provide enough light at that magnification/NA. NA 1.1 actually helped by gathering more light
Hi zzffnn, this is a citation from your interesting post about darkfield in Photomacrography.net, back in 2016. Now that raises a question. You have been using a 40W LED on (which ?) Zeiss microscope ? So perhaps the limiting factor for me when going for NA=1 DF (forgetting the previous unrealistic ambitions) will be the light intensity ?
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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#20 Post by MicroBob » Sat Jun 22, 2019 8:46 pm

Hi Doron,
I clean of my immersion oil with lighter fluid. This is a fairly mild solvent that attacks few paints so you could take basically anything.
A real nice paint would be 2 component car paint. This gets really hard and resistant. It is available in spray cans too, but very expensive. For a completely differnt project I paid 23€ for a spray can, and after mixing the components one has only 24 hours to use it. But if you have a car boy shop around you might talk to them and obtain a bottle cap full.

Bob

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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#21 Post by Hobbyst46 » Sat Jun 22, 2019 9:57 pm

Thanks Bob,
I will get some paint and check its resistance to immersion oil.
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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#22 Post by zzffnn » Sat Jun 22, 2019 10:35 pm

Hobbyst46 wrote:
zzffnn wrote:...My 40w LED barely provide enough light at that magnification/NA. NA 1.1 actually helped by gathering more light
Hi zzffnn, this is a citation from your interesting post about darkfield in Photomacrography.net, back in 2016. Now that raises a question. You have been using a 40W LED on (which ?) Zeiss microscope ? So perhaps the limiting factor for me when going for NA=1 DF (forgetting the previous unrealistic ambitions) will be the light intensity ?
This is my original thread. Last post is relevant:
http://www.photomacrography.net/forum/v ... ring+light

My scope has been a Frankenstein scope based mainly on Nikon Optiphot. At the time I was using an adapted B&L paraboloid (not cardioid) darkfield condenser. It did not work too differently than a cardioid at high NA.

Limiting factor for going up on darkfield objective NA also include (in addition to light intensity):

1) background cleanness (cleaner background is easier);

2) sample thickness (if it is water thinner is better, if mounted diatom then inverted mounted onto cover slip is better);

3) sample thickness and internal structure (thin diatoms with somewhat hollow internals are much easier than a fat paramecium);

4) if your subject moves, because slow shutter speed cannot be used on moving subjects, but no problem with mounted diatoms, with which you can use say 2 seconds of exposure time).

1)-4) only produce difference, after darkfield operation has been optimized.

It is unusual to see cardioid darkfield producing worse result than a simple phase disc stop though. Was condenser centration and condenser height optimized? Err on getting the condenser too high than too low, if necessary. You may have to center the condenser to that specific objective, if your objectives are not perfectly parcentric.

In some cases, going up on objective NA from 0.85 to 0.9-1.1 in darkfield may not produce much image quality difference. Sometimes it would get worse, due to decreased contrast and halos.

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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#23 Post by Hobbyst46 » Sun Jun 23, 2019 6:04 pm

zzffnn wrote:...
Thanks zzffnn, and again, thanks all!

Since I am not yet clear about the resistance of black paint to immersion oil, as first aid, I covered the frustum with the "infamous black insulation tape" (for certain applications, it is fine). Light leakage is mostly blocked. I centered the condenser carefully (I believe I did that OK before, but who knows).

The specimen, as before, was a strew slide of diatoms. I had prepared the slide and mounted it inverted, namely, the diatoms are very near the coverslip.
The medium is NOA61 (it has a high nD of 1.55-1.60). I also have many mounted in Pleurax but did not inspect them yet.

Result: very good darkfield with the 40X(0.6-1.0) iris oil planapo objective! at fully open iris ! for NA=1.0 ! not NA=1.1, but my objective only reaches 1.0. The illumination was sufficiently bright.
I like to think that the improvement relative to the preliminary experiment is because I blocked stray light.

