Setting up AO20: help reqd with Koehler

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sailor_ed
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Setting up AO20: help reqd with Koehler

#1 Post by sailor_ed » Sun Nov 25, 2018 8:02 pm

I am setting up a new to me AO20 and something is wrong that is outside my limited expertise. I am using a 1240 turret with its 1242 condenser lens. I have removed the lower doublet as indicated in the manual I downloaded for the scope. It appears to be properly secured in the yoke. The problem is that when focusing on the field diaphragm the condenser hits the specimen slide before focus is achieved. This is with the 10x dark phase objective and bright field. Other objectives are similar.

Judging by the looks of things the stage height is proper. Is something in the stand out of adjustment or am I!? ;)

Thanks... Ed

photomicro
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Re: Setting up AO20: help reqd with Koehler

#2 Post by photomicro » Sun Nov 25, 2018 8:31 pm

sailor_ed wrote:I am setting up a new to me AO20 and something is wrong that is outside my limited expertise. I am using a 1240 turret with its 1242 condenser lens. I have removed the lower doublet as indicated in the manual I downloaded for the scope. It appears to be properly secured in the yoke. The problem is that when focusing on the field diaphragm the condenser hits the specimen slide before focus is achieved. This is with the 10x dark phase objective and bright field. Other objectives are similar.

Judging by the looks of things the stage height is proper. Is something in the stand out of adjustment or am I!? ;)

Thanks... Ed
At the top of the substage mount, at the right hand side, there is a small grub screw that prevents racking up so far that the condenser top hits the slide. Adjusting this (with an Allen key) will stop that happening, but doesn't solve your focussing issue.

Apologies for asking such a question, but is the specimen in focus?

Another possibility is a diffuser filter in one of the filter turrets...this might be preventing focusing on the diaphragm.

I take it you have got the hang of the rather quirky (to me anyway) diaphragm lever and adjustment controls, and indeed the lamp lens collector control.

The stage can indeed be raised or lowered, using the unlocking mechanism.

It is only very recently that I started using one of these too.

Hope it becomes clear soon!

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75RR
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Re: Setting up AO20: help reqd with Koehler

#3 Post by 75RR » Sun Nov 25, 2018 8:37 pm

Make sure the slide you are using is the right way up - trying to focus through the slide can have this effect.
Zeiss Standard WL (somewhat fashion challenged) & Wild M8
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sailor_ed
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Re: Setting up AO20: help reqd with Koehler

#4 Post by sailor_ed » Sun Nov 25, 2018 9:34 pm

Thanks for the replies. Unfortunately the specimen is right way up, in focus and no filters.

There is a lens in the base and the field diaphragm itself but I am reluctant to start playing under there until I have exhausted all other options.

Thanks again.

einman
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Re: Setting up AO20: help reqd with Koehler

#5 Post by einman » Mon Nov 26, 2018 12:02 am

I have owned many AO20's and the instructions included in the manual should work. I generally skip the part about focusing on a specimen and use a blank slide without a coverslip. The purpose of focusing on a specimen is to bring the objective within the approximate position so you can focus on the leaves of the condenser aperture.

Make sure you are methodical. If you still can get the condenser leaves into focus then you may have to adjust the stage height. That, too, is discussed in the manual.

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Re: Setting up AO20: help reqd with Koehler

#6 Post by sailor_ed » Mon Nov 26, 2018 3:39 am

If I were to focus on the leaves with a blank slide it would lift the slide by several tenths of a millimeter or more. If I start to raise the stage I am almost sure I will hit the 100x objective on the slide. I have never used a scope that seemed to have no room for error. It seems to me there must be some other problem. Thanks all for your input!

PeteM
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Re: Setting up AO20: help reqd with Koehler

#7 Post by PeteM » Mon Nov 26, 2018 4:07 am

One remote possibility -- the screw-on-top condenser lens isn't fully seated? Maybe even dried oil holding a bit unthreaded and high??

MichaelG.
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Re: Setting up AO20: help reqd with Koehler

#8 Post by MichaelG. » Mon Nov 26, 2018 9:08 am

You have my sympathy, Ed ... but regrettably I can offer no insight:

I spent most of yesterday struggling, to no avail, with a similar situation on a Vickers Patholux [with the Trilux condenser] ...
This should be easy, but, like you I am starting to doubt my own 'adjustment' :(

MichaelG.
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Re: Setting up AO20: help reqd with Koehler

#9 Post by wstenberg » Mon Nov 26, 2018 12:48 pm

I was just struggling with a very similar situation yesterday on my Zeiss Standard. I purchased a POL condenser on Ebay. After putting it on, I could not focus on the leaves when adjusting for Koehler. I tried another condenser, and it still wouldn't work. Tried everything I could think of. Finally used a condenser off a working microscope, and it worked beautifully.

Sorted it out: I have two working condensers, and two non-working condensers. All four look identical. All Zeiss West Germany with the exact same markings. Another hour or so, I found that the two non-working condensers were missing the auxiliary lens from the undersurface. No wonder they were such good deals on Ebay!

I know this doesn't help with AO20, except maybe try a condenser that is known to work properly.
William
Astoria, Oregon

Zeiss Axiomat
Zeiss Stereomikroskop
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photomicro
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Re: Setting up AO20: help reqd with Koehler

#10 Post by photomicro » Mon Nov 26, 2018 1:59 pm

sailor_ed wrote:If I were to focus on the leaves with a blank slide it would lift the slide by several tenths of a millimeter or more. If I start to raise the stage I am almost sure I will hit the 100x objective on the slide. I have never used a scope that seemed to have no room for error. It seems to me there must be some other problem. Thanks all for your input!

You should be OK to raise the stage, as the focus mechanism on these models involves the nosepiece moving up and down. Indeed, with the objectives clear of anything, it might be worth just checking if this hasn't stuck.

The trouble with this type of thing is that there could be a dozen things causing it!

apochronaut
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Re: Setting up AO20: help reqd with Koehler

#11 Post by apochronaut » Mon Nov 26, 2018 3:47 pm

I echo photomicro's comments but first, I think you should set your stage to it's default position. Under such conditions the 1242 condenser achieves Koehler illumination about 1/2mm below the slide. There should be no problem. If this does not take place, it would be best to post a picture of the underside of your microscope. Perhaps something in the optical path of the illumination system is awry.

Stage default. There is a hex head cap screw threaded into the male dovetail slide of the stage height dovetail.
The female or stage side of the dovetail has a semi-circular detent on it's top. Sliding the stage up to the cap screw stop establishes normal stage height.
With the stage in this location and the 20X objective focused on a standard slide with a .17mm cover, the 1242 condenser should focus the field diaphragm as stated, about 1/2mm below the slide.
edit.
Thanks Ed, for pointing out that at that default position it is impossible to install the 1240 condenser when the aux. lens is mounted. Raising the stage slightly to the point where the two dovetail sections are level is a better place to start with. This entails backing off the hex cap screw slightly.

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Re: Setting up AO20: help reqd with Koehler

#12 Post by sailor_ed » Mon Dec 03, 2018 3:42 pm

To wrap up this thread : It turned out that a small upwards adjustment of the stage was all that was required to fix my problems. Thanks for all the input, it really helped! The forum is a fantastic resource.

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