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PostPosted: Mon Jul 08, 2019 5:55 pm 
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Sauerkraut wrote:
This thread has inspired great curiosity about condensers. Perhaps this question is off-base, but how does a standard flip top condenser compare to something like this with a swing-out bottom lens? Do the 2 types effectively work the same way with different mechanical designs or are they fundamentally different from an optics standpoint?
To the best of my knowledge:

Low magnification objectives have a wider field of view than the high NA condensers can cover.

One way to completely illuminate the field of view of very low power objectives (say around 4x) is to reduce the NA of the condenser by removing the top lens by flipping or unscrewing it.

This typically leaves a condenser NA of about 32, which unfortunately can not be focused, leaving just the option of Critical illumination.


When the condenser is of a very high NA then even medium power objectives (10x) may not have their field of view fully illuminated.

This is where a swing out bottom lens can help to cover the field of view while maintaining the NA of the condenser.

It also has the advantage that the condenser can still be focused, allowing Köhler to be achieved.

http://www.microscopy-uk.org.uk/index.h ... /index.htm

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PostPosted: Mon Jul 08, 2019 6:06 pm 
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Hi Phil,
thank you for this thorough explanation! So to test different condensers it would be best to use a highly corrected objective and compare the corners of the image.
Did I understand it right that out of focus areas are also dependend on the condenser correction?

@Heather,
the swing in top lenses switch between e.g. numeric aperture 0,3 and 1,25.
The bottom lens is often used with the full condenser to widen the illuminated area. But this is not the same with all microscopes. For my Phomi 1 there is a table that exactly states which of the two lenses has to be used for which objective.


Bob


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PostPosted: Mon Jul 08, 2019 10:11 pm 
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It used to be a common practice that to fill the field of a low magnification objective with light, you removed the top lens. Some condensers had two N.A.s; one with the top lens and one without. A flip top condenser does the same thing but more conveniently.

An auxiliary lens turns the condenser into a wider field version but it usually isn't wide enough for really low magnification objectives, without also lowering the position of the condenser , something that effectively takes place in situ., with the flip top condenser.

Flip tops are more efficient at dealing with lower magnification objectives, down to 2.5X or so. Usually, the aux. lens can't get the illumination circle out that far, unless the original condenser was already of a wider field.


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PostPosted: Mon Jul 08, 2019 10:42 pm 
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Thank you 75RR for the explanation and the link. The bit on condensers aids nicely.

And thanks, Apochronaut and MicroBob for further clarification. It will be fun to try some of these things out first hand to see the explanations in action. Eventually, that is, pending acquisition of misc needed items and time to play.

Heather


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PostPosted: Mon Jul 08, 2019 10:49 pm 
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apochronaut wrote:
It used to be a common practice that to fill the field of a low magnification objective with light, you removed the top lens. Some condensers had two N.A.s; one with the top lens and one without. A flip top condenser does the same thing but more conveniently.

An auxiliary lens turns the condenser into a wider field version but it usually isn't wide enough for really low magnification objectives, without also lowering the position of the condenser , something that effectively takes place in situ., with the flip top condenser.

Flip tops are more efficient at dealing with lower magnification objectives, down to 2.5X or so. Usually, the aux. lens can't get the illumination circle out that far, unless the original condenser was already of a wider field.


The Leitz 600 series uses a 'K4' auxiliary push-in lens with the wide-field of the Orthoplan to fill the objective with light down to as you say Phil, 2.5x (top-lens flipped out below n.a. of about 0.25) - this I know from experience. Below this there's a 'special' condenser for the 1x objective.....

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PostPosted: Tue Jul 09, 2019 11:02 am 
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MicroBob wrote:
Hi Phil,
thank you for this thorough explanation! So to test different condensers it would be best to use a highly corrected objective and compare the corners of the image.
Did I understand it right that out of focus areas are also dependend on the condenser correction?



Bob


No, probably I didn't word it correctly. I was referring to the focus of the condenser. I can't see the condenser affecting the microscope's focus.

The real problem with poorly corrected condensers is that they bring with them a whole bunch of undesirable optical artifacts that increase in severity off axis; ca,coma,astigmatism and spherical aberration. This won't cause out of focus but it would cause undesirable diffraction at the very least.

Creating a perfect lens has been a long sought after goal. The problem was first described thousands of years ago. However, a couple of post doctoral physicists from Mexico have written a formula for a perfect single lens and created a computer simulation of the profile. The Eureka moment came apparently , while one of them was spreading Nutella on bread, no doubt imagining that the Nutella was glass. Turns out the surfaces are incredibly convoluted and the exit surface is a sort of negative image of the first surface, although altered somewhat by the refractive index of the glass. Presumably, he realized that the Nutella( must have been a really hot Mexican day) conformed to the profile of the bread but not exactly. Now , getting that to magnify would be a trick, since I assume that the original would be a 1:1 lens.
The formula is about 1/2 a page long.


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PostPosted: Fri Jul 12, 2019 6:58 pm 
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I picked up these two condensers recently for cheap. After very briefly testing by just holding them in place under the scope (they need serious modding to fit my scope), a few things are apparent. It looks like as Apochronaut and others have stated, the the paraboloid works well at 20x and maybe even 10x (image background is not terribly dark tho), and the cardioid work betters at 40x, although I must make some adapters so the condensers fit well enough to further assess. The "hand-held" method is not endorsed by anyone other than the truly impatient.

