Fixing a Cemented Doublet

Ed T

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Jul 14, 2005, 6:45:50 PM7/14/05

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Hi All - I have an older scope that (in my estimation) deserves a little
investment of time and energy to restore. The main issue currently is the
finder scope eyepiece. It is a three lens system with a cemented doublet
field lens. The cement has become cloudy and I'd like to fix it or have it
fixed. Any of you have ideas? Been down this road?

I'd like to strip the old cement and reglue it or have an optician do it for
me if its beyond me. An alternative would be to replace the doublet, but
how do you "spec" a doublet? I'm guessing I would only be able to
approximate it.

Thanks,
Ed T.

Havr...@aol.com

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Jul 14, 2005, 7:41:52 PM7/14/05

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Most old lenses use Canada Balsam to cement lenses together. CB
melts in hot water and is also soluable in it. Putting it together is
the reverse of taking it apart. I don't know of any way co-center the
optical components except to match up the outside dimensions. Hope
this helps.

Ed T

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Jul 14, 2005, 8:01:03 PM7/14/05

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<Havr...@aol.com> wrote in message
news:1121384512.7...@o13g2000cwo.googlegroups.com...

I wonder if it could be that easy! The lens is Japanese circa 1983 if that
helps ID the cement. I'll give it a try. Thank you.

Ed

Havr...@aol.com

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Jul 14, 2005, 8:42:14 PM7/14/05

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I'm not sure of the water temperature but it is below the boiling
point. What ever it is it shouldent bother the glass any.

Mark S. Holden

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Jul 14, 2005, 8:50:39 PM7/14/05

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Ed T wrote:

One thing I'd look out for is the rotational alignment of the elements.

If you take it apart, and put it back together with the elements rotated
differently with respect to each other it may not perform as well as it
should.

Havr...@aol.com

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Jul 14, 2005, 8:56:41 PM7/14/05

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I tried to look up the proceedure and coulden't find it. I did find
that I was wrong about CB being water soluable. Its soluable in
mineral spirits, turpentine, etc. I was correct about the softening
point in hot water. The suggestion about keeping rotational alignment
is a good one. The lens can be marked on the edges with a magic
marker thats solvent resistant.

jerry warner

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Jul 15, 2005, 2:45:18 AM7/15/05

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makes no difference because Can Balsam melts (to move elements) in
hot water. The key issue is bringing the whole blank to temperature
before trying to slide it apart. Where people get in trouble is trying to
force elements apart before the glass it warmed clear through....
j

jerry warner

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Jul 15, 2005, 2:54:07 AM7/15/05

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hot water is the key... side mark the elements to get them back to their
proper relative position.

The glue could be anything. Balsam (unlikely by 1983). Probably some
water soluable polymer. Send Vixen or Goto an email and ask them
what optical cement they were using in 1983?

When you warm the blank up dont just drop it in boiling water but bring
the temp up gradually. I used to just pop elements in the oven sitting
on tin foil and bring the temp up slowly - that way you can control the
heat/temperature which is advisable if you can. I would not exceed 160F.

jerry

jerry warner

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Jul 15, 2005, 2:56:57 AM7/15/05

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One last thing - when you slide the elements apart hopfully they will
slide easily. Be careful especially at the edges of the elements. No
great pressure downward or upward as you twist or you might break
or take a chip off an element, especially when elements are thin. When
they are hot they are easier to break .... so use common sense in applying
pressure, at all times.
jerry

Ed T

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Jul 15, 2005, 9:03:30 AM7/15/05

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"jerry warner" <jwar...@ai5.net> wrote in message
news:42D75B7E...@ai5.net...

> makes no difference because Can Balsam melts (to move elements) in
> hot water. The key issue is bringing the whole blank to temperature
> before trying to slide it apart. Where people get in trouble is trying to
> force elements apart before the glass it warmed clear through....
> j

Last night I brought a cup of water to boil in the microwave and put the
lens in. Let it set for 15 around minutes. Didn't seem to do much, I may
have to go the solvent route.

Ed T.

Chris.B

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Jul 15, 2005, 1:00:09 PM7/15/05

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Do not bring cold water or even droplets of cold water near hot lenses
or any warm to hot glass for that matter. Wear gloves to protect your
hands from the heat when handling to avoid dropping things on the floor
where they consistently refuse to bounce. Be patient.

Chris.B

Bob May

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Jul 15, 2005, 3:25:11 PM7/15/05

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When you do things like that, put the lens in and then heat it. A low heat
on the stove is a lot better than the microwave as the temp change is
slower.
Go get some Xylene or Xylenol (same stuff) at the hardware store and put the
lens in it for a while. You will see a wavy ring where the index of
refraction changes after a day or so. When that ring gets to the center and
disappears, the Balsam is essentially liquified and you can then get the two
lenses apart.
Don't force the glasses as you will just break them. They will slide apart
with a viscould flow.

--
Why isn't there an Ozone Hole at the NORTH Pole?

ROC

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Jul 15, 2005, 5:31:46 PM7/15/05

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"Chris.B" <chr...@mail.dk> wrote in message
news:1121446809.1...@g14g2000cwa.googlegroups.com...

yep. and remember to go to the bathroom beforehand