Temperature & cemented doublet
Helpful person
optical system that can see a large temperature variation. I have a 5
inch diameter lens with a coefficient of thermal expansion difference
between the two elements of 3.5 x 10-6. I expect a temperature range
of 150C. The glass types are SF and LaF.
I am not concerned with performance degradation due to stress but
mechanical failure, the element breaking or the cement cracking. Does
anyone have any direct experience in this field, preferably by
experiment?
Thanks,
Bob May
of expansion of the two glasses to see how far off they are. Also, the type
of cement will also matter as Canada Balsam will react better than any of
the thinner UV curing cements.
--
Bob May
rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net
Richard J Kinch
> I am not concerned with performance degradation due to stress but
> mechanical failure, the element breaking or the cement cracking.
You mean the cement shearing, correct?
Seems like a real possibility. If I understand your specs right, you could
have 30 microns of shear displacement at the edges of the two pieces.
Helpful person
skrubin
At Thorlabs we heated doublets up to 200C without them breaking apart.
BUT, there is no spec guaranteeing this and I don't think our sales
team would gurentee this, it is just my experience.
Sam
Helpful person
mp...@oscintl.com
> of 150C.
Does
> anyone have any direct experience in this field, preferably by
> experiment?
Yes I have heated smaller diameter doublets from 0.5" diameter to
about 3" diameter, and have had cohesion failures of the cement and
the lenses came apart, either at the edge - like feathering or
fractals in from the outside edge.
Norland cements have an elongation at failure in the range of 30-40%
depending upon the product number chosen.
At your diameter or radius of about 75mm or 0.075m you can expect
about 39um of radial shear in the bond joint between the two surfaces
(you will also see some axial stress due to radii changes of the two
surfaces) at the edge for the full 150C range from room temp. So your
bond joint thickness and optical cement are key to know here, and my
experience is that the bond joints will be smaller so you may be in
trouble here if you want them to survive.
Michael
www.oscintl.com
Phil Hobbs
There's also the fatigue issue--shearing the cement like that is liable
to lead to delamination problems after N cycles even if it survives
proof testing. That's a pretty horrific temperature range for the
mounts, let alone the cement, and it's also wide enough that CTE
nonlinearities are likely to be important. I'd be taking a hard look at
air spacing and flexures, myself.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
914-762-4837
hobbs at electrooptical dot net
http://electrooptical.net
Helpful person
Thanks all for your replies. I was looking for a glimmer of hope that
doublets of this size might survive. I can envisage a few
manufacturing methods that might make it possible but do not want to
undergo a major research effort.
Bob May
variations. The bad side is that you will be wanting to sort of seal the
cell to keep the thin film of oil to stay in the space between the lenses.
One way to cure the problem.
Another is to use a soft cement like Canada Balsam (an old cement used from
way back when) to glue the two elecments together. It will usually fair way
before the glass will fail and you can just reglue the two geter again.
Xylene is a good liquifier for the Balsam.
Helpful person
Thanks, but I do not believe either method is practical without a
major project for verification of process. Oil requires a sealed
reservoir and Canada Balsum is not suitable for for military
environments.
Bob May
the "better" UV curing glues came along and took the time out of the
application of gluing optics together??
mp...@oscintl.com
>
> Thanks, but I do not believe either method is practical without a
> major project for verification of process. Oil requires a sealed
> reservoir and Canada Balsum is not suitable for for military
> environments.
>
> www.richardfisher.com
Richard:
I noticed both were flint glass types.
If you can tell us some general design guidelines you/we might be able
to come up with an airspaced design that will meet all your
requirements, including the thermal ones, or design the system with
matched or closer CTE's. For example the SF class ranges in CTE from
6.1 to 9.5 or by 50%. I know you/we can get a closer match than that
and probably meet your optical requirements. Plus if you air space
the doublet you get two more air glass interfaces for aberration
correction, you do have to ar coat them to keep the xmission high. So
I dont think you have to give up on this idea yet.
Michael
www.oscintl.com
Helpful person
skrubin
Hi sorry for the delay in getting back to you on this. was trying to
dig up the info but could not find it yet. all i recall was that we
had to coat a couple of assembled doublet so put them in teh coating
chamber and they did not break. when i find the exact info i will
shoot you a reply
Sam
mp...@oscintl.com
>
> - Show quoted text -
Richard:
Your welcome.
I understood from your earlier comment " I was looking for a glimmer
of hope that
doublets of this size might survive." that you didnt think it was
possible anymore, without a "major research effort" and I disagree.
How ever only you know the specific requirements for your application.
I believe that it may be possible to solve with some design effort,
and I was trying to provide you some possible design solutions that
are within the realm of an engineering solution. I do this with my
lens design coaching and mentoring clients when they have difficult
problems to solve as well.
Sorry if you interpreted this as a lens design lecture.
Michael
www.oscintl.com