Here is a quick crude comparison. Single images, of a strew slide, where not all diatoms are in focus. Resized, but otherwise untouched.
I wanted to demonstrate a small difference between the condensers in the darkness of the background.
I found it easier to create a uniform darkfield across the FOV, by centration, with the Ultracondenser, than with the phase contrast condenser.
On the other hand , the phase contrast can create DF when used dry, with low power objectives.
Ultracondenser. 40X1.0 oil planapo..jpg
Ultracondenser. 40X1.0 oil planapo..jpg (176.86 KiB) Viewed 3233 times
[/i]
Phase contrast condenser position -D -, 40X1.0 oil planapo.jpg
Phase contrast condenser position -D -, 40X1.0 oil planapo.jpg (188.11 KiB) Viewed 3233 times
[/i]
DF is not as exquisite as DIC, but is funny and aesthetic.

What still did not work was oblique at higher NAs. Failed to observe it with both objectives objective, the 100X1.3 oil Planapo (Ph3) and the 63X1.25 oil Neofluar (Ph3) yielded a very bright image with poor contrast and resolution. This can be attributed to any of the factors listed above by zzffnn, thickness of slide or others, or perhaps because those objectives are phase contrast.
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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#24 Post by zzffnn » Sun Jun 23, 2019 9:11 pm

Congratulations, Hobbyst46, on getting NA 1.0 darkfield!

You said you 100x NA 1.3 phase objective could not produce good COL with ultracondenser;

Can you post a photo of the objective back focal plane (remove eyepiece, and position camera on empty eye tube)? It should look more like fig. 10 of this article (fig. 5 is also an example, but it does not looks as good as fig. 10 in terms of condenser focus, you want to see the central stop in sharp focus): http://www.microscopy-uk.org.uk/mag/ind ... c-col.html

You may have to adjust your condenser up and down slightly to focus it better for COL. Up, more likely, based on your described symptoms.

As for darkfield:
I forgot to mention that when I said, in that old thread, I did not have enough light for NA 1.1 darkfield from a 40w LED, I was testing on live ciliates in water (I could not use long exposure and at the time, I was probably not liberal enough with iso, now I go up to iso 1600 on a m4/3 camera sometimes; previously I don't like to go above iso 400).

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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#25 Post by Hobbyst46 » Sun Jun 23, 2019 9:25 pm

zzffnn wrote:Congratulations, Hobbyst46, on getting NA 1.0 darkfield!

You said you 100x NA 1.3 phase objective could not produce good COL with ultracondenser;

Can you post a photo of the objective back focal plane (remove eyepiece, and position camera on empty eye tube)? It should look more like fig. 10 of this article (fig. 5 is also an example, but it does not looks as good as fig. 10 in terms of condenser focus, you want to see the central stop in sharp focus): http://www.microscopy-uk.org.uk/mag/ind ... c-col.html
Thanks, I will try.
You may have to adjust your condenser up and down slightly to focus it better for COL. Up, more likely, based on your described symptoms.
I had tried that in vain.
As for darkfield:
I forgot to mention that when I said, in that old thread, I did not have enough light for NA 1.1 darkfield from a 40w LED, I was testing on live ciliates in water (I could not use long exposure and at the time, I was probably not liberal enough with iso, now I go up to iso 1600 on a m4/3 camera sometimes; previously I don't like to go above iso 400).
Thanks, that's important. I imaged totally static objects at iso 800, long exposures (>1/10)
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apochronaut
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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#26 Post by apochronaut » Sun Jun 23, 2019 11:36 pm

...a little late to this thread but here are the rules I follow when using a DF oil condenser. If you follow these, you will get good results. I get good results with DF always but I also frequently end up comparing what I find with DF, to various phase and BF techniques in order to get a complete picture of the details I am looking for. It is most useful at high magnification, providing resolution of specific details unachievable by most other contrast techniques; a quality that was observed about 100 years ago, so it became dubbed an ultra microscopy technique, since it utilizes light originating from very high N.A.s to achieve the ultra resolution. With acknowledgement to Fan and others who offered their practical experience to the discussion, so I am going to be redundant at times. I am just trying to put down a set of ground rules for it's use.