Will these condensers take a drop of oil? I assume they do but would prefer to be sure in case they could be ruined. Is there a way to tell?

And also, brushed upon in other posts, is there a way to retrofit or add light to a common microscope to boost the wattage? My system used a 6V 30W halogen bulb and it seems that the need for more light could be part of the reason my scope does not get better DF. And maybe even Rheinberg. Perhaps overall design comes into play also. It seems especially obvious with the cardioid.

I am not opposed to adding a second scope to my collection either, but that's another topic altogether.

Heather


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PostPosted: Fri Jul 12, 2019 8:49 pm 
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Sauerkraut wrote:
Will these condensers take a drop of oil? I assume they do but would prefer to be sure in case they could be ruined. Is there a way to tell?
And also, brushed upon in other posts, is there a way to retrofit or add light to a common microscope to boost the wattage? My system used a 6V 30W halogen bulb and it seems that the need for more light could be part of the reason my scope does not get better DF. And maybe even Rheinberg. Perhaps overall design comes into play also. It seems especially obvious with the cardioid.

First question:
in topic
viewtopic.php?f=5&t=7653&p=67288&hilit=peeling#p67288
Responses no. 5 and 6 down the thread, see photos of the parts of a cardiodid condenser, by 75RR and myself, respectively; mine is a Zeiss, though somewhat different than yours. The "heart" of the cardiodid (pun) is a sealed unit, that is fitted within a round shell that consists of several parts and rings, and there are no o-rings or other elastomer seals. Hence, in principle, oil laid on the top surface might crawl and leak inside, between the heart and the shell. It happened in my condenser once and I suspect it happens every time I use it, especially since my immersion oil is Type A, which is not very viscous. Yet, since the condenser is designed to tolerate oil, I fear not. Moreover, some of the black coating or paint has been peeled off my condenser, and I bought some modelling matt black paint to re-coat it, but did not do it yet. I will report when done.

Second question: more light is always a benefit, provided it does not warm up the other parts of the scope and specimen. For example: on a certain Olympus scope, 20W halogen is very good, 50W has melted the plastic fixtures and damaged the base. If the lamp is external to the microscope, behind it, it will be easy. If the lamp resides within the base, and you stick with halogen, be careful. LED retrofit might be the best way to go, PROVIDED that the LED is very bright. From my experience, for the cardiodid, a 10W LED is the absolute minimum, and 20W may be even better (depending on the distance of the LED from the collector optics) ; 3-5W LED is too dim IMO. Most DIY LED retrofits on the web have been limited to LEDs of 1-5W, I think.

There are at least two suppliers of tailored LED retrofits for old microscopes, one in Texas and one in Germany or the Nederlands, and the prices are roughly 150 USD.
May I suggest, if you consider conversion to LED, that you post the scheme of the optical train of the microscope, if available.

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PostPosted: Sat Jul 13, 2019 3:30 am 
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Those should both be good condensers. I have one of them.
Both cardioids and paraboloids are oil condensers. They must be oiled to the slide to work properly. If such a type leaks oil into the interior, it is defective, not really reparable. I'm surprised you could get any impression by holding them unoiled under your stage but I guess at low magnifications, you might see something.

Retrofits using led into existing halogen or tungsten illumination systems for high resolution microscopy should be considered as experimental. If you know of a system that works for sure and you can copy that system, that's probably a good idea, otherwise you've got some diy'ing to do.
Probably, some of them are o.k. for DF up to 40X but once you jump over the line to a higher resolution objective, typically requiring a minimum of 50 watts, the possibility that an led might not focus the illumination beam properly using the existing optics engineered for a filament and fail to deliver enough light, is high.

Some people have had good results by by-passing the existing illuminator and putting a dedicated high wattage led under the stage. Probably 20 watts is the minimum required, some use 40.
I have used an led in a self-illuminated DF condenser , originally designed for 13 watts right up inside the condenser. 5 watts of led works well there.


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PostPosted: Sat Jul 13, 2019 7:02 am 
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The fist condenser has a kind of well around the top lens. The oil is supposed not to flow beyond this well.


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PostPosted: Sat Jul 13, 2019 7:21 am 
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Quote:
I am not opposed to adding a second scope to my collection either, but that's another topic altogether.
Unless you are an inveterate tinkerer and therefore enjoy adapting condensers and fiddling with the illumination more than viewing through the eyepieces,

this is probably a good time to consider getting yourself a used modular microscope from the big four and just plugging in the module you need.

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PostPosted: Sat Jul 13, 2019 2:30 pm 
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Thank you for the responses - all good information. I probably can't get back to the microscope until Monday, but did have time yesterday to turn a holder for the paraboloid condenser on the lathe. Painted it black to deal with wood splinter bits and it seems ok in that regard.

Lighting is on my to do list - thank you for the suggestions. And so is adding a scope to the mix. I wouldn't mind having phase contrast either, just for the learning experience.


Heather


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PostPosted: Sun Jul 14, 2019 9:39 am 
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The Zeiss Jena condenser has an included excentrical adjustment. That should make it easier to adapt it to other stands.


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