100x objective funnel stop

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daruosha
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100x objective funnel stop

#1 Post by daruosha » Thu Mar 26, 2020 10:05 pm

I found dark field is my very favorite type of illumination and I really like to try it at 100X magnification , however I do not have an objective with NA iris and my condenser is a usual abbe type with no fancy stuff (hopefully in a near future I'll have a nice dedicated DF condenser). I was wondering can I make a DIY funnel stop and insert it into the back of my 100x objective?

I have 2 100x objectives, the plan version doesn't have any space to put anything on its rear lens, but the non-plan achro version is virtually empty and it seems I can insert a funnel stop into it rear portion.

Do you have any experience with funnel stops? Are the any specific requirements? what is the optimum size and design?
Daruosh.

apochronaut
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Re: 100x objective funnel stop

#2 Post by apochronaut » Thu Mar 26, 2020 11:15 pm

Funnel stops are very specifically designed and the aperture allowed for precise performance needs to be dead on. It would be hard to make a diy one that could remain centered as well, unless it could be threaded into the threads for the rear diaphragm of the objective. The factory versions were usually threaded into that same thread. Some older designs used a drop in funnel stop but centering was a problem .

With plan objectives , where the number of glass elements creates a lens stack that approaches the back of the lens housing, there is a rear diaphragm that is usually threaded into the back of the objective housing. If the back diaphragm is also not a lens cell, and is only a threaded plate with a hole in it, that can be replaced with a diaphragm of a slightly smaller diameter and it functions very much as a funnel stop and can be made suitable for DF. Getting the correct diameter and providing the correct thread is a bit of a trick.

Occasionally, an objective and it is usually of the same series, can be found with a rear diaphragm that has a slightly smaller hole but the same thread. In this case, another objective can be converted to a DF objective with the simple act of threading in a smaller rear diaphragm from a sister objective.

The Reichert 63X 1.0 glyz. objective easily receives the rear diaphragm from it's sister 100X objective , which lowers the 63X to about .80 , suitable for DF. Since the 100X has an iris diaphragm, it does not need it's rear diaphragm for DF and can freely donate it to the 63X, with no loss of capability.

BramHuntingNematodes
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Re: 100x objective funnel stop

#3 Post by BramHuntingNematodes » Thu Mar 26, 2020 11:43 pm

I have made one with the help of a lathe custom built for the purpose. Tolerances are tight so it's best to move slowly. Start with stock a little more than two times your concentricity precision limit. You want to open the aperture slowly since if you go over you have to start anew. I used aluminum, but brass might machine better. Keep in mind that there is some decrease in NA even with the best of funnel stops as this is the purpose of them-- probably there will be more with a homemade one.
1942 Bausch and Lomb Series T Dynoptic, Custom Illumination

BramHuntingNematodes
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Re: 100x objective funnel stop

#4 Post by BramHuntingNematodes » Thu Mar 26, 2020 11:46 pm

I can add that if you could find one say in a darkfield kit for your microscope that has been raided for other components that would be a good option as making one is a gigantic pain in the neck
1942 Bausch and Lomb Series T Dynoptic, Custom Illumination

MicroBob
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Re: 100x objective funnel stop

#5 Post by MicroBob » Fri Mar 27, 2020 8:33 am

To work with your existing condenser and a dark field stop the aperture of the objective will have to reduced quite far, loosing lots of resolution. I would suggest the first get the dark field setup for the lower magnifications to work and continue with the 100:1 objective when you have located a real dark field condenser.

Hobbyst46
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Re: 100x objective funnel stop

#6 Post by Hobbyst46 » Fri Mar 27, 2020 9:01 am

I support MicroBob's advice above. In addition, if you happen to find an old 100X/1.25 objective, in which the front lens is very small, may want to try it as is, without any funnel, for DF. The background will not be pitch black but fairly dark. Based on personal experience.
Zeiss Standard GFL+Canon EOS-M10, Olympus VMZ stereo

apochronaut
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Re: 100x objective funnel stop

#7 Post by apochronaut » Fri Mar 27, 2020 11:56 am

MicroBob wrote:
Fri Mar 27, 2020 8:33 am
To work with your existing condenser and a dark field stop the aperture of the objective will have to reduced quite far, loosing lots of resolution. I would suggest the first get the dark field setup for the lower magnifications to work and continue with the 100:1 objective when you have located a real dark field condenser.
There is no loss of resolution in DF. On the contrary, there is an increase in resolving power. One could theorize the same for phase contrast too, due to the condenser working at sub 1.0 N.A. and the objective having an N.A. choke built in as well.

If one uses a for instance a 1.25 N.A. phase contrast objective or a 1.25 N.A. BF objective stopped down to .80, both oiled and mated with a 1.25 N.A. oil immersed condenser for BF, yes, there will be resolution loss with both of them and it will be considerable with the stopped down objective.

The oil immersion DF condenser, especially the mirror condensers achieve illumination of selective N.A.s toward it's theoretical upper limit roughly spanning 20 aperture units, between 1.2 and 1.4 for instance. By introducing a very high N.A., chroma free illumination to the subject as essentially very high N.A. incident light, despite the objective suffering from N.A. throttling, contrast differentials in the subject allow for superior detail in the image. It was for this reason that in the early years of DF implementation that it was referred to as " a super microscope".

MicroBob
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Re: 100x objective funnel stop

#8 Post by MicroBob » Fri Mar 27, 2020 6:46 pm

Hi Phil,
so far Daruosh only has a basic abbe condenser that he could equip with a dark field stop. I would guess that he would have to stop the objective down to n.a. 0,6 or 0,7 before he would get a nice dark field image. It also is a question how the deficiencies of the simple condenser will influence the result. So compared to the achromatic mirror dark field condenser with it's clearly defined light cone the results will be worse in this case here.

BTW: I once played around with Amphipleura pellucida diatoms that regularly live in a certain pond here in Germany and that are difficult to resolve. Here circular oblique lighting with a decentered dark field condenser gave the best resolving power but not a beautiful image.

Bob

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