1) The condenser need not be of any one type, as long as it is an oil condenser. Of the various types around, cardioids and paraboloids are the most common. Cardioids are slightly superior at high N.A.s due to their mirror construction, so have less ca. Paraboloids use tir , so have a llittle more ca but they are typically more useful at lower magnifications.
Cardioids, usually 40X and up. Paraboloids, down to 20X.
2) all objectives must be capable of achieving an N.A. of around .2 less than the minimum N.A. that the DF cone achieves. That specification should be either on the condenser or be on a spec. sheet somewhere. You can get a darkened field at N.A.s closer then .2 below but it will only be darkened, not dark. Part of the ability of DF to achieve the separation of details lies in it's ability to establish high contrast, so achieving true DF , gives better imaging. Objectives of over .80 N.A. can use one of two methods to lower their N.A. ; an adjustable iris diaphragm that can either be added to or is integral to a modified version of the objective and a DF stop or mask, either purpose built for the objective or DIYed at the objective diaphragm. The stop, funnel stop or diaphragm restrictor must be precise. With the optimal N.A. restriction, there will be no loss in the objectives resolution capacity. As the N.A. reduces below the optimum, the resolution will suffer.
3) The condenser must be oiled to the bottom of the slide and whatever immersion spec. the objective being used carries must be adhered to.
4) The condenser must be in a centerable mount and must be centered. Centering a DF condenser can be a bit truicky so DF condensers are fairly easily centered at a magnification below that which they are used at; 10X is good. At this magnification the condenser will provide an illumination circle considerably smaller than the f.o.v. Center this spot of light. This is where the parcentering of your microscope is important. If you know how close your microscope is, using a 10X can work.with the higher power objectives too. If your microscope is not that precise, then you will have to follow the protocol for adjusting the condenser with each objective. Some have a circle marked on the top lens, some produce a cross hatch of light in the field at a certain point of focus and some produce a tiny point of light. All of these can be used with a higher magnification objective to center the condenser.
5) High wattage is required with objectives above .80 N.A. Typically, microscopes with a 100 watt illuminator are required for 100x DF because lowered illumination usually forces the user to open the iris diaphragm, thus impacting the contrast. Skimping on illumination is folly. That's one of the reasons that budget microscope companies claiming DF and offering an oil condenser and iris equipped objective along with a 3 watt led are a scam. Fan is correct; even with led, you need around 40 watts to get good contrasty DF.
5) Since no direct light is entering the objective in DF and all illumination points are reflective, dirt can become reflective too. Slides, coverlips and oil must be totally free of dirt, dust and air bubbles. They can cause considerable flare, even when outside the f.o.v. For this reason, I would cover that ground glass cone on your condenser, Hobbyist. I would try a black permanent marker, at least to determine if it is a problem or not.
6) A very slight blue filtration of the illumination source is best and any ground glass components in the illumination path have a tendency to reduce contrast somewhat.
7) Most condensers have an optimum range for the slide thickness. This will be either stamped on the condenser or in a spec. sheet. Choose a slide in the middle of that range. I keep special slides for DF and micrometer them all, marking the thickness with a fine marker in one corner.

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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#27 Post by Hobbyst46 » Mon Jun 24, 2019 9:52 am

Just a passing thought regarding diatoms and darkfield:

In theory, monochromatic, short wavelength light improves the resolution. Moreover, diatoms disperse light, like tiny prisms, and even when viewed with apochromate objectives, colorful nice halos appear, at the expense of resolution. Hence, viewing the diatoms under monochromatic violet or blue light might improve the observed resolution. On the other hand, highly bright illumination is required for DF. To isolate just the narrow band of violet from the spectrum, an interference filter is desired. The problem with this approach is the very low intensity of the isolated monochromatic light.
Potential solutions:

1. A very powerful LED, not white LED but violet or blue.
2. High-pressure mercury/Xenon lamp, plus interference filter.
3. Tunable laser.
4. Laser diode. If there is an appropriate wavelength.

However, for hobby use:

No 1 - exists; when ready-made off the shelf, complete with housing and power supply and cables and dimmers, they are costly. I think that the wattage is about 20W but not sure.
No 2 - exists, quite costly

For both No 1 and No 2: I wonder if anyone has used them for this purpose. After all, for research, there are more modern and better means to resolve diatom structures.

Nos 3 and 4 introduce several complications (beam width, price, even safety questions), not practical for hobby IMHO.

Notes:
1. these and similar questions might have been raised within the forum in past, so if anyone finds it overly chewn and nagging, please ignore.
2. An alternative and inexpensive approach is imaging with white light and post processing that extracts just one channel (color). not the same though
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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#28 Post by apochronaut » Mon Jun 24, 2019 12:04 pm

The benefit of DF is in being able to isolate structures that are difficult otherwise, due to the extreme N.A. achieved. I suppose that is possible with extreme N.A.s using other illumination sources but aside from COL most of these sources are as expensive or more expensive than DF and in most cases, one would need to acquire specialized objectives of very high correction.

It is true that refracted light is a problem with diatoms, hence my recommendation above that the use of a pale blue filter enhances the image quality and resolution. There isn't so much in the way of illumination loss.

I have tried led illumination as an alternative to lightly filtered halogen and the results have not been encouraging. This is for sure due to led illumination being in it's infancy in terms of it's application to microscopy, not in the nature of led illumination itself. It does seem quite adaptive to DF but curently, not practical. Retrofits are not generally an answer, since the optical engineering of the light pathway is deleterious to the correct functioning of the led source. Fan seems to have found a solution, with a 40 watt remote led.
Since used high grade 100 watt halogen systems can be had from several microscope manufacturers very cheaply ......one could realistically set up a 1000X DF system for 400-500.00, with a pretty short wait window, for me it doesn't seem practical to fiddle with led, hoping eventually to have a successful system, when the existing 100 watt halogen systems are very close to perfection.

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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#29 Post by 75RR » Mon Jun 24, 2019 1:09 pm

To isolate just the narrow band of violet from the spectrum, an interference filter is desired. The problem with this approach is the very low intensity of the isolated monochromatic light.
365nm UV provides spectacular results in Diatom imaging.

See links:

http://www.mikroskopie-ph.de/Frustulia-1-G.jpg

http://www.mikroskopie-ph.de/index-Diatom-3.html

Note: As always Ultraviolet light needs to be treated with extreme caution.
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Re: Zeiss Cardiodid condenser fails with high NAs - peeling paint ?

#30 Post by Hobbyst46 » Mon Jun 24, 2019 1:43 pm

75RR wrote:
To isolate just the narrow band of violet from the spectrum, an interference filter is desired. The problem with this approach is the very low intensity of the isolated monochromatic light.
365nm UV provides spectacular results in Diatom imaging.

See links:

http://www.mikroskopie-ph.de/Frustulia-1-G.jpg

http://www.mikroskopie-ph.de/index-Diatom-3.html

Note: As always Ultraviolet light needs to be treated with extreme caution.
Yes, P. Hubel's photos are first rate.
Those are brightfield photos, taken eith apo objectives and a UV-LED. The diatoms were mounted in Zrax that is transparent to UV. I think that Pleurax is not as transparent.
Anyway, as you say, UV is not for everyone, especially for home use.

I am still thinking of blue light. On dark blackground. As a future project. It will be almost certainly based on LED, at least because buying a used 100W (or even 60W) halogen microscope lamp is not practical for me, even if there is one that mechanically and optically fits my microscope. There are fiber optic halogen lamps, those are a practical option, if found at affordable price.
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