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why aren't porro prisms more often cemented?

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#76 patta

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Posted 09 October 2018 - 04:55 AM

Yes, looks like that the binoculars in the video are definitely NOT made to be serviceable; all is kept together with glue!

They claim 11 G of impact resistance, that I don't know how it translates in actual drop. At 12G, probably the prism basket will deform or crack!

 

Anyway, good to know that the issue "Large adjustment, small tolerances" can be solved with a load of magic glue.



#77 MartinPond

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Posted 09 October 2018 - 06:28 AM

Lots of UV-cured epoxy.

A one-way type construction.

11G is disturbingly low. Rubber jackets won't change the shock much.

Already, the waterproof assembly of many binoculars has become

non-recoverable.  You usually need to slash and peel the 'armor' jacket

to get inside, since everything is glued.



#78 saiph

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Posted 29 March 2019 - 03:20 AM

Hey everyone :)

Resurrecting this post with my input.

 

Just got my hands on a Skypoint Military 8x36 binocular, and it shows the same distinct feature - cemented prisms assembly, with no easy way of re-collimation.

 

I'd like to ask for your thoughts on this, how do you collimate such contraption? 

 

Usually, all Porro prisms i've seen so far were mounted as separate individual optics, with or without adjustment screws on the side of the bino body. This one shows neither of these features..

 

Any kind of help is more than welcome.

 

Thanks.



#79 saiph

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Posted 13 May 2019 - 05:44 AM

Well, nothing important, just that i managed to solve that issue with the SkyPoint 8x36 - there were actually some very very fine screws on the sides of each prism assembly block, that allowed for minute tilt of the pair of prisms. It was a rather difficult procedure, but not exactly as painstaking as it was with the Baigish 12x45, since this one DID in fact have some fine adjustment means to get it close to zero - even though it had to be performed still "in blind".

 

As a side note, i've recently collimated another binocular that had sort of a similar prism sub-assembly, only this time the prisms were not glued, but fixed in place with brackets on the perforated plate, and each individual block could be adjusted separately by using three pairs of push-pull screws (one simple and one spring-loaded).  Collimation was smooth like butter, done in "live view mode", with the eyepieces in place, the only thing i had to remove was the lid under each eyepiece, and then put it back after re-collimation was completed.

I think i like the design on these specific binoculars, too bad it's been phased out - well, to be honest, it's not my super all time favorite (not for astronomy anyway - it's a Bushnell 40mm zoom binocular, with a rather narrow aFOV at 7x and not so great optical quality at 21x), but it's the first of its kind built with a sense of responsibility, a metal casing, and the user in mind. I'm not talking about other binoculars, the ones that took the easy way for collimation (easy come, easy go..). Btw, eyepiece focal length synchronisation is done using gears and not that pitiful steel strap most cheap binos use for the "zoom". Considering the fact that i don't have to trash it in case it accidentally breaks during dismantling (like the "strap" ones do), i don't even mind the minor backlash in the gear teeth when changing back and forth between powers using the zoom lever.


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#80 Cory Suddarth

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Posted 17 May 2019 - 08:37 PM

Frank, do you think balsam-cemented Porro II prism separation rates were higher than lens (objective or eyepiece) separation rates, and if so, any theory why?  Maybe due to the higher and more frequent forces applied to prisms due to cantilevering, etc.  Maybe thermal induced stresses, which perhaps apply more unequal forces on prisms than on lenses?  

I believe Porro type II cement separation occurs at a higher rate (compared to lenses) due to physical shock more than thermal cycles. After all, it's the cemented doublets in the objectives and eyelenses that have two differing glass types and Thermal Coefficients of Expansion, whereas, the two joining prisms in the type II's are of identical glass.  And consider this, when nearly any Porro Type I or II, glass gets bounced across the deck, it's always the prisms that suffer, rarely the lenses. I say inertia, mass, acceleration. Or is it the quick stop?undecided.gif

 

Cory 


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#81 Jon Isaacs

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Posted 17 May 2019 - 08:50 PM

I believe Porro type II cement separation occurs at a higher rate (compared to lenses) due to physical shock more than thermal cycles. After all, it's the cemented doublets in the objectives and eyelenses that have two differing glass types and Thermal Coefficients of Expansion, whereas, the two joining prisms in the type II's are of identical glass.  And consider this, when nearly any Porro Type I or II, glass gets bounced across the deck, it's always the prisms that suffer, rarely the lenses. I say inertia, mass, acceleration. Or is it the quick stop?undecided.gif

 

Cory 

 

:waytogo:

 

I spent the last 30 years as a research engineer doing ballastic and a variety of dynamic impact and dynamic testing .

 

My sense is that Cory is right on the money . There's a lot of surface area bonded in an objective and the masses are not cantilvered,  it would be difficult the break the bond. 

 

A cemented prism has momentum that must be transferred is sheer through the adhesive bond . There is also a rotational component. 

 

Jon


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