dismantling and rebuilding a cemented doublet

Hi all,

I have a small cemented doublet which has some staining between the
elements. Is there a way to dismantle it for cleaning and how should I
rebuild it afterwards?

Thanks
Robin

"Robin Leadbeater" wrote:

I have a small cemented doublet which has some staining between the
elements. Is there a way to dismantle it for cleaning and how should I
rebuild it afterwards?

My advice would be "don't!". However, I'm no optician. You might want
to ask on sci.astro.amateur where a few refractor-makers hang out.


Tim
--
Foo.

"Tim Auton" wrote in message
...
"Robin Leadbeater" wrote:

I have a small cemented doublet which has some staining between the
elements. Is there a way to dismantle it for cleaning and how should I
rebuild it afterwards?

My advice would be "don't!". However, I'm no optician. You might want
to ask on sci.astro.amateur where a few refractor-makers hang out.

Hi Tim,

Oh I am happy to submit this one for experimentation. I got it cheap from
SurplusShed in the US. When I pointed out the flaw, they sent a replacement
by return and told me to keep the old one. (Amazingly efficient and cheap
too)

Robin

Robin Leadbeater wrote:

"Tim Auton" wrote in message
...

"Robin Leadbeater" wrote:

I have a small cemented doublet which has some staining between the
elements. Is there a way to dismantle it for cleaning and how should I
rebuild it afterwards?

Worth thinking about that before you take it apart. You want something
like DPX slide fixative or Canadian balsam - available from your
friendly microscopy shop. Don't be tempted to use cyanoacrylate - it is
clear and watery but it plates out wisps over optical surfaces as it
cures. One of its other uses is for developing fingerprints. Not sure
about clear epoxy - but I suspect bubbles will be an issue with any two
part adhesive.

My advice would be "don't!". However, I'm no optician. You might want
to ask on sci.astro.amateur where a few refractor-makers hang out.

Oh I am happy to submit this one for experimentation. I got it cheap from
SurplusShed in the US. When I pointed out the flaw, they sent a replacement

On the understanding that you may not be able to put it together again
and ignoring the risks associated with aromatic solvents. Immersing in
tepid xylene or toluene should encourage it to fall apart eventually
(Hammerite paint thinner will probably do). But unless you can find
suitable cement I wouldn't do it.

Regards,
Martin Brown

Martin Brown wrote:

On the understanding that you may not be able to put it together again
and ignoring the risks associated with aromatic solvents. Immersing in
tepid xylene or toluene should encourage it to fall apart eventually
(Hammerite paint thinner will probably do). But unless you can find
suitable cement I wouldn't do it.

Regards,
Martin Brown

IF its Canadian Balsam,then gentle heating, possibly in a double water
bath, will part canadian balsam. at around 50-55 C. If its too hot to
hold and hasn't parted, it ain't CB.

Steve

Presumably the refractive index of the cement matters too, if you
want to retain optimal freedom from colour.


Martin
--
M.A.Poyser Tel.: 07967 110890
Manchester, U.K. http://www.fleetie.demon.co.uk

I had some success with a bino objective. I marked the edges to
re-orient them correctly, then heated it gently over a 100W bulb,
presto it came apart and cleaned up ok. Then a blob of Canada balsam
in between and reverse the process with gentle pressure, you don't
need much balsam.
jc

--
http://mysite.wanadoo-members.co.uk/jc_atm/



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Most come apart with heat.

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"Robin Leadbeater" wrote in message
...
Hi all,

I have a small cemented doublet which has some staining between the
elements. Is there a way to dismantle it for cleaning and how should I
rebuild it afterwards?

Thanks
Robin

Why would you want to use an adhesive? You'd only get one chance and
have no idea of the adhesive's effect on the glass. Or it's own opacity
over time.

I cemented a few small hom-made (60mm) achromats with balsam cement
many years ago. I believe I used meths to clean up the residue of the
old cement which clung rather tenaciously. Those who have oiled their
own achromats will know that only a tiny amount of liquid is necessary
to fill the space between the elements.

Have your glass spotlessly clean and handle the lenses only by the
edges. The Pink Marigolds might come in handy at ths point. You place a
blob of balsam cement in the middle of the concave surface. Then
carefully lower your convex onto the blob (the right way round if it's
a biconvex! You did mark the elements permamently before dismantling
didn't you?)

Now watch as the balsam expands magically to finally run stickily off
the edges. So have plenty of paper towels beneath the doublet and meths
handy to clean up the mess. Leave it to harden a bit before attacking
ithe edges with too much meths covered tissue or cloth. Or you could
find the meths creeping into the gap between the elements and have to
start again!

I also remember reading about loading weights onto the achromat to
ensure a close seperation. But this may cause distortion. Balsam is not
usually used for anything over 75mm if I remember correctly. Something
to do with the strain the balsam would cause with changing temperature
on a cemented doublet. The front element being in contact with the cold
dew. While the rear element is warm and cosy in its sealed refractor
tube.

Chris.B

wrote in message
ups.com...
Why would you want to use an adhesive? You'd only get one chance and
have no idea of the adhesive's effect on the glass. Or it's own opacity
over time.

I cemented a few small hom-made (60mm) achromats with balsam cement
many years ago. I believe I used meths to clean up the residue of the
old cement which clung rather tenaciously. Those who have oiled their
own achromats will know that only a tiny amount of liquid is necessary
to fill the space between the elements.

Have your glass spotlessly clean and handle the lenses only by the
edges. The Pink Marigolds might come in handy at ths point. You place a
blob of balsam cement in the middle of the concave surface. Then
carefully lower your convex onto the blob (the right way round if it's
a biconvex! You did mark the elements permamently before dismantling
didn't you?)

Now watch as the balsam expands magically to finally run stickily off
the edges. So have plenty of paper towels beneath the doublet and meths
handy to clean up the mess. Leave it to harden a bit before attacking
ithe edges with too much meths covered tissue or cloth. Or you could
find the meths creeping into the gap between the elements and have to
start again!

I also remember reading about loading weights onto the achromat to
ensure a close seperation. But this may cause distortion. Balsam is not
usually used for anything over 75mm if I remember correctly. Something
to do with the strain the balsam would cause with changing temperature
on a cemented doublet. The front element being in contact with the cold
dew. While the rear element is warm and cosy in its sealed refractor
tube.

Chris.B


Thanks Chris,

Just the information I need. All I need to do now is find a friendly
microscopist to give me a drop of balsam. (The sources I have found so far,
although not unduly expensive, are more than the cost of this surplus
optic!)

Robin
-----------------------------------------------------------------------
Robin Leadbeater
54.75N 3.24W
http://www.leadbeaterhome.fsnet.co.uk/astro.htm
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