Objective repair question: old Zeiss finite 20x Apo

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BramHuntingNematodes
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Objective repair question: old Zeiss finite 20x Apo

#1 Post by BramHuntingNematodes » Tue Mar 30, 2021 5:07 pm

After completely destroying my Zeiss Epiplan 40x last time I took one of these apart, I got a hankering to do it again. Here we have an older Zeiss 20x Apo with bad delamination, but only on the first lens pack! This seems like an ideal candidate for repair as it entails removing only one element, taking apart I think a triplet? and recementing. The objectives from this era seem very easy to take apart: just take off the back baffle and unscrew the cuff, push up gently from the nose in. Now, I am certain I can get these lenses apart once I can get them out of the lens barrel cage, but how do I do that? Any suggestions from the folks I know have done it before?

THanks,

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1942 Bausch and Lomb Series T Dynoptic, Custom Illumination

apochronaut
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Re: Objective repair question: old Zeiss finite 20x Apo

#2 Post by apochronaut » Tue Mar 30, 2021 6:05 pm

I repaired one of those, once, a long time ago. Hopefully you registered the scribed marks against a feature of some kind on the barrel, so everything goes back in the same way.
Those cemented lenses used balsam. Balsam is soluble in ethanol. I took a modern pill bottle , the ones with the child proof push and turn cap and made top and bottom spacers out of wine corks. The best size of bottle, is one that is about 1" across and 3" long. The next size down will work too but you have to cut the corks more.
This involved making a depression in the end of a cork so that it fits evenly and perfectly over the punt in the bottom of the bottle and shaping the other end so that it fits perfectly against one of the lenses of the delaminated complex. Then take another cork , shaping it to fit the other side of the lens pack and cut it off with a sharp knife, so that when the cap is put on, the plastic compression insert just compresses against the cork pushing a little on the lens pack.
The lens pack will be securely sandwiched between the shaped corks under some small pressure.
Fill the bottle with ethanol and leave overnight.
In the morning, drill some holes in the cap and drain the ethanol. Then drill more holes in the bottle here and there and leave for a day or two to dry, perhaps with the benefit of sun or a fan.
This fixed the one I had but other times with a few other lenses it did not, leaving the same swirl pattern behind.I think in the latter instances, there was an imperfect lens fit, the lens leaving the factory already imperfect.
If the lens is lousy anyway, you have nothing to lose. You may still be able to separate the elements and re-cement it.
Your lens looks like it may be sealed in by a type of swaging , so dissolving the cement in situ may be your only way to salvage it.

BramHuntingNematodes
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Re: Objective repair question: old Zeiss finite 20x Apo

#3 Post by BramHuntingNematodes » Tue Mar 30, 2021 6:54 pm

It's the swaging I'm esp curious about. I have it soaking in alcohol, and I can see it infiltrate between the lens elements, but I cant get them out of this brass barrel. If it's more cement, then your method of applying some gentle pressure might be sufficient, but if there is some mechanical locking in place I may need to rethink my strategy.

Photo's were taken prior to disassembly. The previous assembler also has etched some useful marks on the barrel including an arrow indicating which direction to orient the pack!
1942 Bausch and Lomb Series T Dynoptic, Custom Illumination

MicroBob
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Re: Objective repair question: old Zeiss finite 20x Apo

#4 Post by MicroBob » Tue Mar 30, 2021 7:06 pm

These balsams also get soft when warm. In some cases it is enough to warm the lens group and the balsam layer reattatches to the surfaces of the lenses.
If that doesn't help I would suggest a bath in xylene. If the xylene enters the gap I would put the lens group in a warm xylene-canadabalsam bath for a day in a closed bottle. Then let the xylene evaporate and see whether the balsam layer looks good. These old objectives are probably much more repairable than the newer objectives with UV-curing cement.

I one tried to keep track ot the orientation of the lenses in a cemented triplet. The line with a diamond needle worked as a stress riser and the middle lens cracked right through in two lines, inside the still attatched triplet! :roll:

BramHuntingNematodes
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Re: Objective repair question: old Zeiss finite 20x Apo

#5 Post by BramHuntingNematodes » Tue Mar 30, 2021 7:23 pm

wow it just fell out. I had to soak for 24 hours not just 16 I guess.
1942 Bausch and Lomb Series T Dynoptic, Custom Illumination

MicroBob
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Re: Objective repair question: old Zeiss finite 20x Apo

#6 Post by MicroBob » Tue Mar 30, 2021 8:08 pm

It is important to soak over night as the dwarves don't come out in daylight!

BramHuntingNematodes
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Re: Objective repair question: old Zeiss finite 20x Apo

#7 Post by BramHuntingNematodes » Wed Mar 31, 2021 5:30 pm

Destroyed another one :(

I had it all cemented up so carefully did a pretty good job couple bubbles off center but otherwise real good and then one of the lenses completely disintegrated as I tried to get it back into its cage. Hopeless.
1942 Bausch and Lomb Series T Dynoptic, Custom Illumination

BramHuntingNematodes
Posts: 1538
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Re: Objective repair question: old Zeiss finite 20x Apo

#8 Post by BramHuntingNematodes » Wed Mar 31, 2021 5:47 pm

For the future repair attempts: the final lens of the triplet should have been cemented inside the barrel cage. This would have ensured perfect concentricity as well as attaching the lenses to the barrel with the squeeze out.
1942 Bausch and Lomb Series T Dynoptic, Custom Illumination

ScienceMatters
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Re: Objective repair question: old Zeiss finite 20x Apo

#9 Post by ScienceMatters » Thu Apr 01, 2021 1:03 am

Well, good effort. Interesting for the rest of us to read about!

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