Normal Minimum Size of Aperture

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chhejr
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Normal Minimum Size of Aperture

#1 Post by chhejr » Mon May 23, 2022 1:21 pm

I have a Nikon Diaphot inverted microscope, and I've been trying to get it into Köhler illumination which involves closing the aperture iris diaphragm as small as possible. On my microscope it closes to about 1.5 mm in diameter. My question is: Is this normal for this microscope or should it close to a smaller diameter? The problem is that when I try to put it in Köhler illumination using a 40x lwd objective the diaphgram opening when viewed through the microscope is larger than the field of view when the diaphragm is fully closed. Is this normal? Or does this indicate that the diaphragm is broken or needs to be adjusted somehow?

BramHuntingNematodes
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Re: Normal Minimum Size of Aperture

#2 Post by BramHuntingNematodes » Mon May 23, 2022 1:59 pm

You could drape a thin wire over the gap and focus on that if you like
1942 Bausch and Lomb Series T Dynoptic, Custom Illumination

chhejr
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Re: Normal Minimum Size of Aperture

#3 Post by chhejr » Mon May 23, 2022 3:06 pm

Thanks. That could work. But I'm still curious about the minimum diagraphm aperture size. What should the minimum diaphragm aperture size actually be?

BramHuntingNematodes
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Re: Normal Minimum Size of Aperture

#4 Post by BramHuntingNematodes » Mon May 23, 2022 6:12 pm

I think for a field iris a 1.5 mm gap is not.necessarily a sign of malfunction as even that would likely be too tight for almost any circumstance I can think of. I have many irises and while some vanish to a pin prick others have a bit of a gap.

I have also taken apart and repaired a few irises, some successful even, and I wouldn't recommend unless it is absolutely necessary. In the first because it's not a fun exercise and in the second you are most.likely not going to improve performance unless something is very obviously wrong. The most likely candidate here would be that some well meaning doofus oiled the leaves and this dried oil is now keeping it from closing all the way. If you can see a lot of grime you might try some electronics cleaner spray while opening and closing it.
1942 Bausch and Lomb Series T Dynoptic, Custom Illumination

apochronaut
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Re: Normal Minimum Size of Aperture

#5 Post by apochronaut » Tue May 24, 2022 12:29 am

How do you know it is larger than your 40X f.o.v.?

MichaelG.
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Re: Normal Minimum Size of Aperture

#6 Post by MichaelG. » Tue May 24, 2022 10:12 am

apochronaut wrote:
Tue May 24, 2022 12:29 am
How do you know it is larger than your 40X f.o.v.?
Presumably because the edges cannot be seen when the condenser is in its centred position
… or am I missing some subtlety in your question ?

MichaelG.
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apochronaut
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Re: Normal Minimum Size of Aperture

#7 Post by apochronaut » Tue May 24, 2022 11:20 am

I was just wondering if it's perceived absence is based on looking for it or measuring it? If he achieves an image of the fully closed diaphragm with a 20X objective , then with a 40X objective , the image will be twice as big in the field. It is either in the field or not.
As the magnification increases, the image of the diaphragm becomes less distinct and lower in contrast, making it more difficult to see, especially if there is even a small lack of parfocality because the leaves will not be in focus plus sometimes the illumination is high and full of flare.

Closing off the condenser iris will increase contrast and make it easier to see.

I have certainly had problems picking out the edges of the leaves with 100X objectives. It isn't usually that bad with a 40X but using a L.W.D. condenser of low N.A. would alter the imaging some.
Last edited by apochronaut on Tue May 24, 2022 12:27 pm, edited 2 times in total.

MichaelG.
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Re: Normal Minimum Size of Aperture

#8 Post by MichaelG. » Tue May 24, 2022 12:11 pm

Thanks … that makes sense

MichaelG.
Too many 'projects'